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 mv 2

Kinetic energy Example 1 An elephant of mass 2000 kg travelling at 5 m/s has KE = ½ x 2000 x 5 x 5 = 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 = ½ x mass x speed 2 10 x 45 = ½ 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 10 x 45 = ½ x speed 2 speed 2 = 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 = ½ x mass x speed 2 10 x 45 = ½ x speed 2 speed 2 = 10 x 45 x 2 = 900 speed on impact = 30 m / s

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