# Physics + Logic = tired brain :biggrin:

1. Jul 27, 2006

### 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.

Last edited: Jul 27, 2006
2. Jul 27, 2006

### 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?

3. Jul 27, 2006

### 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........

4. Jul 27, 2006

### kristen151027

Yes, thank you!!

5. Jul 27, 2006

### 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 $\mu_s$.

Last edited by a moderator: Jul 27, 2006