OCR - P4: Explaining motion

P4.4 How can we describe motion in terms of energy transfers?
Background to the topic What you should be able to do:

Energy is always conserved in any event or process.

Energy calculations can be used to find out if something is possible and what will happen, but not explain why it happens.

The store of energy of a moving object is called its kinetic energy.

As an object is raised, its store of gravitational potential energy increases, and as it falls, its gravitational potential energy decreases.

When a force moves an object, it does work on the object, energy is transferred to the object; when work is done by an object, energy is transferred from the object to something else, for example:

When an object is lifted to a higher position above the ground, work is done by the lifting force; this increases the store of gravitational potential energy.

When a force acting on an object makes its velocity increase, the force does work on the object and this results in an increase in its store of kinetic energy.

If friction and air resistance can be ignored, an object's store of kinetic energy changes by an amount equal to the work done on it by an applied force; in practice air resistance or friction will cause the gain in kinetic energy to be less than the work done on it by an applied force in the direction of motion, because some energy is dissipated through heating.

Calculating the work done when climbing stairs or lifting a load, and the power output, makes a link back to the usefulness of electrical appliances for doing many everyday tasks.

1. Describe the energy transfers involved when a system is changed by work done by forces including:

a) to raise an object above ground level

b) to move an object along the line of action of the force

During practical work you will:

Use datalogging software to calculate the efficiency of energy transfers when work is done on a moving object.

Measure the work done by an electric motor lifting a load, and calculate the efficiency.

2. Recall and apply the relationship to calculate the work done (energy transferred) by a force:

work done (Nm or J) = force (N) × distance (m) (along the line of action of the force)

3. Recall the equation and calculate the amount of energy associated with a moving object:

kinetic energy (J) = 0.5 × mass (kg) × (speed (m/s))2

4. Recall the equation and calculate the amount of energy associated with an object raised above ground level

gravitational potential energy (J) = mass (kg) × gravitational field strength (N/kg) × height (m)

5. Make calculations of the energy transfers associated with changes in a system, recalling relevant equations for mechanical processes

6. Calculate relevant values of stored energy and energy transfers; convert between newton-metres and joules

7. Describe all the changes involved in the way energy is stored when a system changes, for common situations:


an object projected upwards or up a slope,

a moving object hitting an obstacle,

an object being accelerated by a constant force,

a vehicle slowing down

8. Explain, with reference to examples, the definition of power as the rate at which energy is transferred (work done) in a system

9. Recall and apply the relationship:

power (W) = energy transferred (J) ÷ time (s)