# Homework Help: Help with a question

1. Sep 24, 2006

### jjg242

8. A karate master strikes a board with an initial velocity of 10 m/s, decreasing to 1 m/s as his hand passes through the board. If the time of contact with the board is 0.002 s, and the mass of the coordinated hand and arm is 1 kg, what is the force exerted on the board.

a. 1000 N
b. 1800 N
c. 2700 N
d. 4500 N

- well, I lost my book cuz my backpack got stolen and I have an older physics book i got from a freind and the pages dont match up with my study guide. I cant find the right equation for this. I need an equation that uses change in velocity, contact time, and mass of the object to get F in Newtons. If anyone can show me how I do this or at least point me in the right direction I would appreciate it.

2. Sep 24, 2006

### laminar

Momentum=Impulse

mv=Ft

3. Sep 24, 2006

### jjg242

so...

m = mass
v= change in velocity?
F= force in N
t= time

is v the initial, final, or change in velocity?

if it is change in velocity, then i get:

9*1/0.002 = 4500 N

Is this right?

Last edited: Sep 24, 2006
4. Sep 24, 2006

### jjg242

bump. any feedback appreciated.

5. Sep 25, 2006

### Astronuc

Staff Emeritus