Physics

A 4-kilogram mass has a speed of 6 m/s on a horizontal frictionless surface. The mass collides head-on and elastically with an identical 4-kilogram mass initially at rest. The second 4-kilogram mass then collides head-on and sticks to a third 4-kilogram mass initially at rest.

The final speed of the first 4-kilogram mass is...

The final speed of the two 4-kilogram masses that stick together is...

  1. 👍
  2. 👎
  3. 👁
  1. If they collide elastically the sum of 1/2 m v^2 after has to equal the sum of 1/2 m v^2 s before and the momentums (m1v1+m2v2) must be the same before and after.
    This can only happen if the first one stops and the second one proceeds at 6 m/s.
    I will leave you to prove that to yourself using those energy and momentum equalities.
    Now the second part.
    We have no energy conservation now because it is not elastic.
    momentum is still conserved though and the two stick so they become one 8 kg mass after
    so
    4 (6) + 4(0) = 8 (v final)

    1. 👍
    2. 👎
  2. so for the first part... the initial m1v1 is (4kg)(6m/s). the final is what i can't get. is the final 12kg? arent we only concerned with the first mass in that question?

    1. 👍
    2. 👎
  3. In the first part of the problem
    a 4 kg mass moving at 6 m/s hits another 4 kg mass
    that means
    4 kg * 6 m/s + 4 kg* 0 m/s = initial momentum
    the final momentum is
    4 kg * v1 + 4 kg*v2
    so

    24 + 0 = 4v1 + 4 v2
    or
    v1 = 6 - v2

    then energy
    initial ke = (1/2) m v^2 = 2*36 = 72 Joules
    final ke is also 72 joules
    so
    72 = 2 v1^2 + 2 v2^2
    36 = (6-v2)^2 + v2^2
    36 = 36 - 12 v2 + 2 v2^2
    v2^2 - 6 v2 = 0
    v2(v2-6) = 0
    so v2 = 0 or v2 = 6
    v2 may not equal zero unless mass 1 goes straight through mass 2
    so
    v2 = 6
    (like I tried to tell you :) The first one stops and the second one takes over at the original speed.

    1. 👍
    2. 👎
  4. In the second part of the problem, the 4 kg mass suddenly doubles in mass, so it has to go half as fast to have the same momentum

    1. 👍
    2. 👎
  5. That is it, ten before 11, I am turning into a pumpkin ! Or at least I am turning into sleep.

    1. 👍
    2. 👎
  6. _F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E__F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E_
    _F_U_C_K_M_E__F_U_C_K_M_E__F_U_C_K_M_E__F_U_C_K_M_E_
    v
    v
    _F_U_C_K_M_E__F_U_C_K_M_E_v
    v
    v
    vv
    _F_U_C_K_M_E__F_U_C_K_M_E_v_F_U_C_K_M_E__F_U_C_K_M_E_v

    1. 👍
    2. 👎

Respond to this Question

First Name

Your Response

Similar Questions

  1. physics

    A 15.0-kilogram mass is moving at 7.50 meters per second on a horizontal, frictionless surface. What is the total work that must be done on the mass to increase its speed to 11.5 meters per second?

  2. Physics

    A horizontal force with a magnitude of 3.0 newtons applied to a 7.0 kilogram mass moves the mass horizontally a distance of 2.0 meters. Determine the work done against gravity in moving the mass. What is the formula that you use?

  3. physics

    [20 pts] A 2.00 kg block is pushed against a spring with negligible mass and force constant k = 400 N/m, compressing it 0.220 m. When the block is released, it moves along a frictionless, horizontal surface and then up a

  4. Physics

    two blocks (a and b) are in contact on a horizontal frictionless surface. a 60N constant horizontal force is applied to A. the mass of a is 3.0 kg and the mass of b is 15 kg. what is the magnitude of the force of a on b

  1. Physics- springs

    Block A has a mass 1.00kg, and block B has a mass 3.00 kg. the blocks are forced together, compressing a spring S between them; then the system is released from rest on a level, frictionless surface. The spring which egligible

  2. physics

    A light rope is attached to a block with a mass of 6 kg that rests on a horizontal, frictionless surface. THe horizontal rope passes over a frictionless, massless pulley, and a block of mass m is suspended from the other end. When

  3. physics

    You attach a 2.90 kg mass to a horizontal spring that is fixed at one end. You pull the mass until the spring is stretched by 0.500 m and release it from rest. Assume the mass slides on a horizontal surface with negligible

  4. mechanics and fluids

    a block of mass 6 kg resting on a horizontal surface is connected by a cord passing over a light, frictionless pulley to a hanging block of mass 4 kg. the coefficient of kinetic friction between the block and the horizontal

  1. Physics

    A ball with a mass of 0.50 kilograms is moving at a speed of 1.8 meters/second along the positive x-axis. It collides with another ball of the same mass travelling at a speed of 2.0 meters/second toward the positive y-axis. Both

  2. physic

    Two boxes are connected by by a massless string over a massless, frictionless pulley. Box 1 has a mass of m1 = 15.0 kg and is initially at rest at the bottom of a frictionless surface inclined at an angle θ = 30.0◦ above the

  3. physics

    A solid block of mass m2 = 1.0 $kg$, at rest on a horizontal frictionless surface, is connected to a relaxed spring (with spring constant k = 260 N/m whose other end is fixed. Another solid block of mass m1 = 2.4 kg and speed v1 =

  4. Physics

    A horizontal spring attached to a wall has a force constant of 780 N/m. A block of mass 1.70 kg is attached to the spring and oscillates freely on a horizontal, frictionless surface as in the figure below. The initial goal of this

You can view more similar questions or ask a new question.