Forces Rewind:
Fill in the missing words:
What are forces?
A force can be a ________, pull or a__________. 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______ open a
drawer.
You cannot ______ a force but often you can see what it does. Forces can change the speed
of something, the ________ it is moving in or its shape. For example, an elastic band gets
longer if you pull it.
Measuring forces
The unit of force is called the ________, and it has the symbol N. So 100 N is a bigger
force than 5 N.
Weight, mass and gravity
People often confuse mass and weight. Remember that _________ is a force, and is
measured in newtons. Mass is measured in kilograms (kg).
Mass
The mass of an object is the amount of _______ or "stuff" it contains. The more matter
an object contains, the greater its _______. An elephant contains more matter than a
mouse, so it has a greater mass. Mass is measured in kilograms( ___ ) or grams, (g)
A 100 kg object has a greater mass than a 5 kg object. Remember an object's mass stays
the same wherever it is.
Gravity
All objects have a _______ 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.
Gravitational force increases when:

the masses are ______

the objects are closer
Weight
Weight is a force caused by gravity. The weight of an object is the gravitational force
between the object and the Earth. The more mass the object has the greater its
________will be.
Weight is a force, so it's measured in _________. On the surface of the Earth an object
with a mass of 1 kg has a weight of about 10 N.
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 _______ wherever it is, but its weight can change. This
happens if the object goes somewhere where gravity is stronger, or weaker, such as the
Moon.
The Moon has less mass than the Earth, so its ________ is less than the Earth's gravity.
This means that objects weigh less on the Moon than they do on the Earth.
Balanced forces
Force diagrams We can show the forces acting on an object using a _____ diagram. In a
force diagram, each force is shown as a force ________. An arrow shows:


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 in newtons.
Balanced forces When two forces acting on an object are ______ in size but act in
opposite directions, we say that they are balanced forces.
If the forces on an object are balanced (or if there are no forces acting on it) this is what
happens:


an object that is ______ moving stays still
an object that is moving continues to move at the ____ speed and
in the same direction
YOUR TURN :Complete the diagram below to show Balanced Force (The car is not
moving) Use arrows and label the Newtons
Unbalanced forces
When two forces acting on an object are _______ equal in size,
we say that they are unbalanced forces. If the forces on an object are unbalanced this is
what happens:

an object that is _____ moving starts to move

an object that is moving changes speed or direction
YOUR TURN :
Complete the diagram below to show Unbalanced Force. The car is
moving
Net forces
The size of the overall force acting on an object is called the ________ force. If the forces
are balanced, this is zero. In the example above, the resultant ________ is the difference
between the two forces, which is 100 - 60 = 40 N.
Friction
Whenever an object moves against another object, there is resistance, this is called
_______. These forces act in the opposite direction to the movement. Friction makes it
harder for things to move.
Helpful frictional forces
Friction can be useful:

friction between our shoes and the _______ stop us from slipping

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 ______ surfaces than on rough surfaces, which is why
we slide on ice.
Unhelpful frictional forces
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. The friction _____
them up.
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 ______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.