MOVEMENT AND CHANGE

1
MOVEMENT AND CHANGE

• Calculating Kinetic Energy

2
Kinetic Energy

• A running elephant has more kinetic energy than a
  running man, because it has more mass.

3
Kinetic Energy

• A racing car has more KE than a family car
  because it has higher speed.

4
Kinetic energy

• Kinetic energy depends on the mass and the
  velocity of an object.

5
Kinetic energy

• The formula for KE is
• Kinetic energy ½ x mass x velocity squared
• (joules) (kg) (m/s)2

6
Kinetic energy

• KE ½ mv2

7
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

8
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?

9
Kinetic energy

• This is a tricky one for A students!

10
Kinetic energy

• This is a tricky one for A students!
• Remember energy is conserved!

11
Kinetic energy

• This is a tricky one for A students!
• Remember energy is conserved!
• gravitational energy at the top KE at the
  bottom

12
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 speed2

13
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 speed2

14
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 speed2
• 10 x 45 ½ x speed2

15
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 speed2
• 10 x 45 ½ x speed2
• speed2 10 x 45 x 2

16
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 speed2
• 10 x 45 ½ x speed2
• speed2 10 x 45 x 2
  900
• speed on impact 30 m / s

17
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!

18
Speed of car (m/s) Drivers thinking distance (m) Cars breaking distance (m) Total stopping distance Total stopping distance
Speed of car (m/s) Drivers thinking distance (m) Cars breaking distance (m) 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