Forces and Motion

Force changes motion

Teaching Guidance for 14-16 Supporting Physics Teaching

A common mistake: force  →  motion and motion  →  force

Wrong Track: The car has a force which keeps it moving along the motorway. If it's moving it must carry a force.

Right Lines: A force acting on an object changes its motion.

Giving forces to things

Thinking about the learning

It's a very common belief that objects that are moving must possess a force to keep them going. When questioned, a student might typically argue:

Stephen: The ball is carrying the force you gave it when you threw it to start it off.

Students often label moving objects with force arrows in the direction of motion. You're also likely to meet:

Nathan: The ball will stop when its force is used up.

The idea that a moving object carries a force, usually traced back to the force that was originally exerted, is common sense and it is almost correct. A moving object might indeed have been forced to start its motion. But, and it is a big but, this force changed the motion, in getting it going – it was the starting push. Once the agent doing the pushing stops exerting the force (so when the object no longer interacts with that aspect of its environment) the force is no longer there. The object continues to move but does not carry a force. The force changed the motion.

But both the energy in a kinetic store and the momentum do increase (see episode 03 for more on measures of motion) as a result of the action of the force.

Thinking about the teaching

The big challenge here is to get a visceral belief about the truth of Newton's first law embedded. In the lived-in world we can't escape the effects of frictional interactions. Accurate descriptions of everyday motions always involve frictional forces (grip, slip, drag). So the task is to imagine the natural motion of the objects in a world far removed from students' everyday world, and then – and only then – to add complexity back into the description.

Everyday experience is not on our side – to keep something moving we know we need to keep pushing it. This continued application of a force is required simply to oppose the retarding effect of friction. In a world without friction there would be no need to keep pushing. Objects, once in motion, would carry on moving. Pupils don't live in a world without friction, but there would be other interesting side effects.

You need to engineer the learning environment so that students' imagination can trump their (prior) experience.

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