Question- from https://www.physicsclassroom.com/calcpad/momentum/prob29.cfm
A physics student hurls a $315-g$ ball directly into a $3.54-kg$ box which is at rest on a table top. The baseball strikes the box with a pre-impact speed of $54.1 m/s$. The box is filled with towels to help absorb the blow and effectively catch the ball. The coefficient of friction between the box and the table is $0.714$. Determine the distance which the ball and box slide across the table after the collision.
- First, I use the Law of Momentum conservation which states that the change of momentum of the 1st object is equal and opposite to the change of momentum of the 2nd object, to find the final velocity of both of the object. The velocity after the collision is $4.42m/s$
My question: This is the part where I got stuck on, I know I need to use one of the kinematic equations which is $v^2=u^2+2as$ and solve for $s$. To do this, I am just missing 1 value which is the acceleration $a$. I tried to find it but I couldn't. So, I decided to look for some hints. According to the answer, it said in order to find the acceleration, I need to equate the friction force to the net force. I couldn't see how can you equate the friction force to the net force that is $f net = f fric$, $ma=μ fn$ But how? I do not understand, I thought friction is a force opposing the applied force?