Moving and stopping Friction - bad and good Gravity

Moving always seems to involve forces. In this module, you will investigate the scientific phenomenon of motion, and how it affects you in everyday life.

Moving and stopping
All objects have the tendency to stay where they are, or to continue moving without changing motion unless there is an external force applied. This is Newton's first law of motion, put forward by (Sir) Isaac Newton in the late 1600s. You certainly know that things tend to stay put until a force is used to move them - just think of a broken down car on a flat stretch of road! The other part makes less sense. Why is it that if you are cycling on a flat stretch of road you have to pedal to keep to the same speed? In the mid 300s BC, Aristotle thought that an object only continued to move while a force was applied, and this made sense from his observations. Why does the bicycle slow down? It's all to do with friction.

Friction - bad and good
Friction affects objects moving on surfaces or through fluids (which can be liquid or gas) such as water and air. It's one of the "external forces" referred to by Newton. The bicycle slows because of friction between the tyres and the road, between gears, chains and sprockets and even the air pushing against the rider. All this means that more energy needs to be put in by the rider to apply a large enough force to overcome the combined forces of friction.

There is a positive effect of friction as well. Friction between the tyres and the road moves the bicycle forward - you know what happens when friction is reduced, by oil or ice on the road, you slip and slide! Friction is also necessary when you use the brakes to slow down. Friction is reduced by oiling bearings, reducing wind resistance by getting low over the handlebars, and even by the shape of the helmet.

Gravity
A major force involving motion is the gravitational force, the force tending to pull all objects towards the centre of the Earth. If you drop something, such as a pen, from your desk it falls to the floor then stops again. This brings you to Newton's second and third laws of motion (there are only three!) When the pen drops the existence of the gravitational force and Newton's second law tells us that the pen will accelerate towards the floor. The further the distance the pen falls, the greater its speed will be.

When the pen hits the floor it slows to a stop. It slows because the force of the floor pushing up is greater than the gravitational force on the pen. When the pen is stationary the force upwards from the floor is equal to the gravitational pull of the Earth on the pen.

Gravity applies to anything on Earth, not just falling objects. That's the reason that if you throw a ball straight up in the air, or at an angle, it always comes back down again.

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