1Revise, Reflect, Refine15 questions
Q.1State 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.v
Answer:(i) False (ii) True (iii) True (iv) False (v) True.
Q.2Fill 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.v
Answer:(i) Force; displacement (ii) 1 (iii) 1/2 mv² (iv) mgh (v) rate.
Q.3When 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.v
Q.4For 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.v
Answer:Truck uphill: chemical energy to kinetic and gravitational potential energy. Watch spring: elastic potential to kinetic. Photosynthesis: light to chemical. Water from dam: gravitational potential to kinetic/electrical. Matchstick and firecracker: chemical to heat, light and sound. Microphone: sound to electrical. Bulb: electrical to light and heat. Solar panel: light to electrical.
Q.5A 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?v
Answer:(i) 36,250 J (ii) 36,250 J (iii) potential energy does not depend on the path.
Q.6A 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.v
Answer:Energy required is twice the initial energy; power required is the same as initially.
Q.7Which 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.v
Answer:Energy depends on the flag’s weight and height raised, plus losses. Raising slowly or quickly does not change ideal work against gravity. If the same work is done in half the time, power doubles.
Q.8kg rides a scooter of mass 100 kg. He accelerates the scooter to a velocity v. 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.v
Q.9On 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.v
Answer:For balance, moments about the fulcrum are equal. Since the adult weighs twice the child, the adult should sit half as far from the fulcrum as the child, e.g. child at 2 m and adult at 1 m on opposite sides.
Q.10kg is thrown up with a velocity of 20 m s–v
Answer:(i) Negative during upward motion and positive during downward motion. (ii) –12 J.
Q.11A 10.0 kg block is moving on horizontal floor with negligible friction. As shown in the Fig. 7.37, a variable force is applied (N) 50 on the block in its direction of motion from its position at m till 4 m. 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? 0 1 2 3 4v
Answer:6 m s⁻¹; √66 m s⁻¹; No.
Q.12The gravitational attraction on the surface of the Moon (lunar Displacement (m) 1 Fig. 7.37 surface) is about th of that on the surface of the Earth. 6 m from the surface of the Earth. How far up will the ball thrown with the s–1) A B same upward velocity travel from the surface of the Moon? 35 (mv
Q.13A 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?v
Answer:Between A and B the car moves with constant speed. At A, kinetic energy is 612,500 J. Work done by brakes from B to C is –612,500 J. The kinetic energy becomes mainly heat, sound and internal energy in the brakes, tyres and road.
Q.14The 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.v
Answer:At P: 2√10 m s⁻¹; at Q: 0 m s⁻¹; the ball cannot reach R.
Q.15A 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.v
Answer:(i) 10√2 m s⁻¹ (ii) 0.05 m.