5.3 Identifying Forces

A typical physics problem describes an object that is being pushed and pulled in various directions. Some forces are given explicitly; others are only implied. In order to proceed, it is necessary to determine all the forces that act on the object. The procedure for identifying forces will become part of the pictorial representation of the problem.

Force Notation
General force F⃗
Gravitational force F⃗G
Spring force F⃗Sp
Tension T⃗
Normal force n⃗
Static friction f⃗s
Kinetic friction f⃗k
Drag F⃗drag
Thrust F⃗thrust

TACTICS BOX 5.2 Identifying forces

1 Identify the object of interest. This is the object you wish to study.

2 Draw a picture of the situation. Show the object of interest and all other objects—such as ropes, springs, or surfaces—that touch it.

3 Draw a closed curve around the object. Only the object of interest is inside the curve; everything else is outside.

4 Locate every point on the boundary of this curve where other objects touch the object of interest. These are the points where contact forces are exerted on the object.

5 Name and label each contact force acting on the object. There is at least one force at each point of contact; there may be more than one. When necessary, use subscripts to distinguish forces of the same type.

6 Name and label each long-range force acting on the object. For now, the only long-range force is the gravitational force.

Workbook Exercises 3–8

EXAMPLE 5.1 Forces on a bungee jumper

A bungee jumper has leapt off a bridge and is nearing the bottom of her fall. What forces are being exerted on the jumper?

VISUALIZE

FIGURE 5.11 Forces on a bungee jumper.

The figure illustrates a bungee jumper tied to a cord and falling vertically downward. The steps for identifying the forces acting on the jumper are mentioned.

EXAMPLE 5.2 Forces on a skier

A skier is being towed up a snow-covered hill by a tow rope. What forces are being exerted on the skier?

VISUALIZE

FIGURE 5.12 Forces on a skier.

The figure illustrates a skier being towed up a snow-covered hill by a tow rope. The steps for identifying the forces acting on the skier are mentioned.

NOTE   You might have expected two friction forces and two normal forces in Example 5.2, one on each ski. Keep in mind, however, that we’re working within the particle model, which represents the skier by a single point. A particle has only one contact with the ground, so there is one normal force and one friction force.

EXAMPLE 5.3 Forces on a rocket

A rocket is being launched to place a new satellite in orbit. Air resistance is not negligible. What forces are being exerted on the rocket?

VISUALIZE This drawing is much more like the sketch you would make when identifying forces as part of solving a problem.

FIGURE 5.13 Forces on a rocket.

A drawing of a rocket accelerating upward.