Forces and motion, Year 7: force diagrams and measuring forces：全部卡片
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- On a force diagram, what does the length of an arrow show?
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.
- On a force diagram, what does the direction of an arrow show?
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.
- A book rests on a table. Which two forces should be drawn on it?
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.
- An apple is falling. In which direction does the weight arrow point?
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 pupil draws a parked car with a large forward arrow and nothing else. What is wrong with the diagram?
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.
- Is friction a contact force or a non-contact force?
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.
- Which non-contact force pulls a dropped ball towards the ground?
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.
- Which instrument measures force?
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.
- You need to measure a force of about 3 N. Which newtonmeter is better: one reading 0–5 N or one reading 0–100 N?
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.
- Why do racing cyclists crouch low over the handlebars?
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.
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