Work, Energy, and | Class 9 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 6 min read
Work, Energy, and – this guide gives you a concise, exam-ready overview of Work, Energy, and from Class 9 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
Simple Machines
Simple machines are devices that help us do work more easily by changing the magnitude or direction of the applied force. They do not reduce the total work done but make tasks more convenient.
The force applied to a machine is called effort, and the force to be overcome is called load. Mechanical advantage (MA) is defined as the ratio of load to effort: MA = load / effort.
Three common simple machines are pulleys, inclined planes, and levers.
A fixed pulley changes the direction of the applied force, making it easier to pull downward rather than lifting upward directly. Its mechanical advantage is 1 because effort equals load.
Movable pulleys or pulley systems have mechanical advantage greater than 1, allowing lifting heavier loads with less effort.
An inclined plane allows lifting a heavy object to a height by pushing it along a slope. The force required is less than lifting vertically but must be applied over a longer distance. Mechanical advantage of an inclined plane is MA = length of slope / height.
Levers are rigid bars rotating about a fulcrum. By applying effort at one end, a larger load can be lifted at the other end. The mechanical advantage of a lever is the ratio of effort arm to load arm.
Activities with pulleys, inclined planes, and levers demonstrate these principles. For example, balancing coins on a scale shows the relation effort × effort arm = load × load arm.
Simple machines are fundamental to many complex machines used in daily life, making work easier and more efficient.
📊 Diagram: Fig. 7.23: Pulley; Fig. 7.24: Pulling up a load (a) directly, and (b) using a pulley; Fig. 7.25: A system of pulleys; Fig. 7.26: A box being (a) lifted vertically up, and (b) pushed up the ramp; Fig. 7.27: Measuring the force required to pull up a cart along an inclined plank of (a) smaller length, and (b) larger length; Fig. 7.28: A load being lifted up (a) vertically, (b) along an inclined plane, and (c) along an inclined plane of larger length; Fig. 7.29: Ramp triangle; Fig. 7.30: Climbing ladders; Fig. 7.31: Lifting a heavier object with lighter object; Fig. 7.32: A lever used to lift a heavy rock; Fig. 7.33: Balancing cups hung on a scale; Fig. 7.34: A seesaw
🧪 Activity: Activity 7.3: Measuring force required to pull a cart up an inclined plank of different lengths; Activity 7.4: Investigating lifting heavy objects with a lever; Activity 7.5: Balancing coins on a scale to study lever principle.
🔗 Connection: This section concludes the chapter by connecting work, energy, and machines, leading to exercises and applications.
Frequently asked questions
Which factors determine the energy required to raise a flag from the ground to the top of a tall flagpole using a pulley? Does raising the flag slowly or quickly change the amount of work done? If the speed at which the flag is raised is doubled, how does the power requirement change? Explain your answers.
The energy required to raise a flag depends on the height of the flagpole and the weight (mass) of the flag. The work done is equal to the gravitational potential energy gained by the flag, which is mgh (mass × gravity × height).
Raising the flag slowly or quickly does not change the amount of work done because work depends only on force and displacement, not on time.
If the speed of raising the flag is doubled, the power requirement doubles because power is work done per unit time. Doubling s
A man of mass 60 kg rides a scooter of mass 100 kg. He accelerates the scooter to a velocity v. The next day, his son with a mass of 40 kg joins him as a passenger. If the scooter reaches the same speed on both days in the same time interval, what is the ratio of the fuel of the tank used on the two days? Assume that the energy transfer to the scooter happens entirely due to fuel, and no other losses occur due to air resistance and friction.
First day total mass = 60 + 100 = 160 kg Second day total mass = 60 + 40 + 100 = 200 kg
Kinetic energy on day 1 = (1/2) × 160 × v² = 80 v² Kinetic energy on day 2 = (1/2) × 200 × v² = 100 v²
Since energy transfer is from fuel, fuel used ∝ kinetic energy.
Ratio of fuel used on day 2 to day 1 = 100 v² / 80 v² = 5/4 = 1.25
So, the fuel used on the second day is 1.25 times that on the first day.
On a seesaw with sliding seats, a child is sitting on one side and an adult on the other side. The adult weighs twice that of the child. The seesaw however is balanced. Draw a figure which depicts this situation showing the distances from the fulcrum where the child and the adult are seated.
Let the weight of the child be W and the adult be 2W.
For balance, torque by child = torque by adult
W × d1 = 2W × d2
=> d1 = 2 d2
So, the child sits twice as far from the fulcrum as the adult.
Draw a seesaw with fulcrum in the center, child on one side at distance d1, adult on other side at distance d2 = d1/2.
This shows balance despite adult being heavier.
A ball of mass 2 kg is thrown up with a velocity of 20 m/s. (i) Identify the sign of the work done by gravity on the ball during its upward motion and its downward motion. (ii) If the ball reaches a height of 19.4 m, how much work was done by air resistance (assume g = 10 m/s²).
(i) Work done by gravity during upward motion is negative because gravity acts downward while displacement is upward. Work done by gravity during downward motion is positive because gravity and displacement are in the same direction.
(ii) Initial kinetic energy = (1/2) m v² = 0.5 × 2 × 20² = 400 J
Potential energy at max height = mgh = 2 × 10 × 19.4 = 388 J
Work done by air resistance = Initial KE - Potential Energy = 400 - 388 = 12 J (energy lost due to air resistance)
So, air resistance di
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