Short Intro

This chapter explains the concepts of work, energy, power, and simple machines with practical examples and numerical problems. Students will learn how forces perform work, how energy changes from one form to another, and how machines make our work easier.

Quick Information Box

Topic	Details
Chapter Name	Work, Energy and Simple Machines
Class	Grade 9
Subject	Science / Physics
Important Topics	Work, Energy, Power, Kinetic Energy, Potential Energy, Machines
SI Unit of Work	Joule (J)
SI Unit of Power	Watt (W)
Key Formula	W = F × s

Concepts Used (Topics Covered)

• Work Done by Force
• Positive and Negative Work
• Work-Energy Theorem
• Forms of Energy
• Kinetic Energy
• Potential Energy
• Conservation of Mechanical Energy
• Power
• Pulley
• Inclined Plane
• Lever and Mechanical Advantage

Important Formulas

1. Work Done
   W = F × s
2. Kinetic Energy
   KE = ½ mv²
3. Potential Energy
   PE = mgh
4. Power
   P = W / t
5. Mechanical Advantage
   MA = Load / Effort
6. Inclined Plane
   MA = Length / Height
7. Lever Principle
   Effort × Effort Arm = Load × Load Arm

Think It Over

1. What will be the magnitude of velocity of the child at the bottom of the blue slide?

2. Will two children of different masses reach the bottom of the same slide with the same velocity?

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3. Which of the slides will result in the largest magnitude of velocity for the child at its bottom?

Pause and Ponder

1. In the previous chapter, a weightlifter is shown holding a barbell steady in her hands (Fig. 6.8). Is she doing any work n the barbell while holding it steady?

2. Is the work done by friction on the stack of coins that travels on a rough surface (Fig. 6.13c) — positive, negative or zero?

3. When you pedal a bicycle on a flat road, your muscles supply energy. In what forms does this muscular energy appear as you ride?

4. Two objects A and B of mass m and 4 m have the same kinetic energy. What is the ratio of the magnitude of velocities of A and B?

5. Does the kinetic energy of an object which moves with constant velocity change with its position?

6. Does the potential energy of an object near the surface of the Earth change if it moves with constant velocity in the horizontal direction? What if the object is gradually raised in the vertical direction?

7. For the situation depicted in Fig. 7.19, calculate the mechanical energy of the ball just before it hits the ground and show that even at this position, it is mgh .

8. You may have seen an exhibit like that in Fig. 7.22 in a science park, where a ball is released from the highest point. escribe how the kinetic energy and potential energy change at points A, B and C. Why do subsequent points, such as C, D end E, usually have lower heights compared to the previous ones? Could it have anything to do with the energy lost due to friction?

9. Explain why roads on hills are built to wind around in gentle slopes rather than going straight up (Fig. 4.26)?

10. To reach a higher floor, we find climbing an inclined ladder easier in comparison to climbing a vertical ladder (Fig. 7.30). Explain why.

11. Why is it easier to open the lid of a can by using a spoon as shown in Fig. 7.35?

12. Why do you push an object closer to scissors fulcrum when you want to cut an object which is hard?

13. Throughout history, many designs of perpetual machines (using wheels, weights or magnets) have been proposed but none actually work. Why do all real machines eventually slow down and stop? Explain in terms of work and energy.

Questions & Step-by-step Solutions with Explanation

Revise, Reflect, Refine

1. State whether True or False.
   (i) Work is said to be done when a force is applied, even if the object does not move.
   (ii) Lifting a bucket vertically upward results in positive work done on the bucket. (iii) The SI unit for both work and energy is joule (J). (iv) A motionless stretched rubber band has kinetic energy. (v) Energy can change from one form to another

2. Fill in the blanks.
   

3. When a ball thrown upwards reaches its highest point, tick which of the following statement(s) are correct?
   (i) The force acting on the ball is zero. (ii) The acceleration of the ball is zero. (iii) Its kinetic energy is zero.
   (iv) Its potential energy is maximum.

4. For each of the following situations, identify the energy transformation that takes place: (i) a truck moving uphill, (ii) unwinding of a watch spring, (iii) photosynthesis in green leaves, (iv) water flowing from a dam, (v) burning of a matchstick, (vi) explosion of a fire cracker, (vii) speaking into a microphone, (viii) a glowing electric bulb, and (ix) a solar panel.

5. A student is slowly lifted straight up in an elevator from the ground level to the top floor of a building. Later, the same student climbs the staircase, all the way to the top. Given that the height of the building is h = 72.5 m, acceleration due to gravity is g = 10 m s–2, and student’s mass is m = 50 kg. (i) Find the gain in the potential energy if the student is lifted straight up to the top. (ii) Find the gain in the potential energy when the student climbs the stairs to the same top. (iii) What do you conclude about the dependence of the potential energy on the path taken?

6. A crane lifts a mass m to the 10th floor of a building in a certain time. It then raises the same mass to the 20th floor of the same building in double the time. How much more energy and power are required? Assume that the height of all floors is equal.

7. 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.

8. 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.

9. 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.

10. A ball of mass 2 kg is thrown up with a velocity of 20 m s–1. (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 s–2).

11. A 10.0 kg block is moving on horizontal floor with negligible friction. As shown in the Fig. 7.37, a variable force is applied on the block in its direction of motion from its position at 0 m till 4 m. If the block had a kinetic energy of 180 J when it was at 0 m, find the block’s speed (i) at 0 m, and (ii) at 4 m. Does the block have negative acceleration in any portion of its motion?

12. The gravitational attraction on the surface of the Moon (lunarsurface) is about 1 the 6 of that on the surface of the Earth. An astronaut can throw a ball up to a height of 8 m from the surface of the Earth. How far up will the ball thrown with the same upward velocity travel from the surface of the Moon?

13. A 1000 kg car is moving along a road at a constant speed. Suddenly, the driver notices some obstruction ahead and
    applies the brakes to come to a complete stop. The graphical representation of motion of the car starting from the instant the driver spots the traffic ahead is shown in Fig. 7.38. (i) Describe how the car moves between positions A and B. (ii) Calculate the kinetic energy of the car at A. (iii) State the work done by the brakes in bringing the car to a halt between B and C. (iv) What does the kinetic energy of the car transform into? .

14. The potential energy-displacement graph of a 0.5 kg ball moving along a frictionless track is shown in Fig. 7.39. At O,
    the velocity of the ball is 0 m s–1 and potential energy is 30 J. Calculate the velocity of the ball at P, Q and R.

15. A coconut of mass 1.5 kg falls from the top of a coconut tree onto the wet sand on a beach. The height of the tree is 10 m. On impact, the coconut comes to rest by making a depression in the sand. (i) Calculate the velocity of the coconut just before it hits the sand. (ii) Assume that the average resistive force of sand is 3000 N and all of the coconut’s energy is used to create the depression in the sand. Calculate the depth of the depression the coconut makes in the sand. Assume g = 10 m s–2.

Common Mistakes

• Forgetting SI units
• Using wrong formula for energy
• Ignoring direction in work done
• Confusing power with energy
• Not converting km/h into m/s

Exam Tips

• Learn all formulas carefully.
• Draw diagrams for machines.
• Always write units in answers.
• Use proper sign convention for work done.
• Practice numerical problems regularly.

Practice MCQs

1. SI unit of work is:

A. Newton
B. Joule
C. Watt
D. Pascal

Answer:

B. Joule

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2. Energy possessed due to motion is:

A. Potential Energy
B. Chemical Energy
C. Kinetic Energy
D. Heat Energy

Answer:

C. Kinetic Energy

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3. Formula for power is:

A. F × s
B. mgh
C. W/t
D. ½mv²

Answer:

C. W/t

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4. Mechanical advantage of a fixed pulley is:

A. 0
B. 1
C. 2
D. 10

Answer:

B. 1

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5. Energy stored in stretched spring is:

A. Sound Energy
B. Potential Energy
C. Heat Energy
D. Light Energy

Answer:

B. Potential Energy

FAQ Section

Q1. What is work in physics?

Work is done when force causes displacement in the direction of force.

Q2. What is the SI unit of energy?

Joule (J)

Q3. What is kinetic energy?

Energy possessed by a moving object.

Q4. What is potential energy?

Stored energy due to position or shape.

Q5. What is a simple machine?

A device that makes work easier by changing force direction or magnitude.

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