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Forces and Motion
This Revision Bite is about forces and motion. It has these parts in it:
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What is a force?
Mass and gravity
Weight
Pressure
Balanced forces
Unbalanced forces
Speed
Frictional forces
Moments
What is a force?
A force can be a push or a pull. For example, when you push open a door you
have to apply a force to the door. You also have to apply a force to pull open
a drawer.
You cannot see a force but often you can see what it does. Forces can
change the speed of something, the direction it is moving in or its shape. For
example, an elastic band gets longer if you pull it.
Forces can be measured using a force meter. Force meters contain a spring
connected to a metal hook. The spring stretches when a force is applied to
the hook. The bigger the force applied, the longer the spring stretches and the
bigger the reading. The unit of force is called the newton, and it has the
symbol N. So, 100 N is a bigger force than 5 N.
A force meter is used to measure forces
Mass and gravity
Mass
The mass of an object is the amount of matter or "stuff" it contains. The more
matter an object contains, the greater its mass. An elephant contains more
matter than a mouse, so it has a greater mass. Mass is measured in
kilograms or kg, but it is often easier to measure mass in grams or g. A 100 kg
object has a greater mass than a 5 kg object. An object's mass stays the
same wherever it is.
Gravity
All objects have a force that attracts them towards each other. This is called
gravity. Even you attract other objects to you because of gravity. But you have
too little mass for the force to be very strong. Gravity only becomes noticeable
when there is a really massive object like a moon, planet or star.
Gravitational force increases when:
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the masses of the objects involved increases
the distance between the objects decreases
The Earth has more mass than the Moon, so the gravitational force is greater
on the Earth than it is on the Moon. The gravitational force is greater on the
Earth's surface than it is in space, high above the Earth. The Earth's
gravitational force pulls objects towards the centre of the Earth.
"Down" is towards the centre of the Earth, wherever you are on the planet
Weight
The weight of an object is the result of the gravitational force between the
object and the Earth. The greater the mass of an object, the greater its weight.
An elephant has more mass than a mouse, so it has a greater weight. Weight
is measured in newtons or N. Notice that the unit of weight is the same as
the unit of force (see page 1 of this Revision Bite).
On Earth, an object with a mass of 1 kg has a weight of 10 N. To convert from
kilograms to newtons, we just times by ten.
An object's weight can change if it goes into space or to another planet. This
is because gravity may be weaker or stronger there than it is on the Earth.
Mass and weight
Remember that mass is measured in kilograms, kg, and weight is measured
in newtons, N. The mass of an object stays the same wherever it is, but the
weight of the same object can change. This happens if the object goes
somewhere where gravity is stronger or weaker, such as into space.
The Moon has less mass than the Earth, so its gravity is less than the Earth's
gravity. This means that objects weigh less on the Moon than they do on the
Earth. For example, a 120 kg astronaut weighs 1200 N on Earth (remember to
times kg by 10 to get N). The same astronaut would weigh just 200 N on the
Moon, because the Moon's gravity is one sixth the Earth's gravity. Note that
the astronaut's mass stays the same.
The weight of an object changes if the strength of gravity changes
Pressure
You may get told off if you swing around on one leg of a chair instead of
sitting properly. Apart from the risk that you will damage the chair or hurt
yourself, the chair leg can damage the floor. This is because it puts too much
pressure on the floor.
Working out pressure
To work out pressure, we need to know two things:
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the force or weight applied
the area over which the force or weight works.
This is the equation for working out pressure:
Imagine that a force of 20 N acted over an area of 2 m 2 (two square metres).
This is how we would work out the pressure:
Notice that the unit of pressure here is N/m2 (newton per square metre), but
another unit may also be used. This is called the pascal, Pa. 1 Pa = 1 N/m 2,
so in the example above the pressure is 10 Pa.
Using pressure
Drawing pins have a large round end for you to push the sharp end into a
notice board. Trying to use the sharp end to push the round end into the
board is not a good idea. The round end applies a low pressure to your
thumb, but the sharp end applies a high pressure to the notice board, so it
pushes in.
Drawing pins make good use of pressure
If you swing round on one leg of a chair, you put four times as much pressure
on the floor as you do if you sit properly. This is because your weight stays
the same, but four chair legs spread this over four times more area than one
chair leg can.
Balanced forces
Force diagrams
Forces always act in pairs, with one force acting in the opposite direction to
the other force. We can show the forces acting on an object using a force
diagram. In a force diagram, each force is shown as a force arrow. An arrow
shows:
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the size of the force (the longer the arrow, the bigger the force)
the direction in which the force acts.
The arrow is usually labelled with the name of the force and its size. Text
books often show a force with a thick coloured arrow, but it is best if you just
use a pencil and ruler to draw an arrow with a single line.
The forces on this hanging crate are equal in size but act in opposite
directions
Balanced forces
When two forces acting on an object are equal in size but act in opposite
directions, we say that they are balanced forces. If the forces on an object are
balanced (or there are no forces acting on it):
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the object stays still, if it is not already moving
the object continues to move at the same speed and in the same
direction, if it is already moving.
Floating in water and standing on the ground
Objects float in water when their weight is balanced by the upthrust from the
water. The object will sink until the weight of the water it pushes out of the
way is the same as the weight of the object.
A boat floats because its weight is balanced by the upthrust from the water
When an object rests on a surface such as the ground, its weight is balanced
by the reaction force from the ground. The ground pushes up against the
object.
The weight of a book lying on a table is balanced by the reaction force from
the table top
Unbalanced forces
When two forces acting on an object are not equal in size, we say that they
are unbalanced forces. If the forces on an object are unbalanced:
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the object starts to move, if it is not already moving
the object changes speed or direction, if it is already moving.
Unbalanced forces on a truck make it begin to move
The size of the overall force acting on an object is called the resultant force.
If the forces are balanced, this is zero. In the example above, the resultant
force in the bottom diagram is the difference between the two forces, which is
100 - 60 = 40 N.
Speed
To work out the speed of an object, we need to know two things:
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the distance it has travelled
the time taken to travel that distance.
There are two common situations:
Situation
Fastest object
Two objects travel the same
distance in different times
The one that takes the
shortest time
Two objects travel different
distances in the same time
The one that travels the
longest distance
This is the equation for working out speed:
Imagine that a car travelled 100 m in 5 s. This is how we would work out its
speed:
Notice that the unit of speed is m/s (metre per second).
You should really work in metres and seconds, but you might come across
other units, such as km/h (kilometre per hour).
Frictional forces
Whenever an object moves against another object, it is likely to experience
frictional forces. These are forces that act in the direction opposite to the
direction of movement.
Friction can be useful. Friction between our shoes and the floor stop us from
slipping, and friction between tyres and the road stop cars from skidding.
Friction between the brakes and wheel help bikes and cars slow down.
Frictional forces are much smaller on smooth surfaces than on rough
surfaces, which is why we slide on ice.
Friction can also be unhelpful. If you don't lubricate your bike regularly with oil,
the friction in the chain and axles increases. Your bike will be noisy and
difficult to pedal. When there is a lot of friction between moving parts, energy
is lost to the surroundings as heat. Think of what happens when you rub your
hands together quickly.
Air resistance
Bikes, cars and other vehicles experience air resistance as they move. Air
resistance is caused by the frictional forces of the air against the vehicle. The
faster the vehicle moves, the bigger the air resistance becomes. The top
speed of a vehicle is reached when the force from the cyclist or engine is
balanced by air resistance.
Streamlining
Racing cyclists crouch down low on their bikes to reduce the air resistance on
them. This helps them to cycle faster. They also wear streamlined helmets.
These have special, smooth shapes that allow the air to flow over the cyclist
more easily. Modern cars are also streamlined. Their smooth shapes make
the air resistance smaller, which increases their top speed and allows them to
travel further on the same amount of fuel.
Moments
Forces can make objects turn if there is a pivot. Think of a playground seesaw. When no-one is on it the see-saw is level, but it tips up if someone gets
onto one end. It is possible to balance the see-saw again if someone else
gets onto the other end and sits in the correct place. This is because of
moments.
Working out moments
To work out a moment, we need to know two things:
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the force or weight applied
the distance from the pivot that the force or weight is applied.
This is the equation for working out a moment:
moment = force x distance
Imagine that a force of 10 N acted on a see-saw 2 m from the pivot. This is
how we would work out the moment:
moment = force x distance = 10 x 20 = 200 Nm
Notice that the unit of moment is Nm (newton metre). Don't get confused with
a 'newton meter', which is another name for a forcemeter.
Using moments
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A see-saw will balance if the moments on each side of the pivot are
equal. This is why you might have to adjust your position on a see-saw
if you are a different weight from the person on the other end.
Moments on a balanced see-saw
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Using the same principle, through changing the distance of an object
from a pivot, you change the moment of a tool such as a lever or a
crowbar, and this can help you move heavy objects more easily.
If a nut on a car wheel is difficult to undo with a short spanner, a longer
spanner will help. This is because there will be a bigger moment on the
nut, even if the same force as before is applied.