physics

A mass is dropped from a height of 30m, and at the same time from ground level (and directly beneath the dropped mass), and another mass is thrown straight up with a speed of 15 m/s.
a) How far apart are the two masses after 0.5 s?
b) Will the massses ever meet or pass each other? And if so will the meeting place take place while both masses are falling or when one is falling and the other is rising?

a) I got 30 - 15t
30-15(.5 seconds)=22.5 meters

b) I need help on this part of the queston.

distance apart= 30- 1/2 g (t)^2 - 15t + 1/2 g t^2
distance apart= 30 -15t

t= .5 second

If they meet, distance apart is zero
solve for t.
See what the velocity of the ball thrown upwards is at that time. Is it positive (upward) or not.

I got t= 2 seconds so would they meet when one is rising and the other is falling?


You need to determinse how long it takes for the object thrown upward to reach a top. When you do that you'll know what part of the path the object is in, rising or falling.


You know the falling ball is going down. The question is what is the rising ball doing at time = 2 seconds.

v = initialvelocity +g *time
= 15 - 9.8 *t
a positive answer is going up, a negative answer is going down.


so I would get -4.6 as the answer if I plugged in t=2?


No, look at the equation BobP. gave and use 2 in that equation as he said. Thus
v = initialvelocity +g *time or,
v = 15 - 9.8 *t
Setting t=2 we have
v = 15 - 9.8*2 = -4.6
The negative sign tells us that the object thrown upward is now falling. Therefore the two objects meet (i.e., are at the same height) when the second object is on the way down. Therefore both are falling for b).
In short, after you use the formula for v Bob gave there are no other calculations to do.


After looking at your question again I see that I may've misunderstood what you were asking.
Yes! you should get -4.6 when you put t=2 in that formula. The only thing we were interested in though was the sign of the result -4.6. since it's negative, that means it's falling. It takes approximately 1.53 sec for the thrown object to reach it's peak and then begin to fall back down. Therefore it's on it's way down when the dropped object reaches it.
So yes, you did the correct calculation. All we we're interested to know was whether it was ascending or descending and Bob gave the formula for that.

  1. 👍
  2. 👎
  3. 👁

Respond to this Question

First Name

Your Response

Similar Questions

  1. math

    A ball is dropped from a height of 18 m.On each rebound it rises 2/3 of the height from which it last fell. What distance has it travelled at the instant it strikes the ground for the 5th time?

  2. science

    A ball of mass 1 kg dropped from 9.8m height , strikes the ground and rebounds to a height of 4.9m .If the time of contact between ball and ground is 0.1s,then find impulse and average force acting on ball.

  3. Math

    A ball rebounds 1/2 of the height from which it is dropped. It is dropped 56m from the building and it kept bouncing. How far would the ball have travelled up and down when it strikes the ground for the 6th time?

  4. math

    a ball is dropped from a height of 3m and bounces on the ground . At the top of each bounce, the ball eaches 60% of its previous height. Calculate the total distance travelled by the ball when it hits the ground for the fifth time

  1. physics

    a ball of mass 0.2kg is dropped from a height of 20m on impact on the ground it lose 30j of energy.calculate the height which it reaches on the ground and rebound

  2. math

    A rubber ball is dropped from a height of 729 feet, and at each bounce it rebounds one-third of the height from which it last fell. How far has the ball travelled by the time it strikes the ground for the 5th time?

  3. physics

    a 5kg mass is dropped from a height of 30m above the ground. determine the velocity of the mass when it is 18m above the ground.

  4. physics

    a ball of mass 5kg is dropped from a height of 78.4m,the time taken by the ball to reach the ground is?

  1. Math

    Suppose an object is dropped from a height h0 above the ground. Then its height after t seconds is given by h = −16t 2 + h0, where h is measured in feet. Use this information to solve the problem. A ball is dropped from the top

  2. Physics

    1) A 2.5kg durian falls from a tree with a height of 30m. Determine the energy before it falls in Joule. 2) A 2.5kg durian falls from a tree with a height of 30m. Determine the velocity when it drops 20m from the ground. 3) A

  3. Science

    A ball mass 0.2kg is dropped from the height of 20m on the impact with ground lose 30j of energy calculate the height it reachon the rebound

  4. Physics

    A rubber ball with a mass of 0.130 kg is dropped from rest. From what height was the ball dropped, if the magnitude of the ball's momentum is 0.710 kg · m/s just before it lands on the ground?

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