# Homework Help: Need to find the net force and the average force .

1. Dec 8, 2011

### MsHomework

Need to find the net force and the average force.....

1. The problem statement, all variables and given/known data

A bullet of mass 12 g strikes a stationary fixed block of wood at a speed of 400m/s, penetrating to a depth of 3.0cm. Calculate the average net force acting on the bullet while it is in the wood. Calculate the average force exerted on the wood by the bullet.

2. Relevant equations
Fnet= m x a
W=1/2mv^2

3. The attempt at a solution
I found the force net using the kinetic energy and work equation. I don't really know how to find the average force.

2. Dec 8, 2011

### MsHomework

Re: Need to find the net force and the average force.....

3. Dec 8, 2011

### BetoG93

Re: Need to find the net force and the average force.....

Try using this equation: 2ax = (vf)^2 - (vi)^2.
Find the acceleration.
Use Newton's second law to calculate force.
This is the only possible solution I can see.

Note: use above equation only if a is constant.
I like your approach using work-kinetic energy theorem.
One point: the force exerted on the wood by the bullet isn't the same force exerted by the wood on the bullet with opposite direction? (3rd Newton's Law)

Last edited: Dec 8, 2011
4. Dec 8, 2011

### MsHomework

Re: Need to find the net force and the average force.....

So would the acceleration be 6666.5m/s^2

5. Dec 8, 2011

### BetoG93

Re: Need to find the net force and the average force.....

That would be possible only if the acceleration is constant. I suppose it isn't, so the work-kinetic energy theorem seems to be the only option.

6. Dec 8, 2011

### MsHomework

Re: Need to find the net force and the average force.....

Fnet is equal to 79.998 N. for part b it wants me to find force average. when i think of force average I think of momentum. but i don't have enough information to find force average using momentum. Do you know any other equations???

7. Dec 8, 2011

### MsHomework

Re: Need to find the net force and the average force.....

Woah what. :S Ok you said to use the 2ax = (vf)^2 - (vi)^2. and I got 6666.5 m/s^2. is it wrong? does that mean I have to go back to my original idea? Now I am beyond confused.

8. Dec 8, 2011

### BetoG93

Re: Need to find the net force and the average force.....

Mmmm... I have just tried to solve this problem using the equation of kinematics I gave and the work-kinetic energy theorem and both gave me that the average force is 32000 N. Do you know what is the correct answer (from the textbook, if you took the problem from one)?

About using momentum, you must have a function that describes how it changes over time, in order to derivate it and find force. Then, you will need to integrate the force and divide it by the time interval. Unfortunately, I see no way of doing this for this kind of exercise.

9. Dec 8, 2011

### netgypsy

Re: Need to find the net force and the average force.....

Momentum wouldn't apply because the block is fixed. It seems like it's just an equal opposite thing. There is of course heat generated but I don't see how that could be calculated from the info given.

10. Dec 8, 2011

### BetoG93

Re: Need to find the net force and the average force.....

The heat would be all the kinetic energy lost (neglecting sound) when the bullet comes to rest.

11. Dec 8, 2011

### netgypsy

Re: Need to find the net force and the average force.....

You have mechanical work done in ripping the wood I would think as well as heat but I don't see how it would change the answer.

12. Dec 8, 2011

### netgypsy

Re: Need to find the net force and the average force.....

also the bullet may be flattened. but regardless, you only start with a certain amount of energy available and force ab is - Fba

Last edited: Dec 8, 2011