Chapter 9: Application of Forces and Transfer of Energy

Force

A push or a pull

SI unit of force

Newton (N)

Force meter

Instrument used to measure force

Contact force

A force that acts when two objects are in contact

Friction

A contact force that opposes motion between two surfaces in contact

Positive effects of friction

Walking, writing, lighting a fire with a match, and parachuting

Elastic force

A contact force acting on a stretched or compressed elastic object to return it to its original shape

Non-contact force

A force that acts without objects being in contact

Gravitational force

A force that attracts two objects towards each other

Magnetic force

A force exerted by a magnet on magnetic materials such as iron, steel, nickel and cobalt

Unlike magnetic poles

Attract

Like magnetic poles

Repel

Formula for weight

W = mg

W

Weight in newtons (N)

m

Mass in kilograms (kg)

g on Earth

10 N/kg

Mass when moved from Earth to Moon

Remains unchanged

Weight when moved from Earth to Moon

Decreases

Conversion

1 kg = 10 N

Effects of a force

A force can move a stationary object, change speed, change direction, or stop a moving object

Turning effect of a force

A force can cause an object to turn about a fixed point called a pivot

Pivot

A fixed point about which an object turns

Force and shape

A force can change the size and/or shape of an object

Pressure

An effect of force acting on an object

Energy

The ability to do work

SI unit of energy

Joule (J)

Work done

Energy transfer that occurs when a force acts on an object and the object moves in the same direction as the force

Conditions for work to be done

A force acts on an object and the object moves in the same direction as the force

When holding a stationary object

No work is done because the object does not move

When carrying an object horizontally

No work is done if the force is vertical while the object moves horizontally

Forms of energy

Electrical, sound, kinetic, gravitational potential, elastic potential, chemical potential, thermal and light

Potential energy

Stored energy

Chemical potential energy

Energy stored in a substance due to the position of its atoms or electrons

Examples of chemical potential energy

Food, fossil fuels and batteries

Elastic potential energy

Energy stored in a body due to elastic deformation

Examples of elastic potential energy

Rubber band, spring and bow

Gravitational potential energy

Energy stored in a body due to its height from the ground

Examples of gravitational potential energy

Raised object and hydroelectric dam

Kinetic energy

Energy of a body due to its motion

Electrical energy

Energy of an electric charge due to its motion and position

Thermal energy

Energy stored in a body due to its temperature

Sound energy

Energy transferred from one point to another by vibrations

Light energy

Electromagnetic wave visible to the eye

Energy conservation

Energy cannot be created or destroyed, but can be converted from one form to another

Principle of conservation of energy

The total energy in an isolated system is constant

Bow and arrow energy conversion

Chemical potential → elastic potential → kinetic energy

Swing at highest point

Mainly gravitational potential energy

Swing at lowest point

Mainly kinetic energy

Swing energy conversion

Gravitational potential energy ↔ kinetic energy

Renewable energy

Energy sources that can be sustained or renewed indefinitely

Non-renewable energy

Energy sources that are limited and will eventually run out

Non-renewable example

Fossil fuels

Renewable examples

Solar, hydroelectric, wind, geothermal, biofuels and nuclear

Fossil fuels

Chemical potential → heat → kinetic → electrical energy

Fossil fuel examples

Crude oil, natural gas, petrol, kerosene, diesel and coal

Fossil fuel impact

Burning fossil fuels produces carbon dioxide, which contributes to global warming

Solar energy conversion

Light → electrical energy

Solar energy impact

Solar cells produce no harmful substances or pollutants when generating electricity

Hydroelectric energy conversion

Gravitational potential → kinetic → electrical energy

Hydroelectric impact

Dams can flood forests, wildlife habitats and farmland and affect ecosystems

Wind energy conversion

Kinetic → electrical energy

Wind energy impact

Wind turbines can cause bird and bat deaths and produce low-frequency noise

Geothermal energy conversion

Heat → kinetic → electrical energy

Geothermal uses

Heating homes and generating electricity

Geothermal impact

Can release poisonous substances and cause land to sink

Biofuel conversion

Chemical potential → heat → kinetic → electrical energy

Biofuel impact

Burning biofuels produces carbon dioxide and crops may compete with food production

Nuclear energy conversion

Heat → kinetic → electrical energy

Nuclear impact

Nuclear waste can remain hazardous for thousands of years and accidents can seriously harm the environment