How do you do this physics problem?
A crate with a mass of 7kg is pushed up a ramp forming a 32 degrees angle with the horizontal at a velocity of 25m/s. If the coefficient of friction is 0.2, how far will the crate go up the ramp?
Fgravity=(7)(9.8)=68.6N
Fnormal=(68.6)cos32=58.2N
Fparallel=(68.6)sin32=36.4N
Ffriction=(0.2)(58.2N)=11.6N
After finding the values of these forces, we can find Fnet. However, in my teacher's notes, he added Fparallel with Ffriction. Don't you minus Ffriction from Fparallel to find the net force? Why do you add it?
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Fparallel is the component of Fgravity parallel to the plane. Since Fgravity acts down ,its component parallel to the plane will be directed down the plane.
The crate is moving up the plane and friction opposes this motion. So Ffriction also acts down the plane.
So the two forces parallel to the plane both act down the plane and their net force is just the sum Ffriction + Fparallel. Although you would typically call the sum negative because it acts down the plane.
If the crate stopped and then started sliding down the plate, Ffriction would then act up the plane while Fparallel (due to gravity) would still act down the plane. In that case the net force parallel to the plane would be the difference; Fparallel - Ffriction. Again, you would typically call that net force negative because it would still act down the plane.