# Please give me a suggestion

1. Oct 10, 2008

### foxproff

I am having problesm with this question

a. A 2.5-g bullet traveling at hits a tree and slows uniformly to a stop while penetrating a distance of 12 cm into the tree’s trunk. What force was exerted on the bullet in bringing it to rest?

b You are carrying a backpack across campus. What is the work done by your vertical carrying force on the backpack? Explain.

Thanks anyone

2. Oct 10, 2008

### Rake-MC

travelling at what?

3. Oct 10, 2008

### foxproff

A 2.5-g bullet traveling at 350m/s hits a tree and slows uniformly to a stop while penetrating a distance of 12 cm into the tree’s trunk. What force was exerted on the bullet in bringing it to rest?

4. Oct 10, 2008

### Rake-MC

No worries, well you know initial velocity, final velocity and displacement. Use a constant acceleration formula to solve for acceleration. You can work out force from that [F = ma].

For question two, have a think about it for a little. What is the vertical force (doing work) to keep up the backpack.

5. Oct 10, 2008

### HallsofIvy

Staff Emeritus
So this problem is assuming a constant force or asking for an average force, which is the same thing.
If an object with mass 2.5 g experiences a force F, what will its acceleration be? (F= ma)
With that acceleration, how long will it take the object to slow from 350 m/s to 0 m/s?
How far will the object go in that time? The answers to all of those, of course, depend on F. Set the last equal to 12cm and solve for F. Be careful to use consistent units.

How do you calculate work? The reason for the "explain", I suspect, is that if you really understand the formula I asked for, you don't don't need to do any calculation at all!

6. Oct 17, 2008

### foxproff

A volley ball is traveling toward you. (a) Which action will require a greater force on the volleyball, you catching the ball or you hitting the ball back? Why? (b) A 0.45-kg volley ball travels with a horizontal velocity of over the net. You jump up and hit the ball back with a horizontal velocity of 4.0m/s If the contact time is 0.040 s, what was the average force on the ball?