Tutorial 9 Energy, Work & Power
- A head-on collision between two objects results in these two objects coming to rest
completely.
The following are descriptions of possible candidates for this collision. Say whether each is a possible or impossible situation which fulfils the condition. In each case give
fully detailed reasons.
- Two perfectly elastic balls of the same mass arriving with equal but opposite velocities.
- A collision between two objects which results in destruction of 75% of the total
kinetic energy.
- A collision between an object of mass 5 kg and one of 15 kg.
- A collision between an object of momentum 5 kg ins-1 and one of —7.5 kg ms-1.
- A collision between an object of kinetic energy 17 J and one of kinetic energy 27
- A bullet is fired vertically upwards with a speed of 120 ins-1. What is the maximum
height it can reach?
- The Niagara Falls are about 50 m high. It is estimated that 100 million kilograms of
water pour over the falls every second. If 50% of this energy could be harnessed how much power would be available?
- In loading a lorry a man lifts boxes each of weight 100 N through a height of 1.5 in.
- How much work does he do in lifting one box?
- If he lifts 4 boxes a minute at what power is he working?
- The pendulum of a clock passes its lowest point at a velocity of 0.7 ms-1.
Calculate the height above the lowest point which the pendulum will reach.
- When you learn to drive you need to be able to judge your stopping distance. This is
made up of 2 parts. Your thinking distance where you keep travelling at your original speed while you react to a problem (a slow reaction time would be 0.7 seconds). Then your braking distance where the brakes act to remove the car’s kinetic energy.
Speed (ms-1) | 10 | 20 | 30 | 40 |
Thinking distance (m) | 14 | |||
Braking distance (M) | 10 | 90 | ||
Total distance (m) | 188 |
- Assume your reaction time is 0.7 seconds. Complete the thinking distance values.
- Suppose you and your car have a mass of 1 200 kg. When the brakes are applied
carefully a force of 6 000 N is applied to the wheels.
- Using the idea that the brakes do work to remove kinetic energy, show that
the car’s braking distance, at an initial speed of 10 ms-1, is 10 m.
- Complete the rest of the table.
- If three passengers are given a lift, the mass increases to 1 500 kg. What
difference does this make to your total stopping distance?
- What difference does it make to your stopping distance if you are travelling
downhill. Explain your answer in terms of energy changes.
- A crate is pushed 10 m along a horizontal surface by a force of 80 N. The frictional force
opposing the motion is 60 N.
- Calculate the total work done.
- Calculate the heat energy produced.
- Calculate the kinetic energy gained by the crate.