forces ii return of the forks weight vs. mass mass is the amount of matter in us same on earth and...
TRANSCRIPT
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Forces II
Return of the forks
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Weight vs. Mass
• Mass is the amount of matter in us• Same on Earth and Space
• Weight is the pull of gravity on us• Different on Earth and Space
900kg
900kg
9000N ZeroN
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Newton’s
Cannon
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Weight vs. MassEarth’s Gravitational Field Strength is 10N/kg. In other
words, a 1kg mass is pulled downwards by a force of 10N.
W
GM
Weight = Mass x Gravitational Field Strength
(in N) (in kg) (in N/kg)
1) What is the weight on Earth of a book with mass 2kg?
2) What is the weight on Earth of an apple with mass 100g?
3) Dave weighs 700N. What is his mass?
4) On the moon the gravitational field strength is 1.6N/kg. What will Dave weigh if he stands on the moon?
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Balanced and unbalanced forces
Consider a camel standing on a road. What forces are acting on it?
Weight
Reaction
These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere.
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Balanced and unbalanced forces
What would happen if we took the road away?
Weight
Reaction
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Balanced and unbalanced forces
What would happen if we took the road away?
The camel’s weight is no longer balanced by anything, so the camel falls downwards…
Weight
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Balanced and unbalanced forces
1) This animal is either ________ or moving with _____ _____…
4) This animal is…
2) This animal is getting _________…
3) This animal is getting _______….
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Balanced and unbalanced forces
Fnet=200N
F=200NFnet=100N
Fnet=0N
Fnet=-200N
F=-200N
F=-100N
F=-200N F=200N
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Galileo’s Falling Balls
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Gravity all bodies have gravity we feel it only
from planet sized objects
• Acceleration due to gravity is 10m/s2
• That means every falling body gets 10m/s faster every second
T=0 v=0m/sT=1s
v=10m/s
T=2s v=20m/s
T=3s v=30m/s
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All bodies fall at the same rate
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Gravity is from the earth
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Hookes Law
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Force
Extension
More force means more Extension Force is
proportional to extension
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Hookes Law Calculation123456789
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Force=0N
Length=5cm
Ext.=0cm
Force=6N
Length=8cm
Ext.=3cm
Force=12N
Length=11cm
Ext.=6cm
Force
=24N
Length=17cm
Ext.
=12cm
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Length/m 2.4 3.4 4.6 5.8 7
Weight/N 0 1 2 3 4
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Stopping a car…
Stopping distance
Too much alcohol
Thinking distance
Tiredness
Too many drugs
Wet roads
Driving too fast
Tyres/brakes worn out
Icy roads
Poor visibility
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Work done
When any object is moved around work will need to be done on it to get it to move (obviously).
We can work out the amount of work done in moving an object using the formula:
Work done = Force x distance moved
in Joules in Newtons in metersW
DF
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Work Done Lifting
• Lift a mass of 80kg a height of 6m. How much work is done?
• Work done = Force x distance
moved• Work done = 800N x 6m• Work done = 4800 Joules
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Power• The rate at which work is done• POWER = Work Done
time taken
Example A microlite takes 20s to climb to 100m. If the microlite has mass 2000kg find the work done and the power?
Work Done = Force x Distance = 2000x10 x 100=2000000 Joules
Power = Work Done / Time = 2000000 Joules / 20
= 100000 Watts
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Question
• On a school trip a pupil of mass 50kg climbs the Eiffel Tower. If the tower is 320m high and he takes 50mins (3000s) find his work done and power.
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Pressure
Pressure depends on two things:
1) How much force is applied, and
2) How big (or small) the area on which this force is applied .
Pressure can be calculated using the equation:
Pressure (N/m2 or Pa) = Force (in N)
Area (in m2)
F
APPa is Pascals
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Calculating Pressure
• To use the triangle we have to cover the letter we do not know
• F=PxA• Find the force that
we need to have for a pressure of 10Pa over an area of 2m2?
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Balloon Pressure Expt
• Spread it out share the weight man
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1)A circus elephant weighs 10,000N and can stand on one foot. This foot has an area of 0.5m2. How much pressure does he exert on the floor (in Pa)?
Pressure=Force/area
= 10000N/ 0. 5m2 = 20000 Pa
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• A 50kg woman copies the elephant by standing on the heel of one of her high-heeled shoes. This heel has an area of 1cm2. How much pressure does she exert on the floor?
• Pressure=Force/area • = 500N/ 0.0001m2 = 5000000 Pa
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Pressure – in Fluids
Pressure increases with depth
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Pressure and Depth
As the frog goes deeper
there is a greater weight of water above
it.
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Atmospheric Pressure
• The earth is covered with layer of Gas.
• We are at the bottom of a gas ocean 200km deep.
• The effect of this huge column of gas is 1 Tonne of weight on our shoulders.
• This is called• ATMOSPHERIC
PRESSURE
Heavy!
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The Barometer
The weight of the air holds up the mercury.
If we use water the column is 10.4m high.
1 Atmosphere is 760mm of Hg.
Vacuum
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The Altimeter
• Air is also a fluid• As we go higher
there is less air above us.
• There is less Atmospheric pressure
• We can measure the altitude using a barometer with a different scale.
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Crushing the can
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Boiling Point
• Increase pressure increase B.P.
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• Before altimeters they used BP to find altitude.
• Used to find the source of the Nile.
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H/W
• P310• Q 10-17
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Center of Gravity
• Things stay standing (STABLE) because their Center of Gravity acts through their base.
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Equilibrium
• Things fall over because the center of gravity is outside the base
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Equilibrium
• There are 3 types of equilibrium
STABLENEUTRAL UNSTABLE
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Center of Gravity
• The place where all the mass of the body appears to act.
• How do we find the center of gravity of this flat piece of card?
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Center of Gravity of a LaminaWe must freely suspend the lamina from three
placesThen we trace the plumb line onto the paper.
Center of Gravity
Where all the mass
of the body appears to act
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Levers
A rigid body that rotates around a fulcrum
Used to make work easier
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Moments=Force x Perpendicular
distance
FORCE
Fulcrum
Perpendicular distance
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Moments=Force x Perpendicular
distance
FORCE =10N
Perpendicular distance=5m
= 10N x 5m = 50Nm
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Moments=Force x Perpendicular
distance= 50N x 0.1m = 5Nm
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More Than Two forces
NN NN
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http://www.absorblearning.com/media/attachment.action?quick=63&att=430
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Find the following moments
Moments = F x d
=0.4 x 4 = 1.6Nm
Moments = F x d
=0.25x 4 = 1 Nm
Clockwise Moments = Anticlockwise
F x 0.25 = 0.5 x 5
F = 2.5/0.25 = 10N
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Weight
Moments
Pressure
AtmosphericPressure
C of G
Hooke’s Law
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H/Wp3001-10