Chapter 3, Problem 1RQ

Review Question 3.1 How do we determine how many forces to draw on a force diagram?

To determine

The way in which one can draw the forces in a force diagram.

Answer to Problem 1RQ

Solution:

The number of forces in the force diagram must be equal to the number of objects it is interacting with.

Explanation of Solution

Introduction:

Force is a vector quantity and it can be contacting as well as noncontacting in nature.

Explanation:

Consider a ball $A$ which is hanging from the ceiling with the help of a string:

Considering ball as the system, there are two forces acting on it, as follows:

The string will apply an upward force of contacting nature: $\left({\overrightarrow{F}}_{S\text{ on }A}\right)$.

The earth will apply a downward force of noncontacting nature: $\left({\overrightarrow{F}}_{E\text{ on }A}\right)$.

The force diagram for the system of forces is as shown below:

Here, the length of the arrows indicates the magnitude of the forces, and the direction indicates the direction in which they are acting.

Conclusion:

Hence, to determine the number of forces acting on a system, consider only the number of forces acting on it from the objects it is interacting with.