5.1.7 Free Body Diagrams

Free-body Diagram for a Skydiver

In a free-body diagram, the object is shown as a point.
The forces are drawn as arrows starting at the point.
- The length of the arrow represents the size of the force.
- The direction of the arrow represents the direction of the force.

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Example: Aeroplane Flying at a Constant Altitude and Velocity

Altitude: Height above the ground.
Constant altitude means there must be a force of the same magnitude as the weight acting in the opposite direction (lift).
Thrust: A forward force provided by the engines.
The aeroplane also experiences the force of air resistance (drag) acting in the reverse direction.
Due to constant velocity, the forward and backward forces must be balanced.

Vector Diagrams

Used to calculate the resultant of two forces acting at an angle.

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Example:

Two forces are acting on an object.
One force has a magnitude of 10N.
Another force has a magnitude of 8N.
The angle between the two forces is 30°.
Calculate the resultant force.
1cm = 4N
Resultant force = 17.5N

Resolving Forces

Vector diagrams can be used to resolve a single force into two forces.

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Example:

A 100N force is acting at an angle of 35° with the horizontal.
Determine the horizontal and vertical components of this force.

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