Newton's Second Law
Forces and Motion

When is the motion changing?

Teaching Guidance for 11-14 Supporting Physics Teaching

A resultant force is required to change speed

Wrong Track: I hit the snooker ball with the cue and it speeds up right across the table.

Right Lines: The snooker ball speeds up from rest for as long as the cue is exerting a force on it. Once the ball loses contact with the cue it rolls across the table slowing down all the way (because of friction).

Motion changes for as long as the resultant force acts

Thinking about the teaching

The key point to get over here is that the motion changes for just as long as a resultant force is acting. Very often objects gain their motion through a force acting over a short time (for example in kicking a ball; flicking a paper pellet; firing a catapult). In all such cases, it is worth spending some time in emphasising just when it is that the force is acting and the motion is changing.

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law
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