Consider a rocket in space that ejects burned fuel at a speed of Vex = 1.5 km/s with respect to the rocket. The rocket burns 5% of its mass in 280 seconds. (a) What is the speed V of the

35,623 results
  1. physics

    A rocket is fired in deep space, where gravity is negligible. In the first second it ejects 1/160 of its mass as exhaust gas and has an acceleration of 15.1 m/s^2.What is the speed v_gas of the exhaust gas relative to the rocket?

  2. physics

    A 15.0-kg test rocket is fired vertically from Cape Canaveral. Its fuel gives it a kinetic energy of 1961 J by the time the rocket motor burns all the fuel. What additional height will the rocket rise? m - how do you organize this? how do the J's fit in?

  3. physics

    A rocket engine uses fuel and oxidizer in a reaction that produces gas particles having a velocity of 1380 ms-1. The desired thrust is to be 195000 N. 1-What must be the fuel/oxidizer consumption rate (in kg s-1)? 2-f the initial weight of the rocket is

  4. physics

    A rocket engine consumes 450 kg of fuel per minute. If the exhaust speed of the ejected fuel is 5.2 km/s, what is the thrust of the rocket?

  5. Physics

    A 1.8×104 kg rocket has a rocket motor that generates 2.8×105 N of thrust. What is the rocket's initial upward acceleration? At an altitude of 5000 m the rocket's acceleration has increased to 6.3 m/s2 . What mass of fuel has it burned?

  6. physics

    While taking a plane flight to a meeting, a physicist decides to do a mental calculation of how much fuel the plane would consume in the absence of friction, assuming its engines have an efficiency of 22.0% and the mass of the plane is 200,000 kg. Each

  7. intro to physics

    a toy rocket moving vertically upward passes by a 2.2m- high window whose sill is 7.0m above the ground. the rocket takes 0.17s to travel the 2.2m height of the wondow part a): what was the launch speed of the rocket ? assume the propellant is burned very

  8. Physics

    A small 7.10kg rocket burns fuel that exerts a time-varying upward force on the rocket. This force obeys the equation F = A + Bt^2. Measurements show that at t = 0, the force is 121.0N, and at the end of the first 2.10s , it is 176.0N. Part A Find the net

  9. physic

    A sounding rocket, launched vertically upward with an initial speed of 79.0 m/s, accelerates away from the launch pad at 5.70 m/s2. The rocket exhausts its fuel, and its engine shuts down at an altitude of 1.15 km, after which it moves freely under the

  10. math

    19. a prototype rocket burns 159 gallons of liquid hydrogen fuel each second during a regular launch. Make a table that shows how many gallons of fuel are burned after 2,3,5 and 10 seconds. (3 points) can you help me Ms.Sue

  11. physics

    Model rocket engines are sized by thrust, thrust duration, and total impulse, among other characteristics. A size C5 model rocket engine has an average thrust of 5.26 N, a fuel mass of 12.7 g, and an initial mass of 25.5 g. The duration of its burn is 1.90

  12. Maths

    This question is about the motion of a firework rocket. A firework rocket is fired vertically upwards from the ground. It accelerates uniformly from rest with an acceleration of 8.0 for 5.0 s after which time the fuel of the rocket has all been used. 2

  13. physics

    While taking a plane flight to a meeting, a physicist decides to do a mental calculation of how much fuel the plane would consume in the absence of friction, assuming its engines have an efficiency of 22.0% and the mass of the plane is 200,000 kg. Each

  14. physics

    A rocket is fired at a speed of 75.0 m/s from ground level, at an angle of 60.0º above the horizontal. The rocket is fired toward an 15.0 m high wall, which is located 27.0 m away. The rocket attains its launch speed in a negligibly short period of time,

  15. Physics

    A 4.40-kg model rocket is launched, shooting 53.0 g of burned fuel from its exhaust at an average velocity of 650 m/s. What is the velocity of the rocket after the fuel has burned? (Ignore effects of gravity and air resistance.)

  16. physics

    a 4 kg model rocket is launched, shooting 50 kg of burned fuel from its exhaust at an average velocity of 625 m/s. What is the velocity of the rocket after the fuel has burned.? Ignore effects of gravity and air resistance.

  17. Physics

    A rocket whish is in deep space and initially at rest relative to an inertial reference frame, has a mass of 2.55 X 10^5 kg of which 1.81 X10^5 kg is fuel. The rocket engine is then fired for 250 s during which feul is consumed at the rate of 480 kg/s. The

  18. Physics

    The solid fuel of a 300.kg rocket travelling at 1.64x104 km/h is ignited to correct the rocket trajectory in mid-flight to Mars. 10.00 kg of fuel is burnt in 15.00 s. If the exhaust velocity of the fuel, relative to the rocket, is 2.90x103 km/h, what is

  19. math

    A prototype rocket burns 159 gallons of liquid hydrogen fuel each second during a regular launch. Make a table that shows how many gallons of fuel are burned after 2,3,5, and 10 seconds.

  20. physics

    the solid fuel rocket boosters used to launch the space shuttle are able to lift the shuttle 45 kilometers above earths surface. During the 2.00 min that the boosters operate, the shuttle accelerates from rest to a speed of nearly 7.50 x 10^2 m/s. What is

  21. Chemistry

    2.1 g of a hydrocarbon fuel is burned in a calorimeter that contains 280 grams of water initially at 25.00◦C. After the combustion, the temperature is 26.55◦C. How much heat is evolved per gram of fuel burned? The heat capacity of the calorimeter

  22. Physics

    A model rocket is launched from rest with an upward acceleration of 6.30 m/s2 and, due to a strong wind, a horizontal acceleration of 1.50 m/s2. How far is the rocket from the launch pad 5.30 s later when the rocket engine runs out of fuel?

  23. gr 11 physics

    as a rocket carrying a space probe acceleraes away from earth the fuel is being used up and the rocket's mass becomes less when the mass of a rocket(and its fuel) is M and the distance of the rocket from eart's centre is 1.5rE, the force of gravitational

  24. Math

    A prototype rocket burns 159 gallons of liquid hydrogen fuel each sound during a regular launch. Make a table that shows how many gallons of fuel are burned after 2,3,5, and 10 seconds.

  25. physics

    It is possible for the velocity of a rocket to be greater than the exhaust velocity of the gases it ejects. When that is the case, the gas velocity and momentum are in the same direction as the rocket's. How does the rocket still obtain thrust by ejecting

  26. Exploring Space

    rocket engines which work in space still need oxygen 4 the fuel to bur. space is a vacuum. where does the oxygen come from? I'm stumped on this, can someone please lend me help, thanks

  27. Physics

    A rocket of mass 50 kg is launched vertically. Its fuel is being burnt at a rate of 2 kg s-1 and its exhaust gas is being forced out with a speed of 1000 m s-1. What is the initial acceleration of the rocket?

  28. physics

    A stationary rocket is launched vertically upward. After 4s, the rocket's fuel is used up and it is 225.6 m above the ground. At this instant the velocity of the rocket is 112,8 m/s. Then the rocket undergoes free fall. Ignore the effect of the air

  29. physics

    A mother rocket fired from the ground last year’s eve ejects 25% of its mass 2.0 at the at the rate if so m/s in the first second of its flight. Find the initial acceleration of the mother rocket.

  30. physics

    A rocket is fired straight up and accelerates upward at 24 m/s2 for 12 seconds. The rocket then runs out of fuel and coasts. Ignore any air resistance effects and use -9.8 m/s2 for the local acceleration due to gravity. a) What is the rocket’s maximum

  31. physics

    A rocket is fired at a speed of 75.0 m/s from ground level, at an angle of 59.7° above the horizontal. The rocket is fired toward an 11.0 m high wall, which is located 28.5 m away. The rocket attains its launch speed in a negligibly short period of time,

  32. physics (Urgent)!!!!!!!!!!!!!!!!!!!!!!

    Consider a rocket in space that ejects burned fuel at a speed of vex= 1.5 km/s with respect to the rocket. The rocket burns 8 % of its mass in 280 s (assume the burn rate is constant). (a) What is the speed v of the rocket after a burn time of 140.0 s?

  33. physics Classical Mechanics

    Consider a rocket in space that ejects burned fuel at a speed of vex= 1.5 km/s with respect to the rocket. The rocket burns 8 % of its mass in 280 s (assume the burn rate is constant). (a) What is the speed v of the rocket after a burn time of 140.0 s?

  34. PHYSICS(elena)

    Consider a rocket in space that ejects burned fuel at a speed of vex= 1.5 km/s with respect to the rocket. The rocket burns 8 % of its mass in 280 s (assume the burn rate is constant). (a) What is the speed v of the rocket after a burn time of 140.0 s?

  35. Physics

    Consider a rocket in space that ejects burned fuel at a speed of vex= 2.0 km/s with respect to the rocket. The rocket burns 8 % of its mass in 300 s (assume the burn rate is constant). (a) What is the speed v of the rocket after a burn time of 150.0 s?

  36. Classical Mechanics Physics

    Consider a rocket in space that ejects burned fuel at a speed of vex= 1.5 km/s with respect to the rocket. The rocket burns 6 % of its mass in 310 s (assume the burn rate is constant). (a) What is the speed v of the rocket after a burn time of 155.0 s?

  37. physicS IMP PLS

    Consider a rocket in space that ejects burned fuel at a speed of Vex = 1.5 km/s with respect to the rocket. The rocket burns 5% of its mass in 280 seconds. (a) What is the speed V of the rocket after a burn time of 140 seconds (in m/s)? (b) What is the

  38. physics

    Consider a rocket in space that ejects burned fuel at a speed of vex= 2.0 km/s with respect to the rocket. The rocket burns 10 % of its mass in 340 s (assume the burn rate is constant). (a) What is the speed v of the rocket after a burn time of 170.0 s?

  39. physic

    Consider a rocket in space that ejects burned fuel at a speed of 1.5 km/s with respect to the rocket. The rocket burns 9 % of its mass in 310 s (assume the burn rate is constant). (a) What is the speed of the rocket after a burn time of 155.0 s? (suppose

  40. PHYSICS IMP PLS

    Consider a rocket in space that ejects burned fuel at a speed of v_ex= 2.0 km/s with respect to the rocket. The rocket burns 10 % of its mass in 290 s (assume the burn rate is constant). (a) What is the speed of the rocket after a burn time of 145.0 s?

  41. Physics

    A model rocket is launched vertically upward from the ground. After 4.3s, its fuel is completely burned. Assume uniform acceleration of 3.0 m/s2 while the fuel is burning. What is the velocity of the rocket at the instant that the fuel is completely

  42. Physics

    The MAVEN spacecraft, a NASA mission that is studying the Martian atmosphere, launched from Cape Canaveral in November 2013 on an Atlas V rocket. The rocket burns about 280,000 kg of fuel over 4 minutes (you can assume dm/dt is constant over that interval)

  43. Physics

    A 4.40-kg model rocket is launched, shooting 53.0 g of burned fuel from its exhaust at an average velocity of 650 m/s. What is the velocity of the rocket after the fuel has burned? (Ignore effects of gravity and air resistance.)

  44. Physics

    A 4.40-kg model rocket is launched, shooting 53.0 g of burned fuel from its exhaust at an average velocity of 650 m/s. What is the velocity of the rocket after the fuel has burned? (Ignore effects of gravity and air resistance.)

  45. Physics

    A 4.40-kg model rocket is launched, shooting 53.0 g of burned fuel from its exhaust at an average velocity of 650 m/s. What is the velocity of the rocket after the fuel has burned? (Ignore effects of gravity and air resistance.)

  46. physics

    A 1000m3 rocket weighing 2.0x10^3 kg is going to be launched into space. On ignition, gas is expelled from the bottom of the rocket at a speed of 2.4x10^3 m/s relative to the rocket. The rate at which the fuel is consumed is 8.02kg/s. Is there a time delay

  47. physics

    (ignore the other question i posted this is the right one) A rocket has a pre-ignition mass of 1100kg. On ignition it burns fuel at rate of 20kg s-1 producing a thrust of 10000N. How long after ignition of the fuel, will the rocket lift off? also explain

  48. Physics

    A model rocket with mass 1 kg is launched from the ground, starting from rest, and accelerates vertically due to an upward thrust of 25N being produced by the rocket’s engine. The rocket accelerates vertically for a time of 10 seconds at which point the

  49. physics

    i need help figuring out what formaula to use for finding the final velocity of a rocket. Model rocket engines are sized by thrust, thrust duration, and total impulse, among other characteristics. A size C5 model rocket engine has an average thrust of 5.26

  50. phyicis

    Homer Hickham and his buddies launched model rockets in the late 1950's when the US was trying to get a rocket into space. If homer launches a rocket from rest and its engine delivers a constance acceleration of 8.2 m/s^2 for 5.0seconds after which the

  51. Science/Physics

    An astronaut is about 20 m from her space station when the rocket thrusters run out of fuel. If she is carrying a few tools and can remove the rocket thruster, what should she do to get back to the space station? Explain reasoning.

  52. physics

    A rocket has a per-ignition mass of 1100kgs-1 producing a thrust of 10000N. How long after ignition of the fuel will the rocket lift off? also explain how is it possible for a rocket to accelerate in space. (i cant work it outt. i know it involves the

  53. Physics

    Energy Budget for a Rocket in Free Space: Recall the analysis of the rocket engine in Project I, Flight of the Rocket. Although the momentum of the rocket and its exhaust is conserved (constant), their energy is not; energy must be supplied by the

  54. Physics

    Energy Budget for a Rocket in Free Space: Recall the analysis of the rocket engine in Project I, Flight of the Rocket. Although the momentum of the rocket and its exhaust is conserved (constant), their energy is not; energy must be supplied by the

  55. Math (calculus)

    Prof gave us this question to practice but I do not know how to solve it. If you know please provide step by step with the answer so I can understand it. Thank you very much :) Suppose a rocket is launched from the ground with 10 seconds worth of fuel. The

  56. Math

    A prototype rocket burns 159 gallons of liquid hydrogen fuel each second during a regular launch. Make a table that shows how many gallons of fuel are burned after 2,3,5,and 10 seconds. Please help me

  57. Science

    In a rocket, fuel burns at the rate of 1kg/s. This fuel is Ejected from the rocket with a velocity of 60km/s. What force will be exerted on the rocket?

  58. physics

    A rocket consists of a payload of 4280.0 kg and 1.626 105 · of fuel. Assume that the rocket starts from rest in outer space, accelerates to its final velocity, and then begins its trip. What is the speed at which the propellant must be expelled to make

  59. Physics

    A small rocket is fired in a test range. It rises high into the air and soon runs out of fuel. On the way down it passes near an observer (sitting in a 22.5-m-high tower) who sees the rocket traveling at a speed of 37.9 m/s and moving in a vertical plane

  60. Physics

    Sorry for posting so many, but I just need to make sure I'm on the right track. Just need someone to check my working and answers. A rocket has a mass 3000kg. The fuel used in the rocket has an additonal mass of 500kg. At the launch, the rocket accelerates

  61. Physics

    a 2000kg rocket has a rocket motor thast generates 3.0 x 10^5 N of thrust. What is the rockets initial upward acceleration ? ( I got 140ms^2)???? don't know if its right and then it says at an altitude of 5000m the rockets acceleration has increased to

  62. Physics

    A rocket travels vertically away from the surface of Mars. It is still close to the surface and travelling at a speed of 25 m s when it jettisons an empty fuel tank. The fuel tank initially travels with the velocity of the rocket, but it is attracted to

  63. phys

    The space shuttle, with an initial mass M = 2.41E+6 kg, is launched from the surface of the earth with an initial net acceleration a = 26.1 m/s2. The rate of fuel consumption is R = 6.90E+3 kg/s. The shuttle reaches outer space with a velocity of vo = 4632

  64. computer science

    I need help one how create a spreadsheet that will give an accurate launch profile for a vertical liftoff for the SpaceX Falcon 9 rocket. the information they give me is: •Start with a rocket with a mass, M1, moving with a speed, v1 •The rocket expels

  65. Physics

    A small 6.90-kg rocket burns fuel that exerts a time-varying upward force on the rocket. This force obeys the equation F=A+Bt2. Measurements show that at t=0, the force is 119.0 N , and at the end of the first 2.40 s , it is 193.0 N . Part A Find the net

  66. science

    in the first second of its flight, a rocket ejects 1/60 of its mass with a velocity of 2400 m/s the acceleration of the rocket is

  67. Physics

    The mass of a rocket car plus fuel is 2,000kg. The rocket car starts from rest. The engine expels fuel(in the form of exhaust) over a period of 8 seconds. The exhaust has a velocity of 1,000m/s due west, and the velocity of the rocket car after 8 seconds

  68. Physics

    The mass of a rocket car plus fuel is 2,000kg. The rocket car starts from rest. The engine expels fuel(in the form of exhaust) over a period of 8 seconds. The exhaust has a velocity of 1,000m/s due west, and the velocity of the rocket car after 8 seconds

  69. Physics

    The space shuttle, with an initial mass M = 2.41×106 kg, is launched from the surface of the earth with an initial net acceleration a = 26.1 m/s^2. The rate of fuel consumption is R = 6.90×103 kg/s. The shuttle reaches outer space with a velocity of vo =

  70. physics

    A 2000 kg space probe is moving rightward in empty space along the x axis at 12 m/s. One of the probe’s rockets is fired providing a thrust of 1800j N along the y axis. The rocket fires for 1.5 s. How can I derive the equation for the trajectory of the

  71. physics

    A 2000 kg space probe is moving rightward in empty space along the x axis at 12 m/s. One of the probe’s rockets is fired providing a thrust of 1800j N along the y axis. The rocket fires for 1.5 s. How can I derive the equation for the trajectory of the

  72. physics

    A 2000 kg space probe is moving rightward in empty space along the x axis at 12 m/s. One of the probe’s rockets is fired providing a thrust of 1800j N along the y axis. The rocket fires for 1.5 s. How can I derive the equation for the trajectory of the

  73. dynamics

    (a) A rocket is travelling out of the earth’s atmosphere. During flight it burns fuel at a rate of 80kg/s producing a constant thrust of 500kN which drives the rocket in a vertical trajectory. It has a mass of 3500Kg when it reaches a speed of 1220m/s.

  74. physics

    A 2000 kg space probe is moving rightward in empty space along the x axis at 12 m/s. One of the probe’s rockets is fired providing a thrust of 1800j N along the y axis. The rocket fires for 1.5 s. How can I derive the equation for the trajectory of the

  75. physics

    A 2000 kg space probe is moving rightward in empty space along the x axis at 12 m/s. One of the probe’s rockets is fired providing a thrust of 1800j N along the y axis. The rocket fires for 1.5 s. How can I derive the equation for the trajectory of the

  76. Physics newton

    A rocket weighs 1 x10^7 N while on the surface of the earth. The ignition of its engines results in a lift off force of 1.5 X 10^7 N a) what is the mass of the rocket. b) what is its acceleration at lift -off c)How fast will it be moving if it acceleartion

  77. physiccs

    a rocket that has consumed all of its fuel continues to move upward slowed mostly by the force of gravity. If the rocket's mass is 75.0 g and it takes 1.2 sec for the rocket to stop, what is the change in the rocket's momentum? what is the rockets stopping

  78. physics

    The mass of a rocket car plus fuel is 2,000 kg. The rocket car starts from rest. The engine expels fuel (in the form of exhaust) over a period of 8 seconds. The exhaust has a velocity of 1,000 m/s due west, and the velocity of the rocket after 8 seconds is

  79. ap chemistry

    3.5 g of a hydrocarbon fuel is burned in a ves- sel that contains 250. grams of water initially at 25.00 C. After the combustion, the temper- ature of the water is 26.55 C. How much heat is evolved per gram of fuel burned?

  80. Thermodynamics of steam

    A boiler furnace releases 10000.000kJ of heat for each kg of fuel burned and produces dry saturated steam at 10000.000kPa from feed water at 190.000 degrees Celsius. How many kg of steam will be produced for each kg of fuel burned if the boiler had an

  81. Physics

    A boiler furnace releases 10000.000kJ of heat for each kg of fuel burned and produces dry saturated steam at 10000.000kPa from feed water at 190.000 degrees Celsius. How many kg of steam will be produced for each kg of fuel burned if the boiler had an

  82. MATH

    I need help on number 3.the question is. According to information from NASA the space Shuttles external fuel tank has a capacity of about 500,000 gallons of fuel.Write three numbers that could represent the space Shuttles actual fuel capcity.

  83. physics

    The rate of burning of fuel in a rocket is 50 g/sec and comes out with a velocity 4X1000 sec. then find the force extended by fuel on the rocket.

  84. @ elena - intro to physics

    I posted this question earlier and thank you for respnding, but the answer you give me was incorrect. I try looking at hw you solve and cant find the mistake but when when I check your answer it said it was wrong. Can you please look over it again, I

  85. physics- help please !!1

    A toy rocket moving vertically upward passes by a 2.2m -high window whose sill is 7.0m above the ground. The rocket takes 0.17s to travel the 2.2m height of the window part a) what was the launch speed of the rocker ? assume the propellant is burned very

  86. physics

    A mother rocket fired from the ground last year’s eve ejects 25% of its mass at the at the rate if so m/s in the first second of its flight. Find the initial acceleration of the mother rocket.

  87. Physics

    A mother rocket fired from the ground last year’s eve ejects 25% of its mass at the at the rate if so m/s in the first second of its flight. Find the initial acceleration of the mother rocket.

  88. physics

    While taking a plane flight to a meeting, a physicist decides to do a mental calculation of how much fuel the plane would consume in the absence of friction, assuming its engines have an efficiency of 22.0% and the mass of the plane is 200,000 kg. Each

  89. chemistry

    An intercontinental ballistic missile (ICBM) travels at an average speed of 7.03km/sec. The missile contains 459.0Mg of fuel. The fuel is burned at a rate of 313 pounds per second. What would be the maximum distance (in km) the missile could travel until

  90. Physics

    A rocket is fired vertically with an acceleration of 16m/s^2. After time T, the rocket's engine runs out of fuel and it begins to fall freely. The maximum height of the rocket is 940m, what is the minimum value of T?

  91. MOMENTUM AND SHM

    A(n) 3800 kg rocket traveling at 2500 m/s is moving freely through space on a journey to the moon. The ground controllers find that the rocket has drifted off course and that it must change direction by 9.2 degrees if it is to hit the moon. By radio

  92. PHYSICS MOMENTUM

    A(n) 3800 kg rocket traveling at 2500 m/s is moving freely through space on a journey to the moon. The ground controllers find that the rocket has drifted off course and that it must change direction by 9.2 degrees if it is to hit the moon. By radio

  93. Physics

    A rocket is fired at a speed of 50.0 m/s from ground level, at an angle of 46.0 ° above the horizontal. The rocket is fired toward an 18.7-m high wall, which is located 23.0 m away. The rocket attains its launch speed in a negligibly short period of time,

  94. physics

    A rocket is fired at a speed of 92.0 m/s from ground level, at an angle of 33.0 ° above the horizontal. The rocket is fired toward an 13.4-m high wall, which is located 26.0 m away. The rocket attains its launch speed in a negligibly short period of time,

  95. physics

    A rocket is fired at a speed of 66.0 m/s from ground level, at an angle of 63.0 ° above the horizontal. The rocket is fired toward an 31.2-m high wall, which is located 28.0 m away. The rocket attains its launch speed in a negligibly short period of time,

  96. physics

    A rocket is fired at a speed of 75.0 m/s from ground level, at an angle of 57.6¡ã above the horizontal. The rocket is fired toward an 11.0 m high wall, which is located 25.5 m away. The rocket attains its launch speed in a negligibly short period of

  97. henry ford

    A rocket is fired at a speed of 74.0 m/s from ground level, at an angle of 68.0 ° above the horizontal. The rocket is fired toward an 49.8-m high wall, which is located 32.0 m away. The rocket attains its launch speed in a negligibly short period of time,

  98. physics

    A rocket is fired at a speed of 91.0 m/s from ground level, at an angle of 49.0 ° above the horizontal. The rocket is fired toward an 21.4-m high wall, which is located 25.0 m away. The rocket attains its launch speed in a negligibly short period of time,

  99. physics

    A rocket is fired at a speed of 54.0 m/s from ground level, at an angle of 63.0 ° above the horizontal. The rocket is fired toward an 52.8-m high wall, which is located 37.0 m away. The rocket attains its launch speed in a negligibly short period of time,

  100. physics

    A rocket is fired at a speed of 75.0 m/s from ground level, at an angle of 68.0° above the horizontal. The rocket is fired toward an 11.0 m high wall, which is located 26.5 m away. The rocket attains its launch speed in a negligibly short period of time,

Pages

  1. 1
  2. 2
  3. 3
  4. 4
  5. 5
  6. 6
  7. 7
  8. 8
  9. 9
  10. 10
  11. 11
  12. 12
  13. 13
  14. 14
  15. 15
  16. 16
  17. 17
  18. 18
  19. 19
  20. 20