Transcript Friction, Work and Power - Loreto Science

Force, Work and Power
Forces
• A force is anything which causes an object
to change its velocity
• Forces are measured in Newtons
• Can you think of some forces You could
use to move a ball that is not moving?
Examples of Forces
1. Push – Changes the velocity of an object
2. Pull – Changes the velocity of an object
3. Weight – Moves objects towards the
ground
4. Friction – Slows down moving objects
5. Electric – Causes charged particles to
move
6. Magnetic – Causes magnetic materials to
move
Measuring Forces
• To measure force we
use a newton meter
also called a spring
balance
• Can you find the
force of your pencil
case?
• 1kg has a force of
10N
Force Quantities in every day life
A Newton is a very small unit
•a.
1N to lift an Apple
•b. 12N to lift your textbook
•c.
20N to open a can of Coke
•d.
50N to squash an egg
•e. 100N or more to push a
supermarket trolley
•f. Car brakes can exert a force of
about 5000N on wheels to stop the
car.
5
Forces always occur in pairs
• For every force there is an equal and
opposite force
Friction
• Friction is the force that prevents easy
movement between two objects in contact
• High friction : Sand paper, Car Tyres and
rough soled shoes
• Low Friction: wet soap, ice and glass
What would you use to reduce
friction in the chain of your bike?
• If you said oil you are correct!
• Oil is an example of a LUBRICANT
• A LUBRICANT is anything that reduces
friction
• Can you think of any others?
To investigate friction
• Using a force sensor, a can of coke, some
carpet and some oil how can we show the
effect of friction and lubricants?
What happens if you skid across a
carpet very fast?
• You might notice that you can get a
burning feeling this is because friction
causes heat
• Name 4 advantages and 4 disadvantages
of friction
• Why do you think swimmers, skiers and
cyclists all shave their legs?
Work
• Work is done when a force moves an
object
• We can find the amount of work done by
multiplying the force used by the distance
moved
Work (j) = Force (N) x Distance (m)
Work and energy are measured in joules
Example
• This man uses a force of 300N to push the
lawn mower 20m how much works does he
do?
• Answer: Work (j) = Force (N) x Distance (m)
Work (j) = 300N x 20m =6000 joules
Questions
• Who does more work a woman carrying a
case with a force of 30N for a distance of
100m or a woman carrying a case with a
force of 40N for a distance of 75m?
Power
• Which of these machines would you prefer
to use to cool you down on a hot day?
• If you said the first one it is probably
because you know it would cool you down
more because it is more powerful than the
handheld fan
• Power is the rate at which work is done
• The more powerful an object the faster it
can do work
• Power is measured in Watts where 1 watt =1j/s
• If something has a power of 20watts it can do
20j of work in 1 second
• Why do you think a 100w bulb is brighter than a
60w bulb?
• We can work out power by dividing work done
by the time taken to do the work
Power (w) = Work done (j)
Time (s)
Questions
• A crane lifts a slab of concrete that weighs
60000 N from the ground up to a platform
20m high in 30 s
• (a) How much work does the crane do?
• (b) What is the power of the crane?
• A girl whose weight is 500N runs up a
flight of stairs 6m high in 20s what is her
power?
• A body builder lifts a weight of 6000N 1m
above his head in 1s what is his power?
Remember
• To change centimeters to meters divide by
100
• Eg. 10cm = 0.1m
• To change grams to kilograms divide by
1000
• Eg. 200g = 0.2kg
• To change kg to Newtons multiply by 10
• Eg. 60kg = 600N
Hooke’s Law
• Hooke’s Law states that the extension of a
spring is proportional to the force applied
to it
• This means that for example if you add
50g to a spring it will stretch a certain
amount and if you add another 50g it will
stretch by the same amount
• We can show this by
experiment.
• In Hooke’s law you
should always get a
straight line graph
• A straight line graph
tells you that the two
things on the graph
are proportional
• As soon as the line
goes straight you
know the spring has
become
overstretched
• This should not
happen in your
experiment
Drawing Graphs
• A graph should always have the following
5 point checklist to get full marks
1.A title saying what the graph shows
2.A labelled vertical axis and a labelled
horizontal axis
3.Units written next to the label
4.Correct points on the axes
5.Points with lines joining them