topic 8 Flashcards
a cyclist is riding a bicycle at a steady velocity of 12 m/s. the cyclist and bicycle have a total mass of 68kg. calculate the kinetic energy of the cyclist and bicycle using
ke = 1/2 x m x v^2
1/2 x 68 x 12^2
= 4900
describe the energy transfer that happens when the cyclist uses the brakes to stop
store of kinetic energy decreases and transferred into thermal energy store of surroundings
the cyclist starts to cycle again. does 1600J of useful work to travel 28m. calculate the average force the cyclist exerts
1600 = force x 28
f = 1600/28 = 57
explain what the display shows about the average power of the athlete in each of these two sessions
over the same time (300s) more work is done in session 1 than session 2 therefore more power developed in session 1
the mass of the aeroplane is 750kg. calculate the change in gravitational potential energy of the aeroplane as it descends from 1300m to the ground
gravity = 10n/kg
750 x 10 x 1300
= 9,800,000
the aeroplane is powered by an engine that burns fuel. the fuel supplies a total of 6500kJ of energy every minute. the efficiency of the engine is 0.70
calculate the power output of the engine in kW
efficiency = useful output / input
output = 0.70 x 6500
power = energy / time
= 76
explain why the efficiency of the engine is less than 1
useful output energy is less than input energy. some energy is transferred to less useful forms
which of these is the equation for work done?
A) work done = force / distance moved in direction of force
B) work done = force X distance moved in direction of force
C) work done = force / distance moved at right angles to direction of force
D) work done = force X distance moved at right angles to direction of force
B) work done = force X distance moved in direction of force
a ball has a mass of 0.046kg
calculate the change in gravitational potential energy when the ball is lifted through a vertical height of 2.05m. using
gpe = m x g x h
gpe = 46 x 10 x 2.05
= 0.94
the ball is released. calculate the kinetic energy of the ball when the speed of the ball is 3.5m/s
ke = 1/2 x m x v^2
= 1/2 x 46 x 35^2
= 0.28
use figure 4 to estimate the maximum height that’s the ball reaches after the first bounce
0.8
explain why the ball doesn’t bounce back to its starting height of 2.05m
the ball has lost thermal energy as it was dissipated to the surroundings
a different drone has a mass of 4.5kg with height 20m
calculate the change in gravitational potential energy when the drone rises through a height of 20m
gravity = 10
4.5 x 10 x 20
= 900
state the amount of useful work done by the blades as the drone rises through 20m
900
it takes 4s for the drone to rise through 20m. calculate the useful power developed by the blades in this time of 4s
p = work done / time taken
900 / 4 = 225
