Kinetic energy

MOVEMENT AND CHANGE Calculating Kinetic Energy Kinetic Energy  A running elephant has more kinetic energy than a running man, because it has more mass. Kinetic Energy  A racing car has more KE than a family car because it has higher speed. Kinetic energy  Kinetic energy depends on the mass and the velocity of an object. Kinetic energy  The formula for KE is: Kinetic energy = ½ x mass x velocity squared (joules) (kg) (m/s)2  Kinetic energy  KE = ½ mv2 Kinetic energy Example 1 An elephant of mass 2000 kg travelling at 5 m/s has KE = ½ x 2000 x 5 x 5 = 25 000 joules Kinetic energy Example 2 Galileo drops a stone from the leaning tower of Pisa, which is 45 metres high. A what speed does the stone hit the ground? Kinetic energy This is a tricky one – for A* students! Kinetic energy This is a tricky one – for A* students! Remember energy is conserved! Kinetic energy This is a tricky one – for A* students! Remember energy is conserved! gravitational energy at the top = KE at the bottom Kinetic energy This is a tricky one – for A* students! Remember energy is conserved! gravitational energy at the top = KE at the bottom mass X 10 x height = ½ x mass x speed 2 Kinetic energy This is a tricky one – for A* students! Remember energy is conserved! gravitational energy at the top = KE at the bottom mass X 10 x height = ½ x mass x speed 2 Kinetic energy This is a tricky one – for A* students! Remember energy is conserved! gravitational energy at the top = KE at the bottom mass X 10 x height 10 x 45 = ½ x mass x speed 2 = ½ x speed2 Kinetic energy This is a tricky one – for A* students! Remember energy is conserved! gravitational energy at the top = KE at the bottom mass X 10 x height 10 x 45 speed2 = ½ x mass x speed 2 = ½ x speed2 = 10 x 45 x 2 Kinetic energy This is a tricky one – for A* students! Remember energy is conserved! gravitational energy at the top = KE at the bottom mass X 10 x height 10 x 45 speed2 speed on impact = ½ x mass x speed 2 = ½ x speed2 = 10 x 45 x 2 = 30 m / s = 900 Kinetic Energy Road safety  The formula for KE has the speed squared.  So if a car goes at three times the speed, it has nine times the energy and so the braking distance is nine times as long! Speed of car (m/s) Driver’s thinking distance (m) Car’s breaking distance (m) Total stopping distance in metres in car-lengths 10 20 30 40 6 12 18 24 6 24 54 96 12 36 72 120 3 9 18 30