AQA Science © Nelson Thornes Ltd 2006  1

P3 1 Summary

Turning forces

AQA Science © Nelson Thornes Ltd 2006  2

P3 1 Summary

What is a moment?• A moment is the turning effect of a force.• Moment = force perpendicular

distance from the pivot to the line of action of the force = F d

• F is the force in newtons.• d is the perpendicular distance from the pivot

in metres.• The unit of a moment is newton metres (Nm).

AQA Science © Nelson Thornes Ltd 2006  3

P3 1 Summary

Using levers• Calculating moments is

important when you use levers.• The weight is called the load.• The force you apply to the

crowbar is the effort.• The point about which the

crowbar turns is the pivot.• Levers enable you to lift heavy

loads with little effort.

AQA Science © Nelson Thornes Ltd 2006  4

P3 1 Summary

What is the centre of mass of an object?

• The centre of mass of an object is the point where its mass may be thought to be concentrated.

• When a suspended object is in equilibrium, its centre of mass is directly beneath the point of suspension.

• The centre of mass of a symmetrical object is along the axis of symmetry.

AQA Science © Nelson Thornes Ltd 2006  5

P3 1 Summary

What can you say about the moments of the forces acting on an object in

equilibrium (it isn’t turning)?

The sum of the anticlockwise moments about any point

=the sum of the clockwise moments

about that same point.

Higher

AQA Science © Nelson Thornes Ltd 2006  6

P3 1 Summary

Tilt or topple?

• An object will tend to topple over if the line of action of its weight is outside its base so …

• … bodies with a low centre of mass and a broad base are more stable than bodies with a high centre of mass and a narrow base.

• You can increase the stability of an object by making its base wider and its centre of mass as low as possible.

Higher

AQA Science © Nelson Thornes Ltd 2006  7

P3 1 Summary

How can a body moving at a steady speed be accelerating?

• When it’s moving in a circle at constant speed!

• The object accelerates continuously towards the centre of the circle.

• The centripetal force on it increases as:– the mass of the object increases,– the speed of the object increases,– the radius of the circle decreases.

AQA Science © Nelson Thornes Ltd 2006  8

P3 1 Summary

What is the force of gravity?

• The Earth exerts a force of gravity on all of us – and we exert one on the Earth!

• The force of gravity betweentwo objects:

– is an attractive force, – is bigger the greater the

mass of each object is, – is smaller the greater the

distance between the two objects is.

Digital Vision 7 (NT)

AQA Science © Nelson Thornes Ltd 2006  9

P3 1 Summary

What keeps the Earth orbiting the Sun?

• The force of gravity provides the centripetal force needed to keep one smaller object in orbit around another larger object (e.g. the Earth around the Sun).

• To stay in orbit at a particular distance, a small body must move at a particular speed around a larger body.

• The larger the orbit is, the longer the orbiting body takes to go round it.

AQA Science © Nelson Thornes Ltd 2006  10

P3 1 Summary

What is the period of the orbit of a satellite?

• The period of a satellite is the time it takes to make one complete orbit.

• The Moon is the Earth’s only natural satellite.• A satellite in geostationary orbit (e.g. a

communications satellite) has a period of 24 hours and stays at the same position above the equator.

• Monitoring satellites in low polar orbits have a period of 2–3 hours.