Solving Force and Motion Problems with Friction

[whatsmyname]

I may just be stupid but I am stuck on a few simple force and motion problems where I can't seem to get a numerical value.. everything i try i end up with several unknown variables in my answer.

In one problem there is a 5 gram bullet fired from a .82m rifle barrol, and the velocity of the bullet at the time it leaves the rifle is 280m/s. I got the acceleration in the barrol to be 47,804m/s² (this doesn't seem correct though.. would there even be an acceleration?) and the normal force .049N, but how would you find the force F on the bullet pushing it through the barrol without knowing the friction? Or can you find that?



Another similar problem which may answer the previous one once i figure it out.. there is a 10kg disk pushed along a horizontal surface and it comes to rest 8 meters from the position it was pushed from. How would i find the coefficiant of kenetic friction from just that? I am completely stuck but I am sure its very easy. I end up with F=μ(10)(9.8), but then I am stuck with finding the force.



Just one more having to do with friction.. when there is a block of 5kg attached to a wall by a horizontal string, and it is sitting on top of an accelerating 10kg block and the coefficiant of kenetic friction between the two blocks is given and it asks to find the tension in the string and the acceleration of the 10kg block. Since the coefficiant of friction is given and i know the mass of the objects i think i can find the force on the 10kg block? but the 5kg block sitting on top of the 10kg block is what confuses me. When finding the tension in the string i just used the mass of the block the string is attached to in my equation, since it is the top block, but when finding the acceleration of the bottom block, would you add the masses of both blocks when forming your equation? (The friction is between the two blocks, not on the surface that the larger one sits on).


Thanks!

 

[Euclid]

For the first problem, the force isn't contant in the barrel so you can't find the force (nor the acceleration for that matter). For the second problem, I am almost positive you need more information.

 

[HallsofIvy]

"In one problem there is a 5 gram bullet fired from a .82m rifle barrol, and the velocity of the bullet at the time it leaves the rifle is 280m/s. I got the acceleration in the barrol to be 47,804m/s² (this doesn't seem correct though.. would there even be an acceleration?) and the normal force .049N, but how would you find the force F on the bullet pushing it through the barrol without knowing the friction? Or can you find that?"

Of course there is acceleration- the bullet started from v= 0 didn't it?

What you can calculate is average acceleration. Since v= at= 280, and x= (1/2)at²= .82m you have two equations to solve for t and a. After you know a, you can calculate the net (average) force from f= ma.

If you don't know the friction, then you can't calculate the actual force of the expanding gases on the bullet but do you really need to know that?