How Does Friction Affect Acceleration and Motion?

[William Donald]

Homework Statement

A student exerts a 30 N[E] force to push a 5 kg brick along a floor that exerts a 20 N[W] friction force.
Consider all forces.

a. Calculate the acceleration.
b. If the force is exerted for 5 seconds, calculate the velocity at the end of the
five seconds.
c. Calculate the new acceleration if the student stops pushing he brick after 5
seconds.
d. How long does it take the brick to stop?

Homework Equations

a=Fnet/m (I think?)

The Attempt at a Solution

[/B]
a = 30N(E) / 5kg = 6m/s

But it states consider all forces. How do I consider the friction force into this?

 

[Stephen Tashi]

You find the net force on a "free body" by adding (as vectors) all the forces that act upon it. (You ignore forces exerted by the body itself on other things.) View the mass as the "free body", you add up all forces.

In this problem the force of gravity down on the mass plus the force of the floor upward on the mass add to zero. So there is no upward or downward acceleration. (We ignore the force of the mass downward on the floor because it is a force exerted by the mass itself. We don't ignore the force of gravity because that is a force acting upon the mass.) The force of friction is a force upon the mass, so it must be considered in computing the net force. The East and West forces are unbalanced, so there is horizontal acceleration. You don't have to do anything complicated to add the horizontal forces as vectors since they lie on the same straight line.