# 3.3.3 Tension, Normal force, Upthrust & Friction

### Tension, Normal Force, Upthrust & Friction

• Tension:

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
• Normal Contact Force:

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
• Upthrust:

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

• Friction:

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

### 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
• The net force in a particular direction can be calculated by:
• Using the labelled angles and magnitudes
• Resolving each force into horizontal and vertical components

#### Worked Example

Draw free-body diagrams for the following scenarios:

a) A picture frame hanging from a nail

b) A box being pulled up a slope by a mass on a pulley (resolving the weight into parallel and perpendicular directions)

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 in equilibrium

Part (b)

• In problems such as this, it is best to resolve the forces parallel and perpendicular to the slope
• Usually, an angle will be given to allow calculation of the weight in these directions

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

#### Exam Tip

If you need a reminder on how to combine and resolve vectors, take a look at the notes in ‘3.3 Scalars & Vectors’

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