# Help on Concept Question with F=ma

1. Mar 19, 2016

### sciencecats

1. The problem statement, all variables and given/known data
A truck is pulling a car.
• is the magnitude of the force that the truck exerts on the car
• is the magnitude of the force that the car exerts on the truck
Consider the following scenarios independently.

1. The truck is driving up a mountain with a constant velocity, neglecting friction.
2. The truck is speeding up while driving up a mountain, neglecting friction.
3.The truck is driving with a constant velocity, but as it turns out, the driver of the car left the emergency brake on.
2. Relevant equations
F=ma

3. The attempt at a solution
I know that the answer to all of the scenarios are FT = FC > 0 (the key for this past homework problem is available to me), but I trying to explain the concept to myself. The 3rd one is the hardest one for me. I suppose since the mass of the car and truck are not changing, that the truck must be moving at a faster speed to compensate for the emergency brake? Am I thinking about this correctly?

2. Mar 19, 2016

### sciencecats

FT is the magnitude of the force that the truck exerts on the car
FC is the magnitude of the force that the car exerts on the truck

Sorry that it did not post correctly in the first post!

3. Mar 19, 2016

### Staff: Mentor

Have you drawn separate free body diagrams of the truck and of the car, showing the forces acting on each, or do you feel that you have advanced beyond the point where you need to draw free body diagrams? If you have drawn free body diagrams, please show them for case c.

Chet

4. Mar 19, 2016

### sciencecats

I am not sure if this is correct though!

5. Mar 19, 2016

### Staff: Mentor

The free body diagram for the car is correct. Nice job. The free body diagram for the truck is not separate (and should be). Please draw it separately. Then write down a horizontal force balance on the truck and a separate horizontal force balance on the car. (For case 3)