Expert: Daniel Mazur - 9/25/2006

Question
I'm a senior in college taking college physics. I gotta say, its hard. I thought organic chemistry was hard, but this is crazy. I have a hard time grasping the concepts of HOW things happen the way they do. I just can't figure out some of these questions. Can you help? thanks.

A ball of mass 0.120 kg is dropped from rest from a height of 1.25 m. It rebounds from the floor to reach a height of 0.600 m. What impulse was given to the ball by the floor?
~the only equation i am given for impulse in my book includes force and time...this one confuses me big time.

A 70.0 kg diver performs a dive from a platform 10 m above the water surface. Find the average impact force she experiences in her collision with the water if the collision lasts for 1.20 s.

An archer shoots an arrow toward a 300 g target that is sliding in her direction at a speed of 2.15 m/s on a smooth, slippery surface. The 22.5 g arrow is shot with a speed of 42.0 m/s and passes through the target, which is stopped by the impact. What is the speed of the arrow after passing through the target?

A 29.0 g object moving to the right at 16.0 cm/s overtakes and collides elastically with a 7.0 g object moving in the same direction at 15.0 cm/s. Find the velocity of each object after the collision.

A m1 = 0.400 kg block is released from rest at the top of a frictionless track h1 = 2.90 m above the top of a table. It then collides elastically with a 1.00 kg block that is initially at rest on the table. Determine the velocities of the two objects just after the collision. How high up the track does the 0.400 kg object travel back after the collision? How far away from the bottom of the table does the 1.00 kg object land, given that the table is 2.20 m high? far away from the bottom of the table does the 0.400 kg object eventually land?

i know that is a lot to ask, but it takes me like 2 hours to just get started on a problem. any help would be helpful. you dont have to tell me the answers, just tell me HOW to do them! my book doesnt provide a lot of examples and my teacher doesn't provide examples either. thanks for your help!!!

Answer
Hi,

1)when they ask about impulse transfer, you calculate it as the total change in momentum. From what youre given you determine momentum before and momentum after and the vector difference (or sum of magnitudes) is the answer. Impulse is "F.t", but also  "m.v", that's the key.

2)Now here you use the "F.t" as well. The diver has some momentum before impact and she's slowed to complete rest. Just divide the momentum difference by the time.

3)This is simple momentum conservation "m1v1i-m2v2i = m1v1f+0". Now pull v1f to the left and all the rest to the right.

4)This is combination of momentum and energy conservation. It's simple, but the derivation is lengthy and the final symbolic expression is ugly.

5)First you need to draw a picture of the situation. Solution needs a combination of momentum and energy conservation (like the previous task) and finally the equations of motion of a horizontal and vertical shot.

I hope these clues were enough, but don't hesitate to ask more, if not.

Good luck!
Daniel