1. Sep 11, 2011

### tony.wu

1. The problem statement, all variables and given/known data
Which of the following graphs applies to each situation?
1) An upward moving elevator colliding with a downward moving elevator.
2) A car moving at a steady speed is overtaken by a police car starting from rest.
3) A car, initially at rest, accelerating and a car moving initially at constant speed applying the brakes until it stops.
4) A train, initially moving at a steady speed, has brakes applied to match the speed of a train on a parallel track.
5) A ball thrown upward and a ball thrown downward at the same time with the same initial speed.
6) A race car speeding up to match the speed of a second race car.

the graph attach in the attachments

3. The attempt at a solution

in my opinion, the match should be:
1)-F, when the the elevators colliding, the V become 0.
2)-B, the police's car is faset than than other car. and when the car meet police at first time, it has already moved a part of displacement. so I think B is match this situation.
3)-H, i guest it should be H, because it said one car brakes until it stop, see graph H, it match it.
4)-D, it said a tain brake the match to other so V is reduce and same as another, the displace change rate should be equal after the speed mache. so I choose D.
5)-G, one ball up and one ball down with same speed, the value of V is equal but direction is opposite. two ball just affect by gravitation so the rate of changing should be equal. I choose G.
6)-I, a car speed up to match the speed of second car. so second car is faster. after they match to get same speed. I choose I.

This is my quiz question online. I can't know right answer now but my answer is not right. I do not know which one is wrong. Please help check my answer and helo explain why it is wrong.

thanks every much.

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