# Help With Drawing a Free Body Diagram

1. Oct 9, 2014

### Phoenix23

1. The problem statement, all variables and given/known data
A large box containing your new computer sits on the bed of your pickup truck. You are stopped at a red light. The light turns green and you stomp on the gas and the truck accelerates. To your horror, the box starts to slide toward the back of the truck. (Assume that the truck is accelerating to the right.)

I have to draw a free-body diagram for the truck, and the truck bed is not frictionless.

2. Relevant equations
There are five vectors I have to insert, the normal force, the weight, the force of the road in reaction to the tires pushing backwards, the box's friction force on the truck bed, and the reaction to the normal force on the box from the truck.

3. The attempt at a solution
I drew the normal force up and the weight down. Both vectors were equal in length. The force of the road points to the right, and the friction of the box points to the left. I have no idea what "the reaction to the normal force on the box from the truck" means so I have not included that yet. Is my diagram alright so far, and can someone explain what the above vector means?

Thanks.

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2. Oct 9, 2014

### jack action

1. Draw a truck, resting on the ground, with a box in the bed;
2. Remove the ground and the box;
3. The forces from the ground on the truck (normal force & tire friction force) have to be acting where the ground used to be;
4. The forces from the box on the truck (box weight & its friction force) have to be acting where the box used to be.
5. By definition, the weight of the truck is always acting at its center of gravity.

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