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Types of force and their effectsIB MYP Physics: Revision notes

Section 1

What is a force?

A force is a push or a pull that acts on an object. It is a vector: it has a size and a direction. Forces are measured in newtons (N) using a newton meter, which has a spring that stretches more when a bigger force is applied.

Forces always come from an interaction between objects.

Key termsforcenewtonnewton meter

Section 2

Contact forces

A contact force acts only when objects touch.

  • Friction: a force that opposes motion between two surfaces in contact.
  • Tension: the pulling force in a stretched rope, string or spring.
  • Normal reaction: the upward push of a surface on an object resting on it, at right angles to the surface.
  • Upthrust: the upward force of a fluid (liquid or gas) on an object in it. It allows things to float.
  • Air resistance (drag): friction from air (or water) that opposes motion through it. It increases as speed increases.
Key termscontact forcefrictiontensionnormal reactionupthrustair resistance

Section 3

Non-contact forces

A non-contact force acts at a distance, with no touching.

  • Weight: the force of gravity pulling an object towards the centre of the Earth.
  • Magnetic force: between magnets, or magnets and magnetic materials (iron, steel, nickel, cobalt).
  • Electrostatic force: between charged objects. Like charges repel and unlike charges attract.
Key termsnon-contact forceweightmagnetic forceelectrostatic force
Common mistake

Weight is a force, measured in newtons. Mass is the amount of matter, measured in kilograms.

Section 4

Effects of forces

A force can:

  • change the speed of an object (speed up or slow down)
  • change the direction of an object's motion
  • change the shape of an object (stretch, squash, bend or twist)

Balanced forces (equal and opposite) do not change the motion. A stationary object stays at rest, and a moving object stays at the same speed in a straight line.

Key termsbalanced forces

Section 5

Free-body diagrams

A free-body diagram shows all the forces acting on one object.

  1. Draw the object as a simple box or dot.
  2. Draw an arrow for each force, starting at the object.
  3. Point each arrow in the direction of the force.
  4. Make the length of each arrow match the size of the force (balanced forces have arrows of equal length).
  5. Label each arrow with the name of the force.

Example: a book resting on a table has an arrow down labelled weight and an arrow up labelled normal reaction, the same length.

Key termsfree-body diagram
Exam tip

Only draw forces acting ON the object, not forces the object exerts on other things.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Types of force and their effects

  1. A student in Toronto pulls a heavy wooden box across a classroom floor using a rope. The box moves along the floor at a steady speed.
    State the name of the force between the box and the floor that opposes its motion, and the direction in which it acts.2 marks
  2. A wooden cork floats at rest on the surface of the water in a pond in Kyoto.
    Describe how to draw a free-body diagram of the cork.2 marks
  3. A class in Nairobi investigates how the surface affects the force needed to pull a wooden block at a steady speed. They pull the block along four surfaces using a newton meter. The readings are 3.0 N on tile, 5.5 N on carpet, 7.0 N on sandpaper and 2.0 N on glass.
    Suggest three variables that the class should keep the same to make the investigation a fair test.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).