Easier to push or pull? Image you are moving a 100 kg block across a floor with friction (u_k=.3) at a constant velocity of 2m/s. If you push it, you apply a force at 20 degrees above the horizontal. Compute the required force in each case and the power required to move it at 2m/s.(P=F dot v)

Question

Easier to push or pull? Image you are moving a 100 kg block across a floor with friction (u_k=.3) at a constant velocity of 2m/s.  If you push it, you apply a force at 20 degrees above the horizontal. Compute the required force in each case and the power required to move it at 2m/s.(P=F dot v)

anonymous · Answer

If you're pushing, the force will have a downward component. |dw:1398198497978:dw|

A downward component would increase the normal force between the ground and box. 

If we increase the normal force, what would happen to the frictional force that opposes motion?

anonymous · Answer

The opposite would hold assuming that the pulling force is also applied at 20 degrees above the horizontal. 

This would create for the opposite affect on friction.

anonymous · Answer

yea.. and i know i have to calculate both of the force and then compare...

anonymous · Answer

We'll get to the math, I'm thinking more conceptually now. 

What do we know about friction? It's proportional to the normal force, right? $$F_f = F_N \mu$$

anonymous · Answer

So conceptually, which case do you think will be easier?

anonymous · Answer

i think pulling?

anonymous · Answer

Correct!

anonymous · Answer

Now, let's analyze the forces and prove our thought experiment. 

First, we need a free body diagram. |dw:1398199004088:dw|