Work, Energy, Power and Simple Machines Notes for Competitive

Work, energy and power are the key ideas of physics that connect everything from a falling stone to a running machine. In competitive exams such as SSC, railways, NDA, banking and State services, you will find direct questions on the definitions, units, formulas, the law of conservation of energy and the types of simple machines. This note explains these topics in a very simple way, with examples, small numerical problems, tables and practice questions. If you want to recall the basic ideas of motion and force first, read our notes on physics basics, and for the units, see SI units and measurement.

What Is Work?

In science, work is done when a force acts on an object and the object moves in the direction of the force. If there is no movement, there is no work, even if you feel tired. The formula is Work = Force x Displacement, or W = F x s. The SI unit of work is the joule (J). One joule is the work done when a force of 1 newton moves an object by 1 metre. A larger unit is the kilojoule (1 kJ = 1000 J).

Work can be positive, negative or zero. It is positive when the force and the movement are in the same direction, as when you push a trolley forward. It is negative when the force is opposite to the movement, as in the case of friction or when a ball is thrown up against gravity. It is zero when there is no displacement, such as pushing a wall, or when the force is at a right angle to the movement, such as the work done by gravity on a person walking on a level road, or the force of the earth on the moon in a circular path.

What Is Energy?

Energy is the capacity to do work. It is measured in the same unit as work, the joule. Energy exists in many forms: mechanical, heat, light, sound, electrical, chemical, nuclear and magnetic. A big part of the exam questions are on the mechanical energy, which has two kinds.

• Kinetic energy (KE) is the energy of a body due to its motion. KE = 1/2 x m x v squared. If the speed doubles, the KE becomes four times.
• Potential energy (PE) is the energy due to the position or the condition of a body. Gravitational PE = m x g x h. A stretched spring or a bent bow also has potential energy.

Law of Conservation of Energy

This law says that energy can neither be created nor destroyed; it can only change from one form to another, and the total energy always remains the same. When a ball is dropped from a height, the potential energy goes down and the kinetic energy goes up, but their sum remains constant. In a hydroelectric plant, the potential energy of water becomes kinetic energy, then the mechanical energy of the turbine, and finally electrical energy. In an electric bulb, electrical energy changes into light and heat. In a human body, the chemical energy of food becomes the mechanical and heat energy.

Energy Conversions in Daily Devices

What Is Power?

Power is the rate of doing work, or the work done in one second. Power = Work / Time. The SI unit is the watt (W), where 1 watt = 1 joule per second. A bigger unit is the kilowatt (1 kW = 1000 W). An old unit still used for engines is the horsepower (hp), where 1 hp is about 746 watts. The electricity bill of your home is calculated in kilowatt-hour (kWh), which is called one unit of electricity. One kWh = 3.6 x 10 to the power 6 joules, and it is the energy used by a 1000-watt appliance in one hour. Note that kWh is a unit of energy and not of power.

Small Numerical Examples

• Work: A force of 20 N moves a box by 5 m. Work = 20 x 5 = 100 J.
• Kinetic energy: A body of mass 2 kg moves with a speed of 10 m/s. KE = 1/2 x 2 x 100 = 100 J.
• Potential energy: A 5 kg body is lifted to 10 m (take g = 10). PE = 5 x 10 x 10 = 500 J.
• Power: A machine does 6000 J of work in 20 seconds. Power = 6000 / 20 = 300 W.
• Electricity: A 100 W bulb is used for 10 hours. Energy = 0.1 kW x 10 h = 1 kWh, or one unit.

Simple Machines

A machine is a device that makes work easier by changing the size or the direction of the force. It does not create energy. The main simple machines are the lever, the pulley, the inclined plane, the wheel and axle, the wedge and the screw.

Lever

A lever is a rigid bar that turns around a fixed point, called the fulcrum. The force applied is the effort, and the weight lifted is the load. Levers are of three classes.

Pulley, Inclined Plane and Others

A pulley is a wheel with a rope that changes the direction of force; a fixed pulley gives convenience, while a movable pulley also reduces the effort. An inclined plane (a ramp) makes it easy to lift a load to a height by using a longer path; a road on a hill is made with turns for this reason. A wedge is a double inclined plane, such as an axe or a knife. A screw is an inclined plane wrapped around a cylinder. A wheel and axle is used in a steering wheel and a door knob.

Mechanical Advantage and Efficiency

Mechanical advantage (MA) = Load / Effort. If a machine lifts a load of 100 N with an effort of 20 N, then MA = 5. It has no unit. Efficiency = (Useful work output / Work input) x 100 per cent. The efficiency of a real machine is always less than 100 per cent because some energy is lost as heat due to friction. A machine with 100 per cent efficiency does not exist.

Renewable and Non-Renewable Sources of Energy

The energy that we use comes from different sources. Non-renewable sources, such as coal, petroleum and natural gas, are formed over millions of years and will be finished one day. They also cause pollution when burnt. Renewable sources, such as solar, wind, hydro, tidal, biomass and geothermal energy, are never exhausted and cause far less pollution. India has a large solar potential, and the International Solar Alliance, with its headquarters at Gurugram, was launched by India and France. Wind energy in India is strong in Tamil Nadu, Gujarat and Maharashtra, and hydroelectric power is important in the hill states. The main aim of the country is to increase the share of clean energy and to reduce the use of fossil fuels.

Common Mistakes

Many students think that a person carrying a heavy load on a level road is doing work against gravity. In the scientific sense, the work done by the force of lifting is zero because the displacement is at right angles to that force. Another mistake is to confuse power with energy, and kilowatt with kilowatt-hour: one is the rate and the other is the total amount. Students also forget to square the speed in kinetic energy, or they write that PE depends on the speed. Finally, remember that a machine only gives us convenience; it can never give out more energy than is put into it.

Tips for the Exam

1. Memorise the three units: joule for work and energy, watt for power, and kWh for the commercial unit of energy.
2. Remember that if the speed doubles, kinetic energy becomes four times.
3. Do not forget that work is zero when the displacement is zero.
4. Use the examples of the daily life to remember the lever classes.

Practice Questions With Answers

• Q1. What is the SI unit of power? Answer: Watt.
• Q2. If the speed of a body is doubled, its kinetic energy becomes how many times? Answer: Four times.
• Q3. Which class of lever is a wheelbarrow? Answer: Second class.
• Q4. What is the energy change in a solar cell? Answer: Light energy to electrical energy.
• Q5. One kilowatt-hour is equal to how many joules? Answer: 3.6 x 10 to the power 6 joules.

Quick Revision

• Work = Force x Displacement; energy is the capacity to do work; power is the rate of work.
• KE = 1/2 mv squared, and PE = mgh.
• Energy is conserved; it only changes its form.

Frequently Asked Questions (FAQ)

What is work in physics?

Work is done when a force moves an object in the direction of the force, and W equals force multiplied by displacement.

What is the SI unit of work and energy?

The SI unit of both work and energy is the joule.

What is kinetic energy?

It is the energy of a body due to its motion, equal to half of mass multiplied by the square of the speed.

What is potential energy?

It is the energy a body has due to its position or condition, such as mgh for a height.

What does the law of conservation of energy say?

It says that energy can neither be created nor destroyed, but only changes from one form to another.

What is power?

Power is the rate of doing work, and its SI unit is the watt.

How much is one horsepower?

One horsepower is about 746 watts.

What is one unit of electricity?

One unit is one kilowatt-hour, which equals 3.6 million joules.

Is work done when we push a wall?

No, because the wall does not move, so the displacement is zero.

What are the three classes of levers?

In the first class the fulcrum is in the middle, in the second the load is in the middle, and in the third the effort is in the middle.

What is mechanical advantage?

It is the ratio of load to effort, and it has no unit.

Why is the efficiency of a machine less than 100 per cent?

Because part of the energy is always lost as heat due to friction.