Unit 1: Forces

•Free body diagrams•Newton’s laws•Weight and mass•Kinetic energy•Gravitational Potential energy•Conservation of energy•Work done•Power and efficiency

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.

Newton’s First Law

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

What would happen if we took the road away?

The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces

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

Force and accelerationIf the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers:

Force (in N) = Mass (in kg) x Acceleration (in m/s2)

F

AM

Newton’s 2nd Law

Force, mass and acceleration1) A force of 1000N is applied to

push a mass of 500kg. How quickly does it accelerate?

2) A force of 3000N acts on a car to make it accelerate by 1.5m/s2. How heavy is the car?

3) A car accelerates at a rate of 5m/s2. If it weighs 500kg how much driving force is the engine applying?

4) A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by?

F

AM

Terminal VelocityConsider a skydiver:

1) At the start of his jump the air resistance is _______ so he _______ downwards.

2) As his speed increases his air resistance will _______

3) Eventually the air resistance will be big enough to _______ the skydiver’s weight. At this point the forces are balanced so his speed becomes ________ - this is called TERMINAL VELOCITY

Terminal VelocityConsider a skydiver:

4) When he opens his parachute the air resistance suddenly ________, causing him to start _____ ____.

5) Because he is slowing down his air resistance will _______ again until it balances his _________. The skydiver has now reached a new, lower ________ _______.

Velocity-time graph for terminal velocity…Velocit

y

Time

Speed increases…

Terminal velocity reached…

Parachute opens – diver slows down

New, lower terminal velocity reached

Diver hits the ground

On the Moon

The force on object A = Fba

The force on object B= Fab

Fba Fab

AB

Newton’s 3rd Law

Fba = Fab

Because both skaters are free to move the push on one is the same as the push on the other so the forces on each of them are balanced so they push each other away.

But………….

If we replace one skater with a wall

Fba

B

Fab

A

Friction force

But hold on this is making my head hurt again…………

Fab is the force on the skater

This pushes her back as there’s no friction

Fba and friction force act on the wall in opposite directionsThe friction force balances the push of the skater so wall doesn’t move

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?

Friction1) What is friction?

2) Give 3 examples where it is annoying:

3) Give 3 examples where it is useful:

4) What effect does friction have on the surfaces?

Work doneWhen 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 J in N in m

W

DF

Kinetic energyAny object that moves will have kinetic energy.

The amount of kinetic energy an object has can be found using the formula:

Kinetic energy = ½ x mass x velocity squared

in J in kg in m/s

KE = ½ mv2

Some example questions…

1) A 70kg boy is running at about 10m/s. What is his kinetic energy?

2) A braking force of 1000N is applied by a driver to stop his car. The car covered 50m before it stopped. How much work did the brakes do?

3) What is the kinetic energy of a 100g tennis ball being thrown at a speed of 5m/s?

4) A crane is lifting a 50kg load up into the air with a constant speed. If the load is raised by 200m how much work has the crane done? (The answer isn’t 10,000J)

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Gravitational Potential EnergyTo work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would use the simple equation…

GPE = Weight x Change in height

(Joules) (newtons) (metres)

GPE

HW

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Some example questions…

How much gravitational potential energy have the following objects gained?:

1. A brick that weighs 10N lifted to the top of a house (10m),

2. A 10,000N car lifted by a ramp up to a height of 2m,

3. A 700N person lifted up 50m by a ski lift.

How much GPE have the following objects lost?:

1. A 2N football dropping out of the air after being kicked up 30m,

2. A 0.5N egg falling 10m out of a bird nest,

3. A 10,000N car falling off its 2m ramp.

4. A 60kg student when bungie-jumping off a 110-metre high bridge

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Efficiency

Efficiency is a measure of how much USEFUL energy you get out of an object from the energy you put INTO it.

Efficiency = Useful energy given out by the device

Energy put into it

e.g. if 2000 joules of electrical energy are put into a kettle and 500 joules of heat energy are gained from it, its efficiency is 500/2000 x 100% = 25%

x100%

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Some examples of efficiency…

1) 5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it?

2) A laptop can convert 400J of electrical energy into 240J of light and sound. What is its efficiency? Where does the rest of the energy go?

3) A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it?

Power

Power is the rate of energy transfer

Power = Energy transferred ÷ time to transfer

Remember, energy transferred = work done = Force x distance moved

So Power = force x (distance ÷ time) = force x velocity

Efficiency practicalTotal input power = Force x velocityThis equals useful power PLUSWork done against friction

UsefulPower = force x velocity

Efficiency = (useful power ÷ total input power) x 100%