# 5.1.7 Free Body Diagrams

Higher Tier Only

### Examples of Forces

#### Tension

• Tension is:

The force experienced by a cable, rope, or string when pulled, hung, rotated or supported

• This is normally labelled as T on free body diagrams

Tension always acts away from the mass

#### Normal Contact Force

• The normal contact force is:

The force arising when an object rests against another object acting at a 90° angle to the plane of contact

• It is sometimes also referred to as the reaction force
• This is normally labelled as N or R on free body diagrams
• This force arises from Newton’s Third Law

Normal contact force always acts perpendicular to the surface

#### Upthrust

• Upthrust is:

The upward buoyancy force acting on an object when it is in a fluid

• Upthrust can occur in liquids and gases

Upthrust always acts upwards

#### Friction

• Friction is:

The force that arises when two surfaces are in contact with each other

• Friction always opposes the motion
• This is normally labelled as F or Fr on free body diagrams

Friction always acts at the point where the objects are in contact, and in the opposite direction to the direction of motion

Higher Tier Only

### Free Body Diagrams

• Free body diagrams are useful for modelling the forces that are acting on an object
• Each force is represented as a vector arrow, where each arrow:
• Is scaled to the magnitude of the force it represents
• Points in the direction that the force acts
• Is labelled with the name of the force it represents
• Free body diagrams can be used:
• To identify which forces act in which plane
• To resolve the net force in a particular direction

Free body diagrams can be used to show the various forces acting on objects

#### Worked Example

Draw free-body diagrams for the following scenarios:

a) A picture frame hanging from a nail

b) A box sliding down a slope

c) A man fishing in a stationary boat

d) A car accelerating along a road

Part (a)

• The size of the arrows should be such that the 3 forces would make a closed triangle as they are balanced

Part (b)

• There are three forces acting on the box
• The normal contact force, R, acts perpendicular to the slope
• Friction, F, acts parallel to the slope and in the opposite direction to the direction of motion
• Weight, W, acts down towards the Earth

Part (c)

• As the boat is not moving, the size of both arrows must be the same

Part (d)

• As the car is accelerating, the size of the thrust must be larger than the size of the friction force
• As in part (c), the upwards and downwards forces must be equal

#### Worked Example

Draw a free-body diagram of an toy sail boat with weight 30N floating in water that is being pulled to the right by an applied force of 35N.

Step 1: Draw the object in a simplified diagram

Step 2: Identify all of the forces acting upon the object in the question, including any forces that may be implied

• Weight: 30N down
• Upthrust from the water (since the object is floating): 30N up
• Applied force: 35N to the right

Step 3: Draw in all of the force vectors (arrows), making sure the arrows start at the object and are directed away

• An approximation can be made as to the final resultant force due to all of the forces
• Decide whether the resultant force is approximately up or down
• Decide whether the resultant force is approximately left or right
• For example: The resultant force is directed up and to the right

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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