A15-g bullet pierces a sand bag that is 48cm thick. the initial speed of the bullet is 39 m/s and it emerged from the sandbag at speed 27 m/s. what is the average magnitude of the friction force that slowed the bullet as it traveled through the bag?

We can use the conservation of energy to solve this problem. 

We know the bullet goes in at 39m/s and leaves with 27m/s and some energy lost. We can model this using the kinetic equation energy.



The "U" is the energy lost when the bullet went through the bag. Solving for U, we get:



Next we know the bag is 48cm thick and that F*d=W, rearranging this equation we get:



F = 12.375N