Download Force - Ms. Buicke maths and science

By Miss Buicke
What we must know from the syllabus
OP4 Appreciate the concept of force, recall
that the newton is the unit of force, and
describe forces and their effects.
OP5 Investigate examples of friction and the
effect of lubrication.
OP6 Investigate the relationship between the
extension of a spring and the applied force.
Can you think of a force?
Pushing
Pulling
lifting
Throwing
squeezing
What is happening in the picture? What force is being demonstrated?
What is a force?
A force is anything which causes an object to move
or change its velocity.
The word force is used quite often in everyday conversations.
Even though we use the word force regularly, it can be
difficult to explain its exact meaning. In physics, we are very
precise with the meaning of words and we must use the
proper definitions when answering questions.
A famous scientist, Isacc Newton, wrote a very
exact meaning of force:
• A force can cause a stationery object to move.
• A force can cause a moving object to move more
quickly, to move more slowly or to change the
direction in which it is moving.
Unit of force
The unit used to measure force is the newton and
the symbol is N.
Different types of force and their effects.
1.Force of gravity
2.Magnetic force
1.
4.
2.
3.
3. Electric force
4. Force of friction
1. The force of gravity
If a pen rolls over the edge of a table, it falls to the
ground. This is because the earth pulls objects towards
it. The pull of the earth is due to the force of gravity.
Gravity is pulling the apple to the ground
The story of Newton and the apple. The scientist and
physicist Sir Isaac Newton made one of the greatest
discoveries while sitting down under an apple tree when
an apple fell on his head. Besides giving him a headache,
it made Newton think about why the apple fell down
straight to earth, and later realised that it didn't exactly
“fall”, but rather it was drawn to the Earth's center. This is
because the earth pulls objects towards it. The pull of the
earth is due to the force of gravity.
2. Magnetic force
When a magnet is brought near a nail, you will notice that
the magnet will attract the nail. The force that moved the
nail is the magnetic force of the magnet.
If you bring two magnets near each other, they may attract
or repel. The force of gravity always attracts objects
towards the ground, while the magnetic force may attract
or repel.
3. Electric force
When you rub a plastic pen against the sleeve of your
jumper, you can lift small pieces of paper with the pen.
The pen has become electrically charged and the electric
force can attract the small pieces of paper.
4. Force of friction
How are brakes able to slow down a bicycle?
When you pull the brake handle the brake pad pushes
against the wheel of the bicycle. The friction between
the wheel and the brake pad causes the bicycle to move
more Slowly.
Definition of friction
Friction is a force which prevents easy movement
between two objects in contact. The rougher the two
surfaces in contact, the greater the friction.
All surfaces are rough even though they do not seem to be.
If you could see a close-up photograph of any surfaces,
it would appear rough and jagged. The rough surface is what
causes the force of friction.
How Does friction cause a matchstick to
light?
The red part of the match on the end
of the stick along with the red strip on the side of the box
have rough surfaces which, when rubbed together quickly,
create friction. The friction causes heat, which ignites the
tip of the match.
What happens as a block of wood is pushed along a table?
When a block of wood is placed on a table the rough surface
of the block is caught in the rough surface of the table.
When we push the block, some of your blushing force will
move the block and some of your pushing force will be
needed to overcome the friction between the table and the
block. The amount of your pushing force needed to overcome
the friction will depend on how rough the surface in contact
are.
Advantages of friction:
• Friction is needed to stop a bike. Friction between the
brake pad and the wheel is what stops the bike.
• When driving a car around a corner friction is needed.
If the tyres on a car are worn there is a risk that a lack of
friction could cause the car to skid. An accident while
turning a corner is more likely on a wet day because there
is less friction between the tyres and the road.
• Friction allows grip, so a person can walk, climb and run.
When we walk there is friction between our shoes and
the ground, which stops us from slipping.
If you step onto an oily patch on the ground, you will
most likely slip. The oil patch reduces the friction between
your shoes and the ground.
Disadvantages of friction
Friction can also be a nuisance.
•
Friction causes wearing of moving surfaces in
contact eg. wear and tear of tyres and footwear.
• Getting a blister. The blister is caused by skin
rubbing against the shoe.
• Heating effect
When a space shuttle returns to earth there is a lot of friction
between the surface of the shuttle and the gases in the
atmosphere. This could cause the shuttle to overheat,
with disastrous consequences.
• Friction wastes fuel
When a car is moving there is friction between the air
and the surface of the car. This friction is overcome by the
force of the engine.
However, this means that some of the fuel is needed
to overcome friction. Therefore wasting fuel.
Reducing the force of friction
• To reduce friction between two surfaces you could try to
make the surface smoother.
• There is no perfectly smooth surface so we often
Use special materials to reduce friction
• These materials are called lubricants.
Common examples are oil, liquid polish and soap.
Did you ever slip in the
bathroom over a bar of
soap??
There are many moving parts in a car engine.
Friction would lead to wearing. Friction would also generate
heat which would damage the engine.
Oil and grease are used as lubricants in car engines.
The bones at the joint in the human body are surrounded
by a fluid that acts as a lubricant. This greatly reduces friction.
Experiment: to investigate friction and the effects of lubrication
Equipment: Spring balance, four identical blocks of wood, sheet of
sandpaper,
piece of carpet, oil.
Method:
• place one block of wood on the table attached to a spring balance
• Pull the block along the table and record the force in neutons
• Take the second block and attach a piece of sandpaper.
• pull across the table and record the force in neutons
• Take the third piece of wood and attach the carpet to it
• Pull across the table and record the force
• And finally apply oil to the last piece of wood
• Pull across the table and record the force in neutons.
Result:
The pulling force was different in each case.
Conclusion:
The reading on the spring balance will be different for the
four blocks.
This shows that the force of friction depends on the type
of surfaces that are in contact
Gravity as a force: weight
It is important to understand the difference between the words
mass and weight.
In everyday usage, these words often have the same meaning.
In physics, they have Very different meanings.
weight
mass
The weight of an object is the
pull of the earth on it.
The mass of an object is the
amount of matter in it.
Weight is a force
Mass in measured in grams or
kilograms
Weight is measured in newtons
Mass does not have a direction
Weight has a direction, i.e a force The mass of an object remains
that acts towards the earth
the same
Weight gets smaller as you rise
upwards from ground level.
Calculations
There is a simple formula to convert mass to weight.
Weight (in newtons) = mass ( in kilograms ) x 10
Sample question 1
What is the weight of an object of mass 6.5kg?
Weight = mass x 10
Weight = 6.5kg x 10
Weight = 65 N
Sample question 2
Calculate the weight of an object of mass 200g
Remember to change the unit of mass to kilograms
Weight = mass x 10
Weight = .2kg x 10
Weight 2 N
Sample question 3
What is the mass of a person whose weight is 620 N ?
Weight = mass x 10
Weight/10 = mass
620/10 = mass
62 kg
The effect of a force on a spring
The scientist Robert Hooke discovered the relationship
Between the force applied to an elastic body (spring)
and the extension of the elastic body (spring) caused
by the force. This discovery is known as Hooke’s Law
Definition of Hooke’s law
The extension of an elastic body (a spring) is directly
proportional to the force causing the extension.
This means that if the force on the string doubles so will the
extension; if the force triples then the extension will also
triple, etc.
• When a spring is being extended as in the Hooke’s law experiment,
a problem may arise
• This problem happens if the force used to extend the spring is too
large.
• If the force is too big, the spring will be over-stretched and lose
its elasticity.
• Hooke’s Law will be no more valid. We say that the spring has
exceeded its elastic limit
Experiment:
To investigate the relationship between the extension of a
spring and the force that is stretching it.
1. Use a newton-meter to stretch a spring and note the
extension and the force used.
2. Repeat for lot of different forces.
3. Plot a graph of force used against extension.
Result: Your graph should be a straight line.
Conclusion: because the points gave a straight line through
the origin, it can be concluded that: the extension of an
elastic body is directly proportional to the force causing the
extension.
Graph showing extensions plotted against weight (force)