Running out of driving force

Running out of force

Wrong Track: When I stop pushing the trolley it gradually runs out of force and slows down.

Right Lines: When the driving force on the trolley stops, the only force acting is friction and that force slows down the trolley.

Carrying forces

Thinking about the learning

The very common misconception here is that if you push a trolley to set it moving, the trolley carries that driving force with it and as the force runs out the trolley slows down. The key point here is that the driving force is being applied to the trolley only for as long as you are pushing it. Once you stop pushing the trolley, the only force acting along the direction of motion is friction and that friction force acts to slow down the trolley.

Thinking about the teaching

In your teaching, it is important to be very clear about the forces acting on objects as they speed up and slow down and to tackle head-on those situations where pupils suggest forces which simply don't exist. Here is another such example.

Supposing you throw a ball vertically up into the air. What motion will it have? From the moment it leaves your hand, the ball slows down and actually comes momentarily to a halt as it reaches its top point and then falls back to your hand.

When asked to identify the forces acting on the ball as it is moving upwards, many pupils suggest that there is gravity and air resistance acting downwards and a driving force acting upwards.

The crucial point here is that the driving force acts on the ball only for as long as your hand is in contact with it, throwing it upwards. Once the ball leaves your hand the only forces acting are the downward ones.

So, the ball slows down as it moves upwards due to the resultant force acting on it. Many pupils find it difficult to accept that the ball is moving upwards and the only forces acting on it are in the opposite direction, downwards.

Driving force exists only for as long as you are pushing

Here is another classic: a series of snapshots in the trajectory of a thrown ball. See if you can sketch in the forces acting on the ball for the four steps.

The forces acting on a thrown ball

Here are the forces acting on the ball for the four steps.