# Homework Help: Need Help Drawing a Free Body Diagram

1. Oct 8, 2009

### JoshMP

1. The problem statement, all variables and given/known data

A 1000kg car pushed a 2000kg truck. When the driver steps on the accelerator, the drive wheels of the car push against the ground with a force of 4500 N. What is the magnitude of the force of the car on the truck? What is the magnitude of the truck on the car?

2. Relevant equations

Newton's 2nd and 3rd Laws

3. The attempt at a solution

I can solve the problem once I know what the free body diagram looks like. I don't understand what to do with the 4500 N force. The problem states that 4500 N are "pushing against the ground." Does that mean I should a 4500 N force pointing in the same direction as gravity? Or does this force point in the direction of motion?

2. Oct 8, 2009

### Delphi51

That is too small to be the force of gravity. It must be half of Newton's 3rd law for making the car accelerate:
"The car pushed back on the road with 4500 N so the road pushed forward on the car with an equal and opposite force."

3. Oct 8, 2009

### JoshMP

So for the car, the free body diagram has gravity pointing down, normal force pointing up, 4000N pointing in the direction of motion, and the force from the truck pushing back on the car pointing in the opposite direction of motion.

For the truck, gravity pointing down, normal force pointing up, and the force from the car pointing in the direction of motion.

Are these correct?

4. Oct 8, 2009

### Delphi51

That looks right - carry on!