# Homework Help: Ap Physics Heeeeeeeeeeeeeelllllllllllllllppppppppppp Please!

1. Jan 18, 2005

### urbettybo0p

Hi, I really need help on the following questions and would appreciate it if someone could ASAP!!! Thank you!!!

1) a bullet of mass m is moving horizontally with speed v when it hits a block of mass 100m that is at rest on a horizontal frictionless table. The surface of the table is a height h above the floor. After the impact the bullet and the block slide off the table and hit the floor a distance x from the edge of the table.

a) What is the speed of the block as it leaves the table?
b) What is the change in kinetic energy of the bullet-block system during impact?
c) What is the distance x ?

Suppose the bullet passes through the block instead of remaining in it.

d) State whether the time required for the block to reach the floor from the edge of the table would now be greater, less, or the same. Justify your answer.
e)State whether the distance x for the block would now be greater, less or the same. Justify your answer.

THANK YOU SOOOOOO MUCH!!!! I really needed help.

2. Jan 18, 2005

### urbettybo0p

I just became a member today and had no clue you had to show work...but that's exactly the problem - i don't have any ideas. I've done all of my AP problems assigned except for this one and have been trying to solve it for hours now without any luck

3. Jan 19, 2005

### futb0l

4. Jan 19, 2005

### Moonbear

Staff Emeritus
A start to help on this has been provided on the College Level Forum. Bettyboop is still learning the ropes around here, so please answer her question over there rather than duplicate efforts.

5. Jan 19, 2005

### Curious3141

Fair enough. I got irritated because I took the time and effort to type out a detailed reply to bettyboop's question, only to realise another poster had already unknowingly done the same. This of course, was entirely bb's fault for multiply posting the orig question. I wouldn't have got so mad if I hadn't felt my effort had been wasted.

I've been a noob on forums before and even when I was totally new, I knew better than to cross-post this sort of thing. This may be the internet but common consideration still applies. Nevertheless, I can see some people may not be so sensible.

I'm calm now.

6. Jan 19, 2005

### Curious3141

Here was my original post.

For the first part, you need to find the final velocity of the bullet-block system. Set up an equation using conservation of linear momentum and solve for the final velocity. The initial momentum is mv, when the bullet and the block coalesce, the combined mass is (100m + m). So what is the final horizontal velocity ?

What is the formula for kinetic energy ? Do you know $$E_k = \frac{1}{2}mv^2$$ ? Find the initial and then final kinetic energy of the system. Subtract to find the absolute change in energy. If you want the percentage change, you can take the absolute value of the change and express it as a proportion of the initial value. I would do this just to be safe, because the question is not clear on what it's asking for.

The horizontal velocity is not changed during transit over the frictionless table. Once the block-bullet falls off the table, there is only one force acting on it : gravity. It influences only the vertical motion. Do you know the formula $$s = \frac{1}{2}gt^2$$ ? Use that to find the time of fall. Then use distance = velocity times time to figure out the horizontal distance x.

For the second part, the bullet remains separate from the block and remains travelling in its original trajectory (albeit at reduced speed). Will the final velocity of the block be greater or lower here ?

Would the changed horizontal velocity of the block affect the vertical motion and therefore the time taken for the block to fall to the floor. How about the distance (horizontal motion) ?

Does that help ?