The moment of a force and the principle of moments, how levers and gears multiply force, and pressure in fluids including upthrust and atmospheric pressure. Physics only content.
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1.What is the moment of a force?
The turning effect of a force about a pivot.
2.State the equation for a moment.
moment = force x perpendicular distance from the pivot to the line of action of the force
3.What is the unit of a moment?
The newton metre, Nm.
4.A force of 20 N acts 0.5 m from a pivot. Calculate the moment.
20 x 0.5 = 10 Nm.
5.Why must the distance be perpendicular?
Only the component of the force at right angles to the distance produces a turning effect.
6.State the principle of moments.
If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment.
7.A 4 N force acts 3 m left of a pivot. What force 2 m right balances it?
Anticlockwise moment is 12 Nm, so the force must be 12 divided by 2 = 6 N.
8.Why is a longer spanner easier to use?
The greater distance from the pivot gives a larger moment for the same force.
9.What is a lever?
A simple machine that uses a pivot to increase the distance from the pivot, so a smaller force produces the same moment.
10.What is meant by a force multiplier?
A device that allows a smaller applied force to produce a larger output force.
11.How do gears transmit rotational effects?
Interlocking toothed wheels transfer the turning effect from one to another.
12.What does a large gear driven by a small gear do?
It turns more slowly but with a greater moment.
13.What does a small gear driven by a large gear do?
It turns faster but with a smaller moment.
14.Which way does a driven gear rotate compared with the driver?
In the opposite direction.
15.State the equation for pressure.
pressure = force normal to a surface divided by the area of that surface
16.What is the unit of pressure?
The pascal, Pa, which is one newton per square metre.
17.A force of 200 N acts on 0.5 m2. Calculate the pressure.
200 divided by 0.5 = 400 Pa.
18.Why is a sharp knife more effective?
The small area of the blade means the same force produces a much greater pressure.
19.How does pressure in a liquid change with depth?
It increases with depth, because there is a greater weight of liquid above.
20.State the equation for pressure in a column of liquid.
pressure = height of column x density of liquid x gravitational field strength
21.What is upthrust?
The upward force on an object in a fluid, caused by the pressure being greater on its lower surface than its upper surface.
22.When does an object float?
When the upthrust equals its weight, which happens if its density is less than that of the fluid.
23.Why does atmospheric pressure decrease with height?
There is less air above, so fewer particles and a smaller weight of air pressing down.
24.Why do your ears pop on a plane?
The external air pressure changes quickly and the pressure inside your ears takes time to equalise.
25.Why do submarines need strong hulls?
Water pressure increases rapidly with depth, so the hull must withstand very large forces.
26.Why does a heavy object with a wide base not sink into soft ground?
The large area spreads the force out, so the pressure on the ground is low.
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