Physics + Logic = tired brain :biggrin:

[kristen151027]

This one should be quick and easy, but I just can't seem to get it:

"A college rower can easily push a small car along a flat road, but she cannot lift the car in the air. Since the mass of the car is constant, how can you explain this discrepancy?"

Edit: I'm thinking that friction can be overcome more easily than gravity...but I'm struggling to prove that.

 

[tyco05]

Think about the forces that are acting. What is the difference in direction between lifting it up, and pushing it across?
What are the forces acting in these directions?

 

[tyco05]

Sorry, just read your edit... remember friction force is given by mu * N
(normal force). Where mu is the co-efficient of friction - you might be able to estimate this, or find an approximate value somewhere.
The force acting due to gravity is F=mg.
I hope this gives you enough to go on with...

 

[kristen151027]

Sorry, just read your edit... remember friction force is given by mu * N
(normal force). Where mu is the co-efficient of friction - you might be able to estimate this, or find an approximate value somewhere.
The force acting due to gravity is F=mg.
I hope this gives you enough to go on with...

Yes, thank you!

 

[PPonte]

The force to lift the car up is $F = m.g$.
The force to push the car along a flat road is $F > \mu_s.m.g$

Compare this two forces.

HINT: Between what values is usually $\mu_s$.