Download Gravitational Potential Energy

Chapter 14 – More about Forces:
Friction
Friction is a very common force, whenever one object slides over another,
friction tries to stop it. Friction always opposes the movement of an object. It is
often a nuisance because it converts kinetic energy into heat and wastes it.
Reducing friction
- The slide in the park is polished smooth so you can slide down easily but
even highly polished objects look rough through a microscope so there is
always some fiction.
- You can also reduce friction by lubricating the surfaces with oil which
separates the surfaces so that they do not catch each other.
- A hovercraft works by separating the two surfaces by air.
- Another way of reducing friction is to have the object rolling instead of
sliding which happens with ball bearings.
- Cars, planes and rockets are streamlined to reduce friction with air.
Advantages of friction
Friction allows you to pick up a book. Our lives depend on the friction at the
brakes and tyres of cars and bicycles. Air friction slows down a parachute. We
are able to walk because of friction.
Other forces
Frictional forces only act when two objects are in contact. Other forces can act
at a distance when objects are not touching each other. Magnetic & Electric
forces can act at a distance.
Chapter 15 – Turning Forces:
You can feel that the turning effect on your hand depends on the size of the
force (the weight) and the distance from your hand. The turning effect of a
force is called a moment or a torque. The distance used is always the shortest
distance. Moments are measured in newton-meters.
Moment of a Force = force (N) x perpendicular distance from the force to the
pivot (M)
When a see-saw is balanced (in equilibrium), the clockwise and anti-clockwise
must be equal. This is called the principle of moments.
In equilibrium: total anti-clockwise moment = total clockwise moment
Centre of mass (centre of gravity)
The weight of e.g. a ladder seems to act at its centre. This point of the ladder,
where it balances, is called the centre of gravity or centre of mass. The centre
of gravity is the point through which the whole weight of the object seems to
act.
Stability
When something is in a stable equilibrium, it returns to its original position
when you’ve pushed it. This is because when you push it to one side its centre
of gravity tries to pull it back to its lowest position.
If you try to balance a ruler on your finger it is in an unstable equilibrium
because if it moves slightly, the centre of gravity falls and keeps it falling down.
A billiard ball on a perfectly level table is in neutral equilibrium, because if it is
moved, its centre of gravity does not rise or fall.
Chapter 16 – Work, Energy & Power
If someone pushes a car, he is doing work. He is only doing work if there is
movement against an opposing force. He can only work if he has some energy.
Work done = Force (N) x Distance Moved (M)
The distance is always in the direction of the force. It is measured in Joules (J).
1 joule is the work done when a force of 1 newton moves through 1 metre (in
the direction of the force).
Forms of energy
Thermal energy = The movement energy of the molecules.
Kinetic energy = Movement energy.
Gravitational potential energy = Stored energy of high object falls down.
Elastic potential energy = Stored in catapult etc.
Chemical energy = In food and other fuels.
Sound & Light energy = Common types of energy.
Electrical energy = Common and useful but dangerous.
Magnetic energy = Always connected with electrical energy.
Nuclear energy = Stored in centre of an atom.
The Principle of Conservation of Energy: energy can be changed from one form
to another but cannot be created or destroyed.
Energy and work
Energy is the ability to do work. The amount of work that is done tells us how
much energy has been transferred from one form to another.
Work Done = Energy Transferred
Working against gravity
When someone lifts a brick to a shelf and the work done was 100 J, it means
that 100 J of his chemical (food) energy was transferred to 100 J of gravitational
potential energy.
Falling under gravity
When a diver falls down, his potential energy is transferred into an equal
amount of kinetic energy.
The pendulum
When a pendulum swings, its energy is constantly changing from potential
energy to kinetic energy and back.
Energy Transfer Diagrams
You can show the energy transfer (e.g. chemical -> heat) in an Energy Transfer
Diagram or Sankey diagram. The thickness of an arrow is on scale and shows
the amount of energy.
But although there is the same amount of energy afterwards, not all of it is
useful. Most of the energy is wasted as heat in a light bulb.
Efficiency = Useful Energy Output / Total Energy Output x 100% = ….
Calculation Potential Energy (PE)
1. Gravitational Potential Energy
gravitational PE = work done so
Change in PE (J) = Weight (N) x Change in height (M)
2. Elastic Potential Energy
Energy stored = work done to stretch the bow so:
Elastic energy (J) = Average Force (N) x Distance (M)
Calculating Kinetic Energy
Kinetic Energy (J) = 0.5 x Mass (kg) x Speed squared (m/s2)
Power
The power is the rate of working:
Power = Work done (J) / Time Taken = Energy Transferred / Time taken
It is measured in joules per second or watts (W). Just like electric power.
1 W = 1 Joule per second
Chapter 17 – Machines
Machines transfer energy from one form to another. They also work with
efficiency (see chapter 16). A car is not very efficient, for every 100 J of petrol,
only 25 appear as useful movement energy, the other 75 are wasted as heat.
The efficiency is always less than 100 %.
Efficiency = useful energy output / total energy input
Efficiency = power output / power input
OR
Levers
A lever is a very common machine and it helps to do work more easily.
It is acting as a force magnifier. Read through page 123 and 124.
Gears
Gears can be used as force magnifiers or as distance magnifiers, it depends on
the size of the wheel. When a small wheel move through one complete turn,
the large wheel moves anti-clockwise through only half a turn.
The larger wheel always moves slowly but with a larger force and is the force
magnifier. Smaller wheels with a smaller force are called distance magnifiers.
Gears can also be used as speed magnifiers in e.g. bicycles.
Pulleys
Pulleys are very useful for lifting loads vertically. It is easier to pull downwards
than it is to pull upwards.
Two pulleys
It is easier to life a load when the rope goes round two pulleys. Because there
are two ropes pulling the load, the effort needed is only about half as much.
Three pulleys
When there are three pulleys, there are two at the top and one at the bottom.
The efficiency of a pulley system is always less than 100 % because;
- Friction transfers some of the energy to heat.
- Energy is needed to lift up the bottom pulleys and the ropes.