Energy Changes in a System (AQA GCSE Physics)

Figure 2 below shows a student doing a bungee jump from a stationary platform.

The bungee cord has an unstretched length of 30.0 m.

Figure 2

The mass of the student is 60.0 kg.

The gravitational field strength is 9.8 N / kg.

Write down the equation which links gravitational field strength (g), gravitational potential energy (E_p), height (h) and mass (m).

Using the equation from 1(a), calculate the change in gravitational potential energy from the position where the student jumps to the point 30.0 m below.

Give your answer to 3 significant figures.

75% of this change in gravitational potential energy is transferred to the student’s kinetic energy store.

By how much does the student’s kinetic energy store increase after falling 30.0 m?

Kinetic energy gained = ________________________ J

Calculate the speed of the student after falling 30.0 m.

Give your answer to two significant figures.

What is meant by specific heat capacity?

Quenching is a process used to change the properties of steel by rapidly cooling it.

During the quenching process, the steel is heated to a very high temperature and then placed in a container of cold water.

A steel rod is quenched by heating it to a temperature of 815 °C before placing it in cold water.

The mass of the steel rod is 15 kg.

The final temperature of the rod and water is 60 °C.

Calculate the energy transferred from the steel rod to the water.

Use the correct equation from the Physics Equation Sheet.

Specific heat capacity of steel = 490 J/kg °C.

Give the answer to two significant figures.

The temperature of the steel rod eventually returns to room temperature.

Compare the energies and movement of the particles in the steel rod and in the air at room temperature.

Figure 1 shows a person on a zip wire. The person's weight is 650 N.

The person has 15 000 J in their gravitational potential store at the top of the tower.

Determine the vertical height of the tower.

Give your answer to 3 significant figures.

   Vertical height of tower (3 significant figures) = ...................... m

The height of the zip rope at the end of the ride is 2.5 m above ground.  

Determine the change in gravitational potential energy over the course of the zip rope ride.

Determine the mass of the rider.

Give your answer to the nearest kg.

Write down an expression relating energy in the kinetic store () and energy in the gravitational potential store ().

State the amount of energy in the rider's kinetic store when the rider is at the end of the zip rope. 

You can assume frictional forces to be negligible.

Figure 1 shows a cyclist riding down an incline.

Complete the sentences. 

Choose answers from the box. 

Each answer can be used once, more than once or not at all.

As the cyclist rides down the hill, the amount of energy in his .............................. store decreases.

As the cyclist accelerates, energy in his .............................. store increases.

 When the cyclist uses his breaks to stop, energy is transferred to the .............................. store of the brake pads.

In one journey, the cyclist applies his brakes.

The break pads increase in temperature from 13°C to 51°C.

The material of the break pads has a specific heat capacity of 517 J/kg °C.

The mass of the break pads is 0.043 kg

Calculate the change in thermal energy of the break pads.

Give your answer to 3 significant figures.

   Change in thermal energy (3 significant figures) = .................... J

The cyclist descends the hill on another journey, this time without braking.

The hill the cyclist rode down has a vertical height of 23.5 m.

20 kJ of energy was transferred from the cyclist's gravitational potential energy store as he rode from the top of the hill to the bottom.

Determine the mass of the cyclist.

Give your answer to 3 significant figures.

   Mass of cyclist (3 significant figures) = .................................... kg

Calculate the maximum speed of the cyclist during his descent.

Assume now that there are no energy losses.

Give your answer to 1 significant figure.

   Maximum speed (1 significant figure) = ............................... m/s

A group of students carry out an investigation into the specific heat capacity of water. 

Figure 1 shows the equipment the students use.

Describe a method the students could use to determine the specific heat capacity of water.

State the purpose of insulating the beaker of water.

The students noticed that when the power supply was turned on, the water did not immediately begin to heat up.

Suggest one reason why.

The energy transferred to the heater in 600 s was 36 050 J.

The temperature of the water increased by 17°C.

The specific heat capacity of water is 4200 J/kg °C.

Determine the mass of water the students used.

   Mass of water = .................................... kg

The diagram in Figure 1 below shows a wind turbine.

Figure 1

The relationship between the power output of the turbine and the kinetic energy of the air passing the blades each second is directly proportional. 

Describe the change to the power output when the wind speed is reduced by half.

During a moderate breeze, the average mass of air passing the wind turbine each minute is 14 000 kg.

 The average speed of the wind is 7.6 m/s.

Calculate the average power output of the wind turbine.

You can assume that the process is 100% efficient.

 Average power output = .................................... W 

Figure 3 shows a rollercoaster.

Figure 3

The rollercoaster car is raised to a height of 40m in 35 seconds by an electric motor.

The mass of the rollercoaster and passengers is 550 kg.

The motor has a power rating of 10 000 W.

Gravitational field strength = 9.8 N / kg. 

Calculate the efficiency of the motor.

 Efficiency = ....................................

The CPU (Central Processing Unit) in a desk-top computer controls most of the functions of the computer. 

Modern CPUs generate large amounts of heat. 

Specially designed cooling units such as the one shown in Figure 1 are used to dissipate this heat away from the CPU.

This is to prevent it from overheating and being damaged.

Energy is transferred from the CPU through the metal base of the cooling unit

Explain how this process occurs.

The CPU cooling unit has metal fins that are designed to transfer energy quickly to the surroundings. 

State two features that would help the metal cooling fins to transfer energy quickly to the surroundings.

Salmon are born in freshwater rivers, spend most of their lives in the sea, but return to freshwater rivers to breed. As they swim up rivers, they often have to jump up waterfalls.

Figure 1 shows a salmon attempting to jump up a waterfall with a vertical height of 0.36 m.

Figure 1

The salmon has a mass of 1.84 kg.

The salmon leaves the water with a speed of 3.2 m/s.

Determine whether the salmon can make the height required for the jump.

Another salmon of mass 2.2 kg tries to make the jump. 

Calculate the minimum speed with which the salmon would have to leave the water in order to reach the height required.

   Minimum speed  = .................................... m/s

The salmon swim further upstream and encounter a waterfall that is much higher than the previous one.

The salmon attempts the jump, but fails, and performs a somersault during the descent. The salmon straightens out before it enters the water as shown in Figure 2.

Figure 2

Discuss whether the speed of entry into the water is greater than, less than or equal to the speed with which it leaves the water. Ignore any effects of air resistance.

The temperature of the water at the bottom of a waterfall is greater than the temperature of the water at the top. 

Suggest why this is the case.

Figure 1 shows a student at two stages of a bungee jump.

Complete the sentences. Choose answers from the box. 

Each answer can be used once, more than once or not at all.

Before the student jumps, they have energy in their .............................. store.   

As the student falls, energy is transferred to their  .............................. store.   

At the bottom of the jump, the bungee cord stretches and energy is transferred to the .............................. store of the rope.

At the lowest point of the jump, the student is momentarily stationary. At the lowest point of the jump, the length of the bungee cord is 82 m. The unstretched length of the cord is 40 m.

The bungee cord acts like a spring with a spring constant of 45 N/m.

Calculate the energy stored in the bungee chord at the lowest point of the jump.

Include the correct unit in your answer.

Use the correct equation from the Physics Equations Sheet.

  Energy = ................................. Unit = ..............................

Determine the mass of the student. 

You may assume there are no energy losses in the system.

Determine the speed of the student at the point in the jump where the bungee cord is at its maximum unstretched length. 

You can assume air resistance is negligible.