Forces and Motion

Solving problems – force or energy?

Teaching Guidance for 14-16 PRACTICAL PHYISCS

Physicists aim to understand, and sometimes to predict, physical interactions. They use two abstract concepts a lot: force and energy.


A force is something that can change an object's shape or how it is moving (or not moving). A force can have any size and acts in a particular direction. Forces are something to think about when analyzing things such as:

  • pressure
  • turning effect
  • momentum
  • acceleration

The concept of force is used to explain what causes things to happen. You can analyze the forces acting on macroscopic objects or systems, but also on microscopic objects such as the particles in a gas. When forces acting on an object are in equilibrium (balanced), the velocity of the object does not change. If the object is at rest, it stays there. If the object is moving it doesn’t speed up, slow down or change direction.


Energy is a numerical value that we calculate for an object, or a system, that quantifies the amount of change that has taken, or will take, place. What is important about this quantity is that, in every event or process, there is the same amount of it at the end as there was at the beginning. Energy does not explain why things happen, though we can use it to explain why some things do NOT happen.

The concept of energy has much wider use than the concept of force. The concept of energy can provide insights into movement and materials. It can also be used to analyze electrical and magnetic behaviour, wave behaviour, changes inside the atom, engines that burn fuels … and anything else.

Solving problems

In some situations, you can think in terms of either energy or force. For example, when trying to improve vehicle safety in the event of a crash, you could calculate the energy absorbed in the collision, or calculate the forces acting on the vehicle’s occupants.

Physicists are resourceful and will draw on whatever thinking tools help them understand a particular situation. They prefer to understand things quantitatively.

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