Force, motion and machines


This topic explores the key concepts of force, motion and machines as they relate to:

  • force
  • motion and inertia
  • Newton’s laws of motion
  • force and pressure
  • energy
  • machines.

Key concepts of force, motion and machines

The activities in this topic are designed to explore the following key concepts:

  • A force is just a push or a pull.
  • Forces always come in pairs and involve two objects. One object applies a force (action force) on another object and, in return, the other object applies an equal and opposite force (reaction force) on the first object.
  • Forces can be contact forces involving objects that are in contact or forces.
  • Forces can be field forces where one object sets up a force field in the area around it.
  • There are different types of forces: for example, tension, compression, friction and normal.
  • Multiple forces on an object add up to produce a net force.
  • An object with a zero net force on it is either at rest or moving with constant speed.
  • Inertia is a property of all objects with mass and represents the resistance of the object to change its motion.
  • An object with a non-zero net force will change its motion. This may involve the object speeding up, slowing down and/or changing direction.
  • In an interaction between two or more objects the total energy remains the same. Energy may be transferred from one object to another or it may transform from one type of energy into another.
  • There are different types of energy: for example, kinetic, gravitational potential, elastic potential, heat and sound.
  • A machine is a device that transmits and modifies force. There are different types of machines: for example, incline plane, lever, wheel and axle, pulley and gears.
  • A machine gives mechanical advantage, which is equal to the output force (load) divided by the input force (effort).

Students’ alternative conceptions of force, motion and machines

Research into students’ ideas about this topic has identified the following non-scientific conceptions:

Mass, weight and gravity

    • A kilogram of iron weighs more than a kilogram of feathers.
    • Weight and mass are the same thing; weight is not considered to be a gravitational force.
    • There must be air for gravity to take effect; therefore, there is no gravity on the Moon, nor is there gravity out in space.
    • Earth’s magnetism and spin are connected with gravity.
    • Things fall naturally without the need for a force.
    • Heavier things fall faster than light things.
    • Gravity only acts on an object when it begins to fall and when falling.

Gravity ceases to act when the object lands on the ground and becomes stationary.

  • All objects can be moved with equal ease in the absence of gravity.

Describing motion – kinematics

  • Two objects side by side must have the same speed.
  • Acceleration and velocity are always in the same direction.
  • Velocity is a force.
  • If velocity is zero, then acceleration must be zero too.
  • Heavier objects fall faster than light ones.
  • Acceleration is the same as velocity.
  • The acceleration of a falling object depends upon its mass.
  • If the speed of an object is increasing, so is its acceleration.
  • ‘Velocity’ is another word for speed. An object’s speed and velocity are always the same.
  • Acceleration always means that an object is speeding up. Acceleration is always in a straight line. Acceleration always occurs in the same direction that an object is moving.
  • If an object has a speed of zero (even instantaneously), it has no acceleration.

Explaining motion – dynamics

  • Forces are only associated with movement.
  • Forces get things going rather than making things stop.
  • Forces are associated with living things, physical activity and muscular strength.
  • Inanimate objects do not apply forces.
  • Forces keep objects in motion.
  • When an object is moving, there is a force in the direction of its motion.
  • A moving object has a force within it that keeps it going.
  • Force is a property of a single object rather than a feature of the interaction between two objects.
  • If the pushing force ceases there is a force on the moving object that keeps it moving but which gradually gets used up and then the object stops.
  • Friction is associated with heat.
  • Friction only occurs between solids.
  • If an object is at rest, no forces are acting on the object. Only animate objects can exert a force. Thus, if an object is at rest on a table, no forces are acting upon it
  • A force is needed to keep an object moving with a constant speed.
  • Action-reaction forces act on the same object.
  • Inertia is the force that keeps objects in motion.


  • Energy gets used up or runs out.
  • Gravitational potential energy is the only type of potential energy.
  • Energy is only caused by life/animal activity.
  • Conservation of energy means that energy should be conserved.
  • Energy and force are the same thing.
  • Energy is a thing.
  • An object at rest has no energy.
  • Gravitational potential energy depends only on the height of an object.
  • Doubling the speed of a moving object doubles the kinetic energy.
  • Energy can be changed completely from one form to another (no energy losses).
  • Energy is truly lost in many energy transformations. Things ‘use up’ energy.


  • Only wind, and not still air, has a pressure.
  • Air pressure is a downward influence.
  • A vacuum sucks.
  • Pressure is the same as force.


  • Momentum is the same as force.
  • Momentum and kinetic energy are the same thing.


  • Machines must have a motor.
  • Machines are only inanimate objects.

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