# Homework Help: Kinematics - Bullet moving on a table hitting a block

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

### daster

I'm in a hurry... So I'll keep it short.

a) Conservation of momentum.
b) KE=0.5mv2. Consider the velocities before and after the collision. This will give you two values of KE. Compare them by substraction the one before the collision from the one after. If the difference is negative, KE was lost, and vice versa.
c) Consider vertical AND horizontal projectile motion. Vertical considerations will help you find the time when the system strikes the ground, i.e. when the displacement is -h. Use the standard equations and find t, then substitute this value for t in a horizontal equation.
d) Conservation of momentum again, only this time you have seperate masses and seperate velocities. Compare the new velocity of the bullet to the velocity of the bullet-block. If it's greater, then flight time will be greater too.
e) If flight time is greater, then x will be greater too.

3. Jan 18, 2005

### daster

I haven't really read the question properly, so I could be wrong. :tongue2:

I'm sorry I couldn't help more.

4. Jan 18, 2005

### Curious3141

Why the heck are you double posting this ? Read the stickies ! :yuck:

5. Jan 18, 2005

### urbettybo0p

I don't even know what the heck the stickies are...i needed help with a physics problem and came across this website for the first time. I appreciated the help you gave me before but

STOP BEIN SUCH A JERK

6. Jan 18, 2005

### urbettybo0p

thank you you did help me:)! i appreciate it

7. Jan 18, 2005

### Curious3141

You want help, you come here and ask. By double posting this, you're making other people waste effort and time duplicating their efforts trying to help you. It's selfish and rude of you, and I'm not the jerk here.

Where's your common sense ? :grumpy:

BTW, since you posted this here, and it's been answered, I'm deleting my post in the other thread. So there, you don't have to be appreciative. I don't care.

8. Jan 18, 2005

### urbettybo0p

It's not selfishness it's trying to get a problem done for school tomorrow because this is such and important assignment. Learn how to be nice and get over yourself. THANKS. :yuck:

9. Jan 18, 2005

### Curious3141

I just noticed : it was a quadruple post ! One here, one in the K-12 forum, one in Career Guidance and one in Gen Physics. Nice...

I wonder if your teachers are appreciative when you copy your homework to ten other teachers at the same time to get a head start ?

Instead of telling me to "get over myself" learn some humility and etiquette and COMMON SENSE when asking for help.

Last edited: Jan 18, 2005
10. Jan 19, 2005

### Moonbear

Staff Emeritus
Okay, okay, okay, break it up, nothing more to be seen here.

Bettyboop, welcome to PF. Since you're new, please take note of a few board rules. The "sticky" Curious is referring to is the first post at the top of the list that says "Read This Before Posting." If you haven't done so already, please read it before posting any more questions.

There are MANY students who visit us here every day, all with questions that are important to them, and many of them "urgent." There are a lot fewer people around to help answer those questions, so please be patient and post your questions in just one place in the future.

As a side note, it's also helpful if you use a more informative title for your post, something about what the question is about so those scanning these pages who may be able to help know to stop at your question. Everyone starting threads in the homework help section needs help, we know that.

Above all else, please remember to remain civil to one another here. People volunteer to answer questions out of kindness. Please respect them for their willingness to help. Likewise, Curious, please be patient with the new people and point them to the rules before jumping down their throats.

11. Jan 19, 2005

### futb0l

Meh, Didn't realise she posted like 5 posts on this.