Every card in Forces and motion, Year 7: force diagrams and measuring forces
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- On a force diagram, what does the length of an arrow show?
The size of the force
HintThink about how you would draw a 10 N push next to a 5 N push.
WhyArrows 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
HintAn arrow has a head for a reason.
WhyA 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)
HintOne is from the Earth; the other is from the surface underneath.
WhyThe 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
HintWeight is the pull of gravity.
WhyWeight 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
HintAsk what would happen to a car that really had that arrow.
WhyAn 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
HintAsk whether it could act across a gap.
WhyFriction 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
HintIt acts on everything with mass, with no touching needed.
WhyThe 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
HintIt contains a spring and a scale.
WhyThe 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
HintChoose the scale on which 3 N moves the pointer a long way.
WhyOn 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
HintThink about what the rider has to push through at speed.
WhyAir resistance, or drag, pushes against anything moving through the air. A crouched rider meets less air, so less of the effort is wasted.
1★ KS3-PHYS-FOR-0027
2★ KS3-PHYS-FOR-0028
3★ KS3-PHYS-FOR-0029
4★ KS3-PHYS-FOR-0030
5★ KS3-PHYS-FOR-0031
6★ KS3-PHYS-FOR-0032
7★ KS3-PHYS-FOR-0033
8★ KS3-PHYS-FOR-0034
9★ KS3-PHYS-FOR-0035
10★ KS3-PHYS-FOR-0036
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