Forces and motion, Year 7: force diagrams and measuring forces

The size of the force

提示Think about how you would draw a 10 N push next to a 5 N push.

为什么Arrows are drawn to scale: an arrow twice as long stands for twice as many newtons.

The direction the force acts in

提示An arrow has a head for a reason.

为什么A force has a size and a direction, and an arrow shows both at once. Every arrow should also be labelled with the name of its force.

Weight downwards and the table's upward push (the normal or reaction force)

提示One is from the Earth; the other is from the surface underneath.

为什么The Earth pulls the book down and the table pushes up on it. The book stays still, so the two arrows are drawn equal in length.

Straight down

提示Weight is the pull of gravity.

为什么Weight is the pull of gravity, and it acts vertically downwards whichever way the object happens to be moving.

A parked car has no forward force

提示Ask what would happen to a car that really had that arrow.

为什么An unbalanced forward force would make the car speed up. The correct diagram shows weight downwards and the road's push upwards, equal in size.

A contact force

提示Ask whether it could act across a gap.

为什么Friction comes from surfaces rubbing against each other. Take the surfaces apart and it vanishes.

Gravity

提示It acts on everything with mass, with no touching needed.

为什么The Earth pulls on the ball across the gap between them. Magnetism and static electricity are the other everyday forces that act at a distance.

A newtonmeter

提示It contains a spring and a scale.

为什么The bigger the force, the more the spring inside stretches, and the stretch is read off a scale. It is also called a force meter.

The 0–5 N one

提示Choose the scale on which 3 N moves the pointer a long way.

为什么On the 0–100 N meter, 3 N would hardly move the pointer and could not be read accurately. A meter should be chosen so that the reading falls well up its scale.

To reduce air resistance

提示Think about what the rider has to push through at speed.

为什么Air resistance, or drag, pushes against anything moving through the air. A crouched rider meets less air, so less of the effort is wasted.