OCR A Level Physics

Revision Notes

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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