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Solved problems for the chapter:

Power

View the theory »

Power for A-Levels

1. Power in a linear motion

2. Power in rotational motion

2.1. Pulley

3. Power and energy

4. Forum

↓ Show theory [1] ↓

Detailed solution steps are available at:

https://en.openprof.com/wb/chapter:power/5050/

Example 1.

Which statements are correct?

The acceleration of a car is determined by measuring the time it takes for the car to accelerate from 0 to 100 . The more powerful the engine is, the more time the car needs to accelerate.
If we want to shorten the time of doing the work, we must do the work with more power.
Two machines worked for one hour. If the first machine did more work during this time than the second, then, the first machine worked with more power.
A machine that works at a higher power always does more work than one that works at a lower power.
If two locomotives pull wagons with the same amount of force, the locomotive with higher power will have a higher speed.
An elevator with more power can lift a heavier load.

Example 2.

Solve the following problems without calculation, only by reasoning:

If we have two identical cars with different engine powers. The first has a maximum power of 100 kW and the second has a maximum power of 180 kW. Which of them can accelerate faster from 0 to 100 ?
If we have two cranes at a construction site. The first lifts a load of 2 tons while the second lifts a load of 3 tons. Which of the cranes will lift the load to the top of the building faster if they work with the same power?
Two machines can do the same job. The first machine does the job in 3 hours and the second machine does the same job in 4 hours. Which machine works with more power?

Calculate the power of a device when it does 900 J of work in 20 seconds.

An electric drill performs 270 kJ of work in 15 minutes while drilling. Calculate the power with which the drill works.

How much work does a motor do in 1 hour if it runs at 7.5 kW?

How long does a 420 W machine have to work to do 630 kJ of work?

A car drives uphill and accelerates while driving. In doing so, its potential energy increases by 660 kJ in 20 seconds, and its kinetic energy by 100 kJ. At what average power did the engine run for those 20 seconds?

A student pushes a cart with a force of 28 N for 20 seconds. The cart moves uniformly and travels a distance of 15 m. Calculate the power used by the student.

When we chop cooked vegetables with an electric blender, the blender is set to the first level and does 5 kJ of work in 20 seconds. When chopping hazelnuts, the electric blender is set to the maximum speed at which the power is three times as much as that in the first level.

With how much power does the blender chop the cooked vegetables?
How long will it take to chop the hazelnuts if the blender has to do 90 kJ of work?

10.

A swimmer swam for 4.5 minutes with a power of 60 W. How far did he swim if he pushed the water with an average force of 540 N?

11.

How long does it take a car to cover a distance of 6 km if it moves uniformly in a straight line with a power of 50 kW? 2.5 kN of frictional force is applied to the car while driving.

12.

A boy weighing 40 kg climbs a pole. How high does he climb in 28 seconds if he climbs with an average power of 50 W?

13.

Determine the correctness of the statement:

An electric machine that does the same job in less time has more power.
The unit of electrical power is joules per second [J/s].
An electric drill that has more power will do more work.
Two bulbs are connected to the same voltage. The current through the first bulb is twice as much as the current through the second bulb, so the first bulb has twice the power than the second.
The power of the electric furnace is greater with a longer heating wire.

14.

The car travelled in a straight line for 6 minutes. At what speed was the car travelling if the braking force was 2.1 kN at all times and the engine power was 42 kW?

15.

An electric stove is connected to a voltage of 230 V. A current of 6 A flows through the stove.

How much power does the electric stove take from the voltage source?
What does this power turn into?

16.

A lady is cooking lunch. She uses a 3 kW stove for two hours and a 800 W stove for four hours. What will be the total cost of the electrical energy needed to prepare lunch if 1 kWh costs 6 cents?

17.

An electric saw was used on a board. The teeth of the saw acts on the board with a force of 1600 N. The board moves under the saw at a speed of 0.1 m / s. What is the power of the saw motor?

18.

An elevator transports passengers to a height of 40 m in 20 s. If the total mass of the passengers on the lift is 800 kg, what is the power of the elevator's electric motor?

19.

A train is moving at a constant speed of with a power of . What is the traction force of the locomotive of the train?