Constant motion requires no resultant force
Teaching Guidance for 11-14
Constant speed is a result of no resultant force
Wrong Track: It's obvious! I've got to push here to keep it going at a constant speed. That's all that's needed, my push at the back.
Right Lines: For an object to move with constant speed the forward force is opposed by the effects of retarding forces. There is no resultant force acting.
Explaining the role of friction
Thinking about the learning
The common line of thinking that takes pupils down the Wrong Track here is that a constant push is all that is needed to keep an object moving at a constant speed. In the world around us few objects move at a constant speed for any extended period of time. A pupil's everyday experience is more commonly of changing speed. We also live in a world where all motion is influenced by friction. However, pupils are so accustomed to the effects of friction that it is easy for them to take friction for granted, or to ignore it altogether.
Whether pupils are skateboarding, cycling or travelling in a car or bus, they know that when the driving force is removed the motion slows down and eventually stops. So it is natural and common sense to assume that a driving force must be present to act on any moving object to keep it moving. In the real world of friction this idea is correct, but the crucial point is that the driving force is not the only force acting. The driving force is opposed by the retarding forces.
Thinking about the teaching
When thinking about objects moving at constant speed it is very helpful to consider how they got to that speed in the first place. It is important to separate the initial force which started the motion from any subsequent forces which might be present to maintain the motion.
To start something moving there is the need for a force, acting for a period of time, which is often called an impulse. For example, you provide an impulse to a boat as you push it away from the jetty. The impulse lasts for just as long as you are in contact with the boat.
Once moving, if there is no friction, then the object will continue to move at a constant speed unaided. Where friction is present there will be a need for a driving force to maintain the motion. The driving force balances the effect of friction. When this happens, there is no resultant force acting on the object, but this doesn't mean that there are no forces at all.