Physics - newton's 2nd law question

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.


Junior Member
15+ Year Member
Mar 18, 2006
Reaction score
I suck at physics.. and got confused on this line in the TPR.

if the direction of "a" is always the same as the direction of "Fnet" then the object will accelerate in the direction of the net force. BUT the object does NOT always move in the direction of the Fnet force.

Can someone help clear that up for me? examples? Thanks....

Members don't see this ad.
For example: a ball is rolling on the floor. The law of inertia says the ball will keep rolling unless another force acting on it. Obviously the ball doesn't roll forever, so the force acting on the ball is friction. And if you remember frictional force is alway in the opposite direction of the movement. So if ball rolls to the right, friction force is to the left. And since friction force is our net force in this case, acceleration is to the left.

BUT REMEMBER, ball is still moving to the right, just getting slower and slower and eventually stop. the ball accelerate to the left (in another word - deccelerate), but still moving to the right. Make sense?
Yea, think of throwing the ball up in the air. Gravity ("a") is pointed downwards and so is the force, but the ball is moving up in the opposite direction for awhile.