Class 9 › Science › Exploration Work, Energy, and Simple Machines Class 9 Science · Exploration · Chapter 7
Summary Questions & answers Practise this chapter → This chapter gives the scientific meanings of work, energy and power and links them through energy conservation, then introduces simple machines. In science, work is done only when a force moves an object along its direction: \(W = F \times s\), measured in joules (J); no work is done if there is no displacement or if the force is perpendicular to the motion. Energy is the capacity to do work, also measured in joules. Kinetic energy is the energy of motion, \(KE = \tfrac{1}{2}mv^2\), while gravitational potential energy is the energy of position, \(PE = mgh\). The law of conservation of energy states that energy can neither be created nor destroyed but only transformed; for a freely falling or sliding body the sum of kinetic and potential energy stays constant, which is why two children of different masses reach the bottom of the same frictionless slide with the same speed. Power is the rate of doing work, \(P = \dfrac{W}{t}\), measured in watts (W); commercial electrical energy is sold in kilowatt-hours (units). Simple machines — the lever, pulley, inclined plane, wheel and axle, wedge and screw — do not create energy but make work easier by changing the size or direction of the effort needed, illustrating mechanical advantage in everyday tools.
Work
Done when a force moves an object along its own direction, \(W = F \times s\); measured in joules (J).
Kinetic energy
The energy possessed by a body because of its motion, \(KE = \tfrac{1}{2}mv^2\).
Potential energy
The energy possessed by a body because of its position or configuration, e.g. gravitational \(PE = mgh\).
Law of conservation of energy
Energy can neither be created nor destroyed, only changed from one form to another; the total stays constant.
Power
The rate of doing work or transferring energy, \(P = \dfrac{W}{t}\); measured in watts (W).
Simple machine
A device such as a lever, pulley or inclined plane that makes work easier by changing the magnitude or direction of the effort.
Q1 When is work said to be done in science? Answer Q2 Write the formula for work and its SI unit. Answer Q3 Give an example where a force acts but no work is done. Answer Q4 Write the expressions for kinetic and potential energy. Answer Q5 State the law of conservation of energy. Answer Q6 Why do two children of different masses reach the bottom of the same frictionless slide with the same speed? Answer Q7 Define power and give its SI unit. Answer Q8 What is a kilowatt-hour? Answer Q9 Do simple machines reduce the total work needed? Answer Q10 Name some simple machines and one use of a lever. Answer Q1 What will be the magnitude of velocity of the child at the bottom of the blue slide? Answer Q2 Will two children of different masses reach the bottom of the same slide with the same velocity? Answer Q3 Which of the slides will result in the largest magnitude of velocity for the child at its bottom? Answer Q1 1. In the previous chapter, a weightlifter is shown holding a barbell steady in her hands (Fig. 6.8). Is she doing any work on the barbell while holding it steady? Answer Q2 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? Answer Q1 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? Answer Q1 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? Answer Q2 5. Does the kinetic energy of an object which moves with constant velocity change with its position? Answer Q1 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? Answer Q1 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. Answer Q2 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. Describe how the kinetic energy and potential energy change at points A, B and C. Why do subsequent points, such as C, D and E, usually have lower heights compared to the previous ones? Could it have anything to do with the energy lost due to friction? Answer Q1 9. Explain why roads on hills are built to wind around in gentle slopes rather than going straight up (Fig. 4.26)? Answer Q2 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. Answer Q1 11. Why is it easier to open the lid of a can by using a spoon as shown in Fig. 7.35? Answer Q2 12. Why do you push an object closer to scissors (fulcrum) when you want to cut an object which is hard? Answer Q3 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. Answer Q1 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. Answer Q2 2. Fill in the blanks. (i) Work done = ______ × ______ (in the direction of force). (ii) 1 joule of work is done when a force of ______ newton displaces an object by 1 metre in the direction of the force. (iii) The expression for kinetic energy of a body of mass m and velocity v is ______. (iv) The potential energy of an object of mass m at a small height h from the Earth’s surface is ______. (v) Power is defined as the ______ at which work is done. Answer Q3 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. Answer Q4 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. Answer Q5 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? Answer Q6 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. Answer Q7 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. Answer Q8 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. Answer Q9 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. Answer Q10 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). Answer Q11 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? Answer Q12 12. The gravitational attraction on the surface of the Moon (lunar surface) is about 1/6 th 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? Answer Q13 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? Answer Q14 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. Answer Q15 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. Answer
In science, work is done only when a force produces:
A A loud sound B A change in colour C Heat D A displacement along its direction
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