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# Free Body Diagrams

## Representation of all the forces acting on an object.

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Free Body Diagrams

For every problem involving forces it is essential to draw a free body diagram (FBD) before proceeding to the problem solving stage. The FBD allows one to visualize the situation and also to make sure all the forces are accounted. In addition, a very solid understanding of the physics is gleaned and many questions can be answered solely from the FBD.

### Review

1. Draw free body diagrams (FBDs) for all of the following objects involved (in bold) and label all the forces appropriately. Make sure the lengths of the vectors in your FBDs are proportional to the strength of the force: smaller forces get shorter arrows!
1. A man stands in an elevator that is accelerating upward at 2 m/s2\begin{align*}2 \ m/s^2\end{align*}.
2. A boy is dragging a sled at a constant speed. The boy is pulling the sled with a rope at a 30\begin{align*}30^\circ\end{align*} angle.
3. The pictureshown here is attached to the ceiling by three wires.

4. A bowling ball rolls down a lane at a constant velocity.

5. A car accelerates down the road. There is friction f\begin{align*}f\end{align*} between the tires and the road.

2. For the following situation, identify the 3rd\begin{align*}3^{rd}\end{align*} law force pairs on the associated free body diagrams. Label each member of one pair A\begin{align*}A\end{align*}, each member of the next pair B\begin{align*}B\end{align*}, and so on. The spring is stretched so that it is pulling the block of wood to the right. Draw free body diagrams for the situation below. Notice that we are pulling the bottom block out from beneath the top block. There is friction between the blocks! After you have drawn your FBDs, identify the 3rd\begin{align*}3^{rd}\end{align*} law force pairs, as above.

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