# Physics help

Discussion in 'Community Discussion' started by Jasonbot, Sep 11, 2007.

1. ### Jasonbot macrumors 68020

Joined:
Aug 15, 2006
Location:
The Rainbow Nation RSA
#1
Hi again, got more assistance requirements again. BTW, last time MR helped me with maths I got 96% for the exam- the highest in the grade.

So...

A toy cannon (0.4kg) is at rest. The cannon shots a ball (0.025kg). Immediately after firing the shot the cannon rolls back at 0.2m.s-1, then comes to rest.

1. Calculate teh magnitude of the velocity of the ball. (I got 3.2m.s-1)
2. If the acceleration of the cannon is 0.05m.s-2 calculate the magnitude of the frictional force of the ground on teh cannon.

so far I have got
F=m.a=Fball - Ffriction
=0.4 * 0.05 =Fb-Ff

At this point I cant get Fb since no time is given.

2. ### squeeks macrumors 68040

Joined:
Jun 19, 2007
Location:
Florida
#2
i failed my physics final...still got a C in the class though, but that was from association with some of my teachers favorite students being my best friends

3. ### elfin buddy macrumors 6502a

Joined:
Sep 16, 2001
Location:
Tuttlingen, Germany
4. ### Dandaman macrumors 6502

Joined:
Jan 31, 2004
Location:
soCal
#4
someone correct me if i'm wrong, but I dont think you have to take the force of the ball into consideration. The frictional force will equal that of the force from the cannon. so F = ma = Ffriction.

again, I could be wrong. It's been awhile since i've done a physics problem.

daniel

5. ### Jasonbot thread starter macrumors 68020

Joined:
Aug 15, 2006
Location:
The Rainbow Nation RSA
#5
to restate; Fresultant=Fball exerts on cannon - Ffriction

If you look @ the vectors:

Fres= --->
Fball= ---->
Ffriction= <-

so: Fres=Fball-Ffric
or: ---> = ----> - <-

6. ### Dandaman macrumors 6502

Joined:
Jan 31, 2004
Location:
soCal
#6
hmmm, not sure if your force vectors are 100% correct, but I could be wrong. the Fres should be opposite of the direction of the ball.

if you need time, you can use (vfinal = vinitial - at) where vfinal = 0 since the cannon comes to rest.

daniel

EDIT: hmm, i just realized that you're looking for the time for the ball. I don't think you can find this because you are not given acceleration for the ball.

7. ### Jasonbot thread starter macrumors 68020

Joined:
Aug 15, 2006
Location:
The Rainbow Nation RSA
#7
I was meaning force of ball on cannon as it is equal but opposite

And as for the time I need the time of the flight of the ball or something to calculate the Fres of the ball as it affected the cannon.