Balanced forces - but not quite

Just a little bigger

Wrong Track: If it's moving, the forward force must be just a little bit bigger than friction, to keep it going.

Right Lines: If an object is moving at constant speed the forward force must exactly equal the friction forces acting in the opposite direction.

Resultant forces for changing motion

Thinking about the learning

Pupils can sometimes hang onto inconsistencies when talking about constant speed. Thus some pupils may accept that the forces must add to zero for stationary objects, whilst arguing that in the case of objects moving at constant speed the driving force must be at least a little bit bigger than the friction forces. This line of thinking goes back to the idea that an object remains stationary if the forces acting on it add to zero. Intuitively then, since the object is moving, the force acting in one direction must, they argue, have an advantage.

Thinking about the teaching

When confronted with this kind of learning challenge go back to thinking about how the object got started moving in the first place. While initially the driving force must have been bigger than the retarding force in order to start the object moving in one direction (see episode 02), when the two forces add to zero the object travels at a constant speed, neither speeding up nor slowing down.