Forces II

Return of the forks

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

Newton’s

Cannon

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?

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.

Balanced and unbalanced forces

What would happen if we took the road away?

Weight

Reaction

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 _____ _____…

4) This animal is…

2) This animal is getting _________…

3) This animal is getting _______….

Balanced and unbalanced forces

Fnet=200N

F=200NFnet=100N

Fnet=0N

Fnet=-200N

F=-200N

F=-100N

F=-200N F=200N

Galileo’s Falling Balls

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

All bodies fall at the same rate

Gravity is from the earth

Hookes Law

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Force

Extension

More force means more Extension Force is

proportional to extension

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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

Length/m 2.4 3.4 4.6 5.8 7

Weight/N 0 1 2 3 4

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

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

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 100=2000000 Joules

Power = Work Done / Time = 2000000 Joules / 20

= 100000 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

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

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

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

• 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

Pressure – in Fluids

Pressure increases with depth

Pressure and Depth

As the frog goes deeper

there is a greater weight of water above

it.

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.

If we use water the column is 10.4m high.

1 Atmosphere is 760mm of Hg.

Vacuum

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

STABLENEUTRAL 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?

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

Internet Demo

Levers

A rigid body that rotates around a fulcrum

Used to make work easier

Moments=Force x Perpendicular

distance

FORCE

Fulcrum

Perpendicular distance

Moments=Force x Perpendicular

distance

FORCE =10N

Perpendicular distance=5m

= 10N x 5m = 50Nm

Moments=Force x Perpendicular

distance= 50N x 0.1m = 5Nm

More Than Two forces

NN NN

http://www.absorblearning.com/media/attachment.action?quick=63&att=430

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

AtmosphericPressure

C of G

Hooke’s Law

H/Wp3001-10