Drawing FreeBody Diagrams
We will need a way to organize forces that are acting on a particular object. The easiest way to do this is by using a free body diagram.
- A free body diagram is just a simple sketch of the object showing all the forces that are acting on it.
- To draw a proper free body diagram, you must follow these steps:
- Draw a quick sketch of the object. Often a simple box will do.
- Place a dot in the centre of the object. We basically treat this as the spot that all the forces are thought to act upon.
- For every force acting on that object (we don't care about forces acting on any other objects), draw a vector that shows the size and direction of the force. Each vector must start from the dot and point outwards.
- Label each vector based on the type of force it is. Do not include numbers or calculations!
Expand Each Step to See the Details
Example 1 Holding a Book
We draw a quick sketch of the book, and then put a dot in the centre.
Next, we identify that there are two forces acting on this book;
the force of gravity pulling it down, and the applied force of a person pushing it up.
Since the book is just being held up (not accelerating up or down),
we can assume that the two forces are equal, so we will draw the vectors the same size.
Example 2 Book on a Desk
- We know that there will be a force due to gravity (Fg) pulling the laptop down, but if that was the only force it should be dropping down towards the ground.
- There must be a force acting against the force of gravity that is holding the laptop up. This is happening because the table top is strong enough to hold up the laptop.
We call this upwards force the normal force (FN). A normal force is exerted upwards by a surface (like a table or a floor) and is perpendicular to the surface.
Example 3 Pushing a Heavy Object
Pusing an object across a rough surface
- We still have force due to gravity and normal force, since it is still an object on a surface.
- You are trying to push it sideways, so that will be an applied force that you are exerting.
- It's tough to push because of friction between the box and the floor, so we'll also need to draw a force due to friction.
- Force due to friction (fr)
- Always opposes the motion of an object. It is parallel to the surface that the object is on.
- Notice that the normal force and the force due to gravity are still equal.
The applied force and the force due to friction are also equal to each other.
This would mean that the object is moving at a constant velocity or is at rest.
Example 4 Object on an Incline
Object Sliding Down an Incline
Notice that …
- Fg Still points straight down
- FN points upwards at an angle, since it has to be
perpendicular to the surface of the slope.
- fr points back up the hill, since friction will try to slow down the sled.
Common Forces that we will use
Free body diagrams must show a wide variety of forces acting on an object.
These are the common forces acting on objects that you need to memorize: