Transcript No Slide Title

P3 1 Summary
Turning forces
AQA Science © Nelson Thornes Ltd 2006
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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
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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.
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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.
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P3 1 Summary
Higher
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.
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P3 1 Summary
Higher
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.
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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.
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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 between
two objects:
– is an attractive force,
– is bigger the greater the
mass of each object is,
– is smaller the greater the
distance between the two
Digital Vision 7 (NT)
objects is.
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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
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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.
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