8/12/2019 Physics 4 O Level

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8/12/2019 Physics 4 O Level

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iv. explain why the ball does not reach its original height when it bounces upagain i. gravitational potential energy = mgh = 3 x 10 x 5 = 150J ii. The kinetic energy of the ball on hitting the ground is equal to the ball'soriginal gravitational potential energy so the kinetic energy of the ball on

hitting the ground = 150J If the ball hits the ground with speed v, 1/2 mv2 = 150  v2 = (150 - 2)/3 = 100  v = 10ms-1 iii. The kinetic energy of the ball on leaving the ground is equal to itsgravitational potential energy on rising to its maximum height, that is 3m. Thegravitational potential energy of the ball 3m above the ground = 3 x 10 x 3 =90 J The kinetic energy of the ball leaving the ground = 90 J  If the ball leaves the ground with speed v, 1/2 mv2 = 90v2 = (90 x 2)/3  v = 7.746ms-1 iv. Because part of its kinetic energy is changed into other forms of energy likesound and heat when it hits the ground Eg. A pendulum bob of mass 0.5kg is moved sideways until it has risen by a vertical height of 0.2m. Calculate the speed of the bob at itsi. highest point ii. lowest point i. at the highest point, the kinetic energy of the bob = 0 if the speed of the bob at its highest point is v, 1/2 mv2 = 0 1/2 x 0.5 x v = 0  v2 = 0  v = 0 ii. according to the principle of conservation of energy, the kinetic energy of atthe lowest point is equal to the gravitational potential energy at the highestpoint. If the speed of the bob at its lowest point is v, 1/2 mv2 = mgh  v2 = 2 x 10 x 0.2 = 4  v = 2 m/s Power and efficiency  •  Power is defined as the rate of doing work  •  Power = work done/time taken •  SI unit is watt (W) 

8/12/2019 Physics 4 O Level

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•  Efficiency is the ratio of useful output energy to the total input energy orthe ratio of useful power to the total input power. •  Efficiency = (useful output energy / input energy) x 100% Eg. A crane can lift a 200kg mass through a vertical height of 5m in 4s.Calculate i. the power output of the motor driving the crane ii. the efficiency of the motor if the power input is 5kW  i. power output = work done/time taken = (200 x 10 x 5)/4 = 2500W  ii. efficiency of motor = (power output/power input) x 100% = (2500/5000) x100% = 50% Friction •  The net force that slows down moving objects •  Acts in the opposite direction of motion of objec 1. Static friction •  related to objects which are not moving. •  amount of force applied = amount of friction 2. Moving friction •  applied force does not affect friction •  it can be affected by surface or sudden change in mass  Advantages of friction •  enables walking •  brakes of vehicles Disadvantages •  reduce efficiency of machinery  •  energy wasted as heat Methods to reduce friction •  lubricants •  ball bearings