Question

A rifle bullet with a mass of 11.5 g traveling toward the right at 251 m/s strikes a large bag of sand and penetrates it to a depth of 23.7 cm. determine the magnitude and direction of the friction force (assumed constant) that acts on the bullet.

Answer 1

To look for the acceleration, it will come from:

vf^2=v0^2+2ad 
where:
vf = final velocity = 0 
v0 = initial velocity =251 m/s 
a = acceleration 
d= distance traveled = 0.237 m 

0=251^2+2a(0.237 ) 
a= -251 ^2 / (2*0.237) =-132 913.502 m/s/s 

we find the force from: 

F = ma = 0.0115kg*(-1.32x10^5m/s/s) = -1518 N 

the negative sign shows that the force is in the direction contradictory the bullet's motion