Question

A 15-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?

Answer 1

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.


`(1)/(2)(0.015)(39)^2 = (1)/(2)(0.015)(27)^2 +U_(L)`

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


`U= (1)/(2)(0.015)(39)^2- (1)/(2)(0.015)(27)^2= 5.94J`

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


`F= (W)/(d) = ((5.94J))/(0.48m) = 12.375N`

F = 12.375N