Newton's Second Law
Forces and Motion

The motion changes for just as long as a resultant force is acting

Physics Narrative for 11-14 Supporting Physics Teaching

Speed changes require a resultant force to act

An important point to recognise in thinking about objects speeding up and slowing down is that the motion changes only for as long as a resultant force is acting on the object. Taking our first example of the ice skater, she speeds up only while she is pushing with her driving leg. As soon as she stops pushing, she stops speeding up. If she wants to gain a high final speed, she pushes with a big force and maintains her push for as long as she can.

If you hit a ball with a tennis racquet, the ball speeds up just as long as the strings are in contact with it. If you want the ball to fly away at top speed, you must hit it hard and for as long as possible. Expert tennis players achieve this by following through on their stroke (as do golfers, footballers and so on).

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