CBSE Class 9 Science Revision Notes Chapter 7: Work, Energy, and Simple Machines

Work is done when a force produces displacement in its direction, while energy represents the capacity to perform work. CBSE Class 9 Science Chapter 7 explains kinetic energy, potential energy, power and simple machines used in daily life.

Force can change the motion of an object, but work is done only when that force produces displacement. The amount of work depends on the force applied and the displacement produced in its direction.

This CBSE Class 9 Science Chapter 7 notes explain work, energy, power and simple machines through clear concepts, formulas and everyday examples. They also help students understand how energy changes form and how machines make tasks easier.

Key Takeaways

• Work: It equals force multiplied by displacement in the force’s direction.
• Energy: It is the capacity to perform work and is measured in joules.
• Mechanical energy: It is the sum of kinetic and potential energy.
• Simple machines: They change the magnitude or direction of effort but do not reduce total work.

Access Class 9 Science Chapter 7 Work, Energy, and Simple Machines Notes in 30 Minutes

Revise the chapter in three parts:

• First 10 minutes: Work, zero work, positive and negative work, and the work-energy theorem
• Next 10 minutes: Forms of energy, kinetic energy, potential energy and conservation of mechanical energy
• Final 10 minutes: Power, mechanical advantage, pulley, inclined plane and lever

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Work Done in Class 9 Science Chapter 7

In science, work has a specific meaning.

Work is done on an object when:

• A force acts on the object.
• The object undergoes displacement.
• At least part of the displacement occurs in the direction of force.

Lifting a wheat bag from the floor involves work because an upward force produces upward displacement.

Pushing a wall does not involve work on the wall because it does not move.

Work Done by a Force

For a constant force acting in the direction of displacement:

Work done = Force × Displacement

W = F × s

Where:

• W = work done
• F = constant force
• s = displacement in the direction of force

The amount of work increases when:

• A larger force acts over the same distance.
• The same force acts over a larger distance.

If three identical bags are lifted through the same height, the total work is three times the work required for one bag.

SI Unit of Work

The SI unit of work is the joule.

Its symbol is J.

One joule of work is done when a force of 1 newton produces a displacement of 1 metre in its direction.

1 J = 1 N × 1 m

Since:

1 N = 1 kg m s⁻²

Therefore:

1 J = 1 kg m² s⁻²

Work from a Force-Displacement Graph

The work done by a force is represented by the area under a force-displacement graph.

For a constant force, this area forms a rectangle.

Example:

Force = 10 N
Displacement = 1 m

Work done = 10 × 1
Work done = 10 J

The same idea can be used when force changes with displacement. The area under the graph gives the total work.

Zero Work in Work, Energy, and Simple Machines

Work done on an object can be zero even when a person applies force.

There are three important cases.

No Force Acts

If force is zero, work is zero.

W = 0 × s = 0

No Displacement Occurs

If the object does not move, displacement is zero.

W = F × 0 = 0

A person pushing a rigid wall does no mechanical work on the wall.

The person may still feel tired because muscles use internal energy while applying force.

Force Is Perpendicular to Displacement

Work is zero when the force and displacement are perpendicular.

A person carrying a box horizontally applies an upward force. The displacement is horizontal.

Since there is no displacement in the direction of the upward force, that force does no work on the box.

Positive and Negative Work in Class 9 Science Chapter 7

The sign of work depends on the directions of force and displacement.

Positive Work

Work is positive when force and displacement act in the same direction.

Examples include:

• Pushing a wheelchair forward
• Lifting a bag upwards
• A moving striker pushing a carrom coin
• Gravity acting on a falling object

When positive work is done, the object usually gains energy.

Negative Work

Work is negative when force acts opposite to displacement.

Examples include:

• Friction acting on a moving object
• A goalkeeper stopping a ball
• Gravity acting on an upward-moving object
• Brakes slowing a vehicle

Negative work usually reduces the object’s kinetic energy.

Example of Negative Work

A goalkeeper applies a force of 200 N while their hands move backwards by 15 cm.

Displacement = −15 cm = −0.15 m

Work done = F × s

Work done = 200 × (−0.15)

Work done = −30 J

The negative sign shows that the force acts opposite to the displacement of the ball.

Work-Energy Theorem in Work, Energy, and Simple Machines

Energy is the capacity to perform work.

A moving ball can strike a wicket and make it fall. A raised flowerpot can damage an object if it falls.

These objects possess energy because they can perform work.

The work-energy theorem states:

Work done on an object = Change in its energy

Positive work increases the object’s energy. Negative work decreases it.

The theorem can be applied even when:

• Several forces act on an object.
• The force is not constant.
• A system contains more than one object.

The SI unit of energy is also the joule.

Energy Transfer Through Work

When one object performs work on another, energy is transferred.

In a carrom game:

• The striker performs positive work on a coin.
• The coin gains energy.
• The coin may transfer energy to another coin.
• The object doing negative work loses energy.

Energy can also be transferred as heat, sound, radiation or electricity.

Forms of Energy in Class 9 Science Chapter 7

Energy exists in several forms.

Energy can change from one form to another.

Examples include:

• A bulb converts electrical energy into light and thermal energy.
• Food supplies chemical energy to muscles.
• A bell converts mechanical energy into sound.
• A water heater converts electrical energy into thermal energy.
• A solar panel converts light energy into electrical energy.

Mechanical Energy in Work, Energy, and Simple Machines

Mechanical energy is the energy an object possesses because of its motion or position.

It has two main forms:

• Kinetic energy
• Potential energy

Mechanical energy = Kinetic energy + Potential energy

Kinetic Energy Formula in Class 9 Science Chapter 7

Kinetic energy is the energy possessed by an object due to its motion.

All moving objects possess kinetic energy.

Examples include:

• A moving bicycle
• A rolling ball
• A flying aircraft
• Flowing water

The kinetic energy of an object depends on its mass and velocity.

The kinetic energy formula is:

K = ½mv²

Where:

• K = kinetic energy
• m = mass
• v = velocity

The SI unit of kinetic energy is the joule.

Effect of Mass on Kinetic Energy

For the same velocity, a heavier object has greater kinetic energy.

If mass doubles while velocity remains unchanged, kinetic energy also doubles.

Effect of Velocity on Kinetic Energy

Kinetic energy depends on the square of velocity.

If velocity doubles:

New kinetic energy = ½m(2v)²

New kinetic energy = 4 × ½mv²

Therefore, the kinetic energy becomes four times greater.

If velocity triples, kinetic energy becomes nine times greater.

Numerical Example

Mass of a cricket ball = 0.2 kg
Velocity = 43 m s⁻¹

K = ½mv²

K = ½ × 0.2 × 43²

K = 0.1 × 1849

K = 184.9 J

Potential Energy in Class 9 Science Chapter 7

Potential energy is stored energy.

It may be stored because of:

• The shape or deformation of an object
• The position of objects in a system
• Gravitational, magnetic or electric interactions

A stretched rubber band has potential energy. A compressed spring and a bent bow also store energy.

When released, this stored energy may change into kinetic energy.

Elastic Potential Energy

Stretching or compressing a spring changes its shape.

Work done while deforming the spring is stored as potential energy.

When released:

• The spring returns to its original shape.
• Stored energy is transferred.
• An object in contact with it may gain kinetic energy.

A slingshot and bow work in a similar way.

Energy Due to Relative Position

A system can store energy because of the relative positions of its objects.

Examples include:

• Two separated unlike magnetic poles
• Electric charges placed apart
• An object raised above the Earth

The energy stored due to relative position is potential energy.

Gravitational Potential Energy and Its Formula

An object raised above the Earth possesses gravitational potential energy.

Work must be done against gravity to raise it.

Near the Earth’s surface:

Gravitational potential energy = Mass × g × Height

U = mgh

Where:

• U = gravitational potential energy
• m = mass
• g = acceleration due to gravity
• h = height above the reference level

Its SI unit is the joule.

Effect of Height

Potential energy increases with height.

A ball dropped from a greater height creates a deeper depression in sand because it had more gravitational potential energy.

Effect of Mass

For the same height, an object with greater mass possesses greater potential energy.

Numerical Example

Mass of ball = 0.2 kg
Height = 10 m
g = 10 m s⁻²

U = mgh

U = 0.2 × 10 × 10

U = 20 J

Kinetic Energy and Potential Energy Comparison

Conservation of Mechanical Energy

The conservation of mechanical energy states that the total mechanical energy remains constant when no external force such as friction acts.

As an object falls:

• Potential energy decreases.
• Kinetic energy increases.
• Total mechanical energy remains constant.

Object Falling from a Height

Suppose an object of mass m is raised to height h.

At the top:

• Potential energy = mgh
• Kinetic energy = 0
• Mechanical energy = mgh

As it falls:

• Its height decreases.
• Its velocity increases.
• Potential energy changes into kinetic energy.

Just before reaching the ground:

• Potential energy is nearly zero.
• Kinetic energy is nearly mgh.
• Mechanical energy remains mgh.

Simple Pendulum

At one extreme position:

• Potential energy is maximum.
• Kinetic energy is zero.

At the lowest point:

• Potential energy is minimum.
• Kinetic energy is maximum.

At the opposite extreme:

• Kinetic energy becomes zero again.
• Potential energy becomes maximum.

In real life, the pendulum eventually stops because friction and air resistance transfer mechanical energy into other forms.

Child Moving Down a Slide

A child at the top of a slide possesses gravitational potential energy.

Ignoring friction:

mgh = ½mv²

Mass cancels from both sides.

v = √2gh

The final speed depends on the vertical height, not the child’s mass or the exact shape of the slide.

Power in Work, Energy, and Simple Machines

Power tells us how quickly work is done.

Two people may perform the same work but take different amounts of time.

The person who completes the work faster uses greater power.

The power formula is:

Power = Work done ÷ Time taken

P = W/t

Where:

• P = power
• W = work done
• t = time taken

The SI unit of power is the watt.

Its symbol is W.

1 W = 1 J s⁻¹

One watt means one joule of work is done every second.

Numerical Example

A weightlifter raises a 75 kg mass through 2 m in 5 seconds.

Work done = mgh

Work done = 75 × 10 × 2

Work done = 1500 J

Power = W/t

Power = 1500/5

Power = 300 W

Horsepower

Horsepower is another unit used for engines and pumps.

1 horsepower = 746 W

Simple Machines in Class 9 Science Chapter 7

Simple machines make tasks easier by changing the magnitude or direction of the force applied.

They do not reduce the total work required.

The force applied to a machine is called effort.

The force that must be overcome is called load.

The chapter covers three simple machines:

• Pulley
• Inclined plane
• Lever

Mechanical Advantage of Simple Machines

Mechanical advantage shows how much a machine multiplies the applied force.

Mechanical advantage = Load ÷ Effort

A mechanical advantage greater than 1 means that the effort is smaller than the load.

Machines reduce effort by increasing the distance over which effort acts.

They do not create energy.

Pulley in Work, Energy, and Simple Machines

A pulley is a wheel with a groove through which a rope moves.

Fixed Pulley

A fixed pulley changes the direction of effort.

When a person pulls the rope downward, the load rises upward.

A fixed pulley does not reduce the magnitude of effort.

For an ideal fixed pulley:

Mechanical advantage = 1

Examples include:

• Raising a flag
• Drawing water
• Lifting small loads

Movable Pulley

A movable pulley moves along with the load.

It can provide a mechanical advantage greater than 1.

Systems of fixed and movable pulleys are used in:

• Cranes
• Elevators
• Construction equipment

Inclined Plane in Class 9 Science Chapter 7

An inclined plane is a sloping surface used to move an object between different heights.

A ramp allows a heavy box to be moved upward with less force than direct vertical lifting.

The effort becomes smaller, but it acts over a larger distance.

For an ideal inclined plane:

Mechanical advantage = Length of plane ÷ Vertical height

MA = L/h

Where:

• L = length of the inclined plane
• h = vertical height

A longer and less steep plane requires less effort.

Numerical Example

Length of ramp = 50 cm
Vertical height = 30 cm

MA = L/h

MA = 50/30

MA = 1.67

Winding roads on hills use gentle slopes because a smaller force is required over a longer distance.

Lever in Work, Energy, and Simple Machines

A lever is a rigid bar that rotates about a fixed point.

It has three main parts:

• Fulcrum: Fixed point about which the lever turns
• Load: Force to be overcome
• Effort: Applied force

The distance between load and fulcrum is the load arm.

The distance between effort and fulcrum is the effort arm.

For a balanced lever:

Effort × Effort arm = Load × Load arm

The mechanical advantage is:

Mechanical advantage = Effort arm ÷ Load arm

A longer effort arm reduces the effort needed.

Why a Lever Makes Work Easier

A smaller force moves through a larger distance at one end.

The other end applies a larger force through a smaller distance.

The total work remains the same in an ideal machine.

Classes of Levers

Levers are classified according to the relative positions of effort, fulcrum and load.

Class I Lever

The fulcrum lies between effort and load.

Example: Seesaw

Class II Lever

The load lies between fulcrum and effort.

Example: Wheelbarrow

Class III Lever

The effort lies between fulcrum and load.

Example: Tweezers

Pulley, Inclined Plane and Lever Comparison

Class 9 Work, Energy and Simple Machines Formulas

Work Energy and Simple Machines Notes: Quick Revision

These Work Energy and Simple Machines notes can be revised through the following points:

• Work requires force and displacement.
• Work is zero when displacement is zero.
• Positive work increases energy.
• Negative work usually reduces kinetic energy.
• Work done equals the change in energy.
• Kinetic energy is given by K = ½mv².
• Potential energy near Earth is U = mgh.
• Mechanical energy is the sum of kinetic and potential energy.
• Mechanical energy remains constant when friction is absent.
• Power is the rate of doing work.
• Simple machines change force magnitude or direction.
• Machines do not reduce total work.
• Mechanical advantage equals load divided by effort.
• The main simple machines are pulley, inclined plane and lever.

Useful Links for Class 9 Science