Isolate

How to identify forces on an object

1. Decide on the object
2. Use physical clues in the environment of the object to identify the forces
3. Draw the object isolated, replacing the environment with the forces you have identified.

A simple example

A rock resting on the bottom of a pond.

First separate the environment from the object, at least in your imagination, as a first step to thinking about the forces acting on the object, exerted by the environment.

Now focus on just one aspect of the environment. Think about the interactions between the water and the rock. These interactions lead to a buoyancy force acting on the rock, exerted by the water.

Now focus on a second aspect of the environment. Think about the interactions between the bottom and the rock. These interactions lead to a compression force acting on the rock, exerted by the bottom.

a second example

A rock hanging on a rubber band in a glass of water.

First separate the environment from the object, at least in your imagination, as a first step to thinking about the forces acting on the object, exerted by the environment.

Now focus on just one aspect of the environment. Think about the interactions between the water and the rock. These interactions lead to a buoyancy force acting on the rock, exerted by the water.

Now focus on a second aspect of the environment. Think about the interactions between the rubber band and the rock. These interactions lead to a tension force acting on the rock, exerted by the band.

Now focus on the final aspect of the environment. Think about the (at-a-distance)interactions between the Earth and the rock. These interactions lead to a gravity force acting on the rock, exerted (almost entirely) by the Earth.

Now you have a complete description of all the forces acting on the rock, exerted by its environment. You've replaced each interaction with the environment, by thinking about how the environment affects the object, with a force.

For three objects, treat in pairs

Here are three interacting objects.

First deal with the left hand pair: the green and red objects. As there are two objects, and you should isolate each one, there will be two diagrams, one for each object. Each diagram shows the forces exerted on one isolated object.

Now deal with the right hand pair: the red and brown objects. Again there are two objects, so there will be two diagrams, one for each object. A diagram shows the forces exerted on one isolated object.

Now you have a complete description of all the forces acting on the rock, exerted by its environment. You've replaced each interaction with the environment, by thinking about how the environment affects the object, with a force.