Answers to KE

Kinetic energy

1.       Calculate the kinetic energy of

 A a 1500 kg car moving at 20 m/s 300 000J (3 x 10⁵J)

B  a 3000 kg van moving at 10 m/s 150 000J (1.5 x10⁵J)

C a 10 kg shell fired at 500 m/s. 1 250 000J (1.25 x 10⁶ J)

D the 90 000 kg space shuttle landing at 100 m/s. 450 000 000J (4.5 x 10⁸J)

 

2.     You throw a ball straight up into the air.

(a) What happens to the ball’s kinetic energy as it rises? falls

(b) What is the ball’s kinetic energy at its highest point? 0

(c) What happens to the ball’s kinetic energy as it falls? increases

(d) When does the ball have maximum kinetic energy? The instant it hits the ground

 

3.     A car, a lorry and a motor bike are travelling side by side at 25m/s. Explain which has the most kinetic energy. Lorry as it has the greatest mass

 

4.     An empty lorry has a mass of 3 tonnes. It travels at a steady speed of 15m/s.

(a)   Calculate the lorry’s kinetic energy  ½ x 3000 x15² = 135 000J

(b)   What would its KE be if its mass were 6 tonnes and it still travelled at 15m/s? ½ x 3000 x15² = 270 000J (c) What will happen to the 3 tonne lorry’s kinetic energy if its speed goes up from 15 m/s to 30m/s? ½ x 3000 x30² = 1 350 000J

 

5.     A 800kg car travels at a steady 35m/s. Calculate its kinetic energy. ½  x 800 x 352 = 490 000 J

 

6.     An air rifle pellet has a mass of 10 g. It travels at 120m/s.

(i)            Write down the pellet’s mass in kilograms. 10/1000 = 0.01kg

(ii)           Calculate the pellet’s kinetic energy ½ x 0.01 x 120² = 72J

 

7.     A football has a mass of 800 g. Its speed is10m/s. Calculate its kinetic energy ½ x 0.8 x 10² = 40J

 

8.     A car has a mass of 750 kg and a velocity of 15m/s. (a)Calculate its KE. ½ x 750 x 15² = 84375 J (b) What is the work done by the brakes to stop the car 84375 J The brakes exert a maximum force of 5600 N. (c) Calculate the distance travelled before the car stops. Work = F x d, so, d = work / F = 84375/ 5600 = 15m(d) Repeat this for a speed of 30m/s KE = ½ x 750 x 30² = 337 500 J               Work = F x d, so, d = work / F = 337 500/ 5600 = 60.3m