Forces and Motion

Does equilibrium mean standing still?

Teaching Guidance for 11-14 Supporting Physics Teaching

Resultant forces of zero—and motion

Wrong Track: If the forces all balance each other it can't be moving.

Right Lines: An object moving at a steady speed is one example of forces in equilibrium.

Explaining motion in equilibrium

Thinking about the learning

Objects that move at a steady speed in a straight line are often seen as having a force pushing them. The idea that there might be several forces, the sum of which is zero, is not at all obvious. It is certainly possible to have an equilibrium state which involves motion. A child sliding down a playground slide at a steady speed is being pulled by gravity and also being opposed by friction. These two forces balance and the child continues to move at a steady speed. With forces balanced, the child is in equilibrium but is still moving.

Children certainly have some interesting thoughts about equilibrium, and listening to what they have to say, and seeing what they do as they seek to explain situations involving equilibrium can be very instructive.

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Thinking about the teaching

Here are two examples of moving objects in equilibrium. For each one, identify the most significant forces that are acting. A carefully drawn diagram will help you to show that these forces might add to zero.

A car moving at 70 mph along a straight motorway.

Emergency food supplies falling with a parachute.

Try deliberately building these challenging situations into your lessons. By creating a challenging situation you will offer learners an opportunity to talk to you and to each other about what they think is happening. It is through this discussion that you will be able to listen, explore and direct thinking about dynamic equilibrium.

can be analysed using Force
has the special case Thermal Equilibrium
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