# Forces II Return of the forks.

## Presentation on theme: "Forces II Return of the forks."— Presentation transcript:

Forces II Return of the forks

Weight vs. Mass Mass is the amount of matter in us
900kg 900kg Mass is the amount of matter in us Same on Earth and Space 9000N ZeroN Weight is the pull of gravity on us Different on Earth and Space

Newton’s Cannon

Weight vs. Mass Earth’s Gravitational Field Strength is 10N/kg. In other words, a 1kg mass is pulled downwards by a force of 10N. W G M Weight = Mass x Gravitational Field Strength (in N) (in kg) (in N/kg) What is the weight on Earth of a book with mass 2kg? What is the weight on Earth of an apple with mass 100g? Dave weighs 700N. What is his mass? On the moon the gravitational field strength is 1.6N/kg. What will Dave weigh if he stands on the moon?

Balanced and unbalanced forces
Reaction Consider a camel standing on a road. What forces are acting on it? These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere. Weight

Balanced and unbalanced forces
Reaction What would happen if we took the road away? Weight

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

Balanced and unbalanced forces
1) This animal is either ________ or moving with _____ _____… 2) This animal is getting _________… 3) This animal is getting _______…. 4) This animal is…

Balanced and unbalanced forces
Fnet=200N F=-100N F=200N Fnet=100N F=-200N F=200N Fnet=0N F=-200N Fnet=-200N

Galileo’s Falling Balls

Gravity all bodies have gravity we feel it only from planet sized objects
T=0 v=0m/s Acceleration due to gravity is 10m/s2 That means every falling body gets 10m/s faster every second T=1s v=10m/s T=2s v=20m/s T=3s v=30m/s

All bodies fall at the same rate

Gravity is from the earth

More force means more Extension Force is proportional to extension
Hookes Law Force 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Extension More force means more Extension Force is proportional to extension

Hookes Law Calculation
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Force =24N Length=17cm Ext. =12cm Force=0N Length=5cm Ext.=0cm Force=6N Length=8cm Ext.=3cm Force=12N Length=11cm Ext.=6cm

Length/m 2.4 3.4 4.6 5.8 7 Weight/N 1 2 3 4

Stopping a car… Tiredness Too much alcohol Thinking distance
Too many drugs Poor visibility Wet roads Icy roads Stopping distance Tyres/brakes worn out Driving too fast

Work done Work done = Force x distance moved W D F
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 meters W D F

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

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 = Joules Power = Work Done / Time = Joules / 20 = Watts

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.

Pressure F A P Pressure depends on two things:
How much force is applied, and How big (or small) the area on which this force is applied . Pressure can be calculated using the equation: F A P Pressure (N/m2 or Pa) = Force (in N) Area (in m2) Pa is Pascals

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?

Balloon Pressure Expt Spread it out share the weight man

A circus elephant weighs 10,000N and can stand on one foot
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 = Pa

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/ m2 = Pa

Pressure – in Fluids Pressure increases with depth

As the frog goes deeper there is a greater weight of water above it.
Pressure and Depth As the frog goes deeper there is a greater weight of water above it.

Atmospheric Pressure 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!

The Barometer The weight of the air holds up the mercury.
Vacuum 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.

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.

Crushing the can

Boiling Point Increase pressure increase B.P.

Before altimeters they used BP to find altitude.
Used to find the source of the Nile.

H/W P310 Q 10-17

Center of Gravity Things stay standing (STABLE) because their Center of Gravity acts through their base.

Equilibrium Things fall over because the center of gravity is outside the base

Equilibrium There are 3 types of equilibrium STABLE NEUTRAL UNSTABLE

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?

of the body appears to act
Center of Gravity of a Lamina We must freely suspend the lamina from three places Then we trace the plumb line onto the paper. Center of Gravity Where all the mass of the body appears to act

Internet Demo

Levers A rigid body that rotates around a fulcrum
Used to make work easier

Moments =Force x Perpendicular distance
Fulcrum Perpendicular distance FORCE

Moments =Force x Perpendicular distance
= 10N x 5m = 50Nm Perpendicular distance=5m FORCE =10N

Moments =Force x Perpendicular distance
= 50N x 0.1m = 5Nm

More Than Two forces N N N N

http://www. absorblearning. com/media/attachment. action

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

Weight Moments Pressure Atmospheric C of G Hooke’s Law

H/W p