🔍 Detailed Explanation of Work Done and Energy Transfer in Year 11 Physics
Understanding the concepts of work done and energy transfer is essential in Year 11 Physics as part of the UK National Curriculum. These topics help explain how forces cause changes in energy, which is a fundamental idea in physics.
⚙️ What Is Work Done?
Work done in physics means transferring energy by applying a force that moves an object. In simple terms, work is done when a force causes displacement. The basic definition is:
Work Done (W) = Force (F) × Distance (d) moved in the direction of the force
- Work done (W) is measured in joules (J).
- Force (F) is measured in newtons (N).
- Distance (d) is measured in metres (m).
This formula tells us that if you push or pull something and it moves, you have done work on it. However, no movement means no work done, even if force is applied.
📐 Formula and Units for Work Done
\[
W = F \times d
\]
You must ensure the force and distance are measured in the same direction. If the force is not fully along the line of movement, we use the component of the force in the direction of displacement:
\[
W = F \times d \times \cos(\theta)
\]
where \( \theta \) is the angle between the force and the direction of motion.
📝 Examples of Work Done
- Pushing a box across the floor: If you apply a force of 10 N and the box moves 5 m, the work done is:
\[
W = 10\, N \times 5\, m = 50\, J
\] - Lifting a book: To lift a book upwards against gravity, work is done to increase its gravitational potential energy.
🔄 Energy Transfer
Work done always involves energy transfer. Energy can be transferred when work is done on an object, causing its energy to change form—such as kinetic, potential, or thermal energy.
For example:
- When you lift an object, mechanical work is done against gravity, transferring energy to the object as gravitational potential energy.
- If you push a car and it moves, your work increases the car’s kinetic energy.
- When friction acts during work, some energy is transferred as thermal energy (heat).
🔗 Relationship Between Work Done and Energy Transfer
Work done on an object results in energy being transferred to or from the object. This means:
- Positive work (force and displacement in the same direction) transfers energy to the object.
- Negative work (force opposite to displacement) takes energy away from the object.
This concept is crucial when solving Year 11 physics problems related to energy conservation and efficiency.
By learning these definitions, formulas, and examples, students at Key Stage 4 can confidently understand how work done and energy transfer relate to forces and motion in everyday situations. Remember always to check the direction of forces when calculating work and think about the energy changes involved.
❓ 10 Examination-Style 1-Mark Questions with 1-Word Answers on Work Done and Energy Transfer
- What is the unit of work done?
Answer: Joule - What type of energy is stored in a stretched spring?
Answer: Elastic - Which quantity is calculated by force multiplied by displacement?
Answer: Work - What is the formula symbol for work done?
Answer: W - What is transferred when work is done on an object?
Answer: Energy - What type of energy does a raised object possess?
Answer: Gravitational - What is the symbol for force in physics equations?
Answer: F - What do you call energy changes due to movement?
Answer: Kinetic - What is the term for energy transfer by heating?
Answer: Thermal - Which vector quantity direction affects work done?
Answer: Displacement
✏️ 10 Examination-Style 2-Mark Questions with 1-Sentence Answers on Work Done and Energy Transfer
- Question: Define work done when a force moves an object.
Answer: Work done is the product of the force applied and the distance moved in the direction of the force. - Question: State the unit of work done in the SI system.
Answer: The unit of work done is the joule (J). - Question: Explain what energy transfer occurs when a ball is thrown upwards.
Answer: Chemical energy from the muscles is transferred to kinetic energy and then to gravitational potential energy as the ball rises. - Question: Calculate the work done when a force of 10 N moves an object 3 metres in the direction of the force.
Answer: Work done = force × distance = 10 N × 3 m = 30 J. - Question: Describe the energy transfer in a moving car that suddenly applies brakes.
Answer: Kinetic energy is transferred to thermal energy due to friction between the brakes and wheels. - Question: Why is no work done if a force is applied but the object does not move?
Answer: Because work requires displacement in the direction of the force, no movement means no work done. - Question: State the relationship between work done and energy transfer.
Answer: Work done on an object results in energy being transferred to or from the object. - Question: What type of energy is stored in a stretched spring?
Answer: Elastic potential energy. - Question: Explain why energy is conserved in a closed system during energy transfer.
Answer: Because energy cannot be created or destroyed, only transformed, so total energy remains constant. - Question: How much work is done when a 5 N force moves an object 4 m at 90° to the force direction?
Answer: No work is done because the displacement is perpendicular to the force direction.
📝 10 Examination-Style 4-Mark Questions on Work Done and Energy Transfer with Detailed Answers
Question 1:
Explain what is meant by ‘work done’ in physics and how it relates to energy transfer.
Answer:
Work done in physics is defined as the energy transferred when a force moves an object over a distance. It occurs when a force causes displacement in the direction of the force. The formula to calculate work done is Work = Force × Distance × cosθ, where θ is the angle between force and displacement. Work done transfers energy from one object to another or changes the energy of the object. For example, pushing a box causes work to be done on the box, transferring energy to it and increasing its kinetic energy. Therefore, work done is a way energy is transferred or transformed in systems.
Question 2:
A person lifts a 10 kg box vertically upwards by 2 metres. Calculate the work done against gravity. (Take g = 9.8 m/s²)
Answer:
The force required to lift the box is equal to its weight, which is mass × gravitational field strength. So the force is 10 kg × 9.8 m/s² = 98 N. The distance moved in the direction of the force is 2 metres upwards. Work done against gravity is calculated as Work = Force × Distance = 98 N × 2 m = 196 J. This work done transfers energy to the box as gravitational potential energy. Hence, the total work done in lifting the box is 196 joules.
Question 3:
Describe the energy transfers taking place when a moving car comes to a stop due to braking.
Answer:
When a moving car brakes, the kinetic energy of the car decreases. This kinetic energy is transferred mainly into thermal energy due to friction between the brake pads and the wheels. The brakes heat up, which can be felt after heavy braking. Some energy may also be lost as sound energy during braking. No energy is destroyed; it is transferred from the car’s kinetic energy to other forms of energy. Thus, work done by the brakes results in an energy transfer causing the car to slow down.
Question 4:
Why does the work done by a force not always result in the movement of an object?
Answer:
Work done requires force and displacement in the direction of the force. If the object does not move, the displacement is zero. Since work is Force × Distance, zero displacement means no work is done. For example, pushing against a wall with great force but no movement results in no work done on the wall. The energy used by the person pushing may be lost as heat in their muscles but is not transferred to the wall. Therefore, no energy transfer to the object occurs if there is no displacement.
Question 5:
A force of 50 N pulls a crate 3 metres across a floor at an angle of 30° to the horizontal. Calculate the work done by the force.
Answer:
The component of the force in the direction of the movement is important. This is calculated using cosθ: 50 N × cos 30° = 50 × 0.866 = 43.3 N. The distance moved is 3 metres, so work done = Force component × Distance = 43.3 N × 3 m = 129.9 J. This means 129.9 joules of work are done by the force in moving the crate. The angle reduces the effective force doing work on the crate. Therefore, only the horizontal component of the force contributes to work done.
Question 6:
Explain how energy transfers occur when a spring is stretched by applying a force.
Answer:
When a spring is stretched, work is done on it by the applied force. This work transfers energy to the spring by increasing its elastic potential energy. The spring stores this energy as it is deformed from its original length. If the force is removed, the stored energy can be converted back to kinetic energy or work done on other objects. The amount of energy stored depends on the extension and the spring constant. Thus, stretching a spring transfers mechanical energy into elastic potential energy.
Question 7:
What is the difference between kinetic energy and gravitational potential energy in terms of energy transfer?
Answer:
Kinetic energy is the energy an object has due to its motion, while gravitational potential energy is due to an object’s position in a gravitational field. When an object is raised, work is done to transfer energy into gravitational potential energy. When the object falls, this potential energy is transferred into kinetic energy as the object speeds up. Both forms are mechanical energy but represent different types of energy stored or transferred. Energy transfer between these forms happens continuously in many physical systems involving motion and height changes.
Question 8:
Calculate the kinetic energy of a 2 kg ball moving at 6 m/s.
Answer:
Kinetic energy (KE) is given by the formula KE = ½ mv². The mass (m) is 2 kg and the velocity (v) is 6 m/s. Substituting these values: KE = ½ × 2 × 6² = 1 × 36 = 36 J. Therefore, the ball has 36 joules of kinetic energy. This energy represents the work the ball can do due to its motion. It is an example of energy being transferred through movement.
Question 9:
Explain why decreasing the speed of a moving vehicle reduces its kinetic energy.
Answer:
Kinetic energy depends on the square of the speed, given by KE = ½ mv². When speed decreases, kinetic energy decreases more significantly because of the square term. For example, halving the speed reduces kinetic energy by a factor of four. This means the vehicle has less energy available to do work, such as moving or overcoming resistive forces. Decreasing speed reduces the energy stored in the vehicle’s motion. Energy is transferred out of the system mainly through work done against friction or brakes.
Question 10:
Describe how energy is transferred when a cyclist pedals uphill.
Answer:
When a cyclist pedals uphill, chemical energy from the cyclist’s muscles is transferred to mechanical energy. The muscles exert a force on the pedals, doing work that transfers energy to the bicycle and cyclist. This energy increases the cyclist’s gravitational potential energy as they move to a higher position. Some energy is also lost as thermal energy due to friction between the bike’s moving parts and air resistance. Overall, pedalling uphill transfers chemical energy into mechanical and gravitational potential energy. This is an example of energy transfer through work done against gravity.
📝 10 Examination-Style 6-Mark Questions with 10-Sentence Answers on Work Done and Energy Transfer
Question 1
Explain how work done is related to energy transfer when a person lifts a box from the ground to a shelf.
Answer:
When a person lifts a box, they apply a force upwards against gravity. The box gains gravitational potential energy because it is moved to a higher position. Work done is calculated by multiplying the force applied by the distance moved in the direction of the force. Here, the force equals the weight of the box, and the distance is the height of the shelf. The energy used to do the work is transferred from the person to the box. This transfer of energy increases the box’s potential energy. If the box weighs 10 Newtons and is lifted 2 metres high, the work done is 20 Joules. This means 20 Joules of energy have been transferred to the box. No energy is destroyed; it just changes from chemical energy in muscles to potential energy in the box. The process illustrates the law of conservation of energy.
Question 2
Describe the energy transfers when a car accelerates from rest to a certain speed.
Answer:
When a car accelerates, chemical energy from the fuel is converted into kinetic energy. The engine does work on the car, applying a force that increases its velocity. The work done by the engine transfers energy to the car’s moving parts. As the car speeds up, its kinetic energy increases according to the formula \( \frac{1}{2}mv^2 \). Some energy is lost as heat and sound due to friction and engine noise. The tyres also do work against the road, providing traction for motion. Energy transfer continues until the car reaches the desired speed or runs out of fuel. The work done must be enough to overcome resistive forces such as air resistance. Thus, accelerating involves work done to increase the car’s kinetic energy and overcome resistance. Energy is conserved, changing forms during the process.
Question 3
Calculate the work done when a force of 50 N moves a crate 3 m along the floor.
Answer:
Work done is calculated using the formula: work done = force × distance moved in the direction of the force. Given the force is 50 Newtons and distance is 3 metres, the work done is: 50 N × 3 m = 150 Joules. This means 150 Joules of energy is transferred by the force to move the crate. The energy transfer happens mostly as kinetic energy of the crate. Some energy may be lost overcoming friction. The work done is a measure of the energy transferred. Units are Newton-metres, which are equal to Joules. This calculation assumes the force is applied in the same direction as the movement. If frictional force is present, more work might be needed. Hence, work done indicates the energy required for moving objects.
Question 4
Explain how power relates to work done and energy transfer.
Answer:
Power is defined as the rate of doing work or the rate of energy transfer. It tells us how quickly energy is transferred or work is done. Mathematically, power = work done ÷ time taken. If the same amount of work is done in less time, the power output is greater. Power is measured in Watts, where 1 Watt = 1 Joule per second. For example, lifting a box quickly requires more power than lifting it slowly, even if the work done is the same. Power describes how fast energy changes from one form to another. Machines with higher power can transfer energy more rapidly. In physics, power helps compare efficiency and performance. Understanding power is important for designing engines and electrical devices.
Question 5
A cyclist applies a force of 200 N to pedal a bike. If the bike moves 10 metres, how much work is done?
Answer:
The formula for work done is force multiplied by distance in the direction of the force. The force applied by the cyclist is 200 Newtons, and the distance moved is 10 metres. So, work done = 200 N × 10 m = 2000 Joules. This means the cyclist does 2000 Joules of work on the bike. This work transfers energy from the cyclist’s muscles to the bike’s motion. Some energy is lost as heat because of friction between the bike parts and air resistance. The work done increases the kinetic energy of the bike and the cyclist. This process converts chemical energy in the cyclist’s body to mechanical energy. The unit Joule measures the energy transferred by the cyclist’s effort. Calculating work done quantifies energy consumption during physical activities.
Question 6
How does friction affect the total work done by a force when sliding a box across a floor?
Answer:
When sliding a box, friction works against the direction of motion. The force applied must overcome friction to move the box. Therefore, some work goes into overcoming friction instead of just moving the box. Friction converts some energy into heat, causing energy loss. This means more work must be done by the applied force to slide the box than if friction were absent. The total work done is the sum of work to overcome friction plus work to move the box. This increases the energy required to move the box the same distance. Friction reduces the efficiency of energy transfer. The presence of friction means energy is transferred to the environment as heat. Understanding friction’s impact helps explain why machines consume extra energy in real situations.
Question 7
State and explain the principle of conservation of energy in the context of work done.
Answer:
The principle of conservation of energy states energy cannot be created or destroyed, only transferred or transformed. When work is done, energy is transferred from one form to another. For example, lifting an object increases its gravitational potential energy. The work done by the applied force equals the gain in potential energy. Energy lost to non-useful forms like heat still exists as energy. Throughout any process, total energy remains constant. This means all the work done on a system appears as energy changes in different forms. The principle ensures energy accounting is balanced in physics problems. Understanding this helps solve work done and energy transfer questions accurately. It also shows why energy efficiencies are never 100% perfect in real life.
Question 8
Describe an example where energy transfer involves kinetic energy and thermal energy due to work done.
Answer:
When braking a car, the brakes apply a force to stop the wheels. Work is done by the friction force between the brake pads and the wheels. This work transfers kinetic energy from the moving car to the brake pads. The kinetic energy decreases as the car slows down. The brake pads heat up because friction converts kinetic energy into thermal energy. This thermal energy raises the temperature of the brakes. This example shows how work done by friction causes energy transfer from kinetic to thermal energy. The total energy is conserved during the process. Energy is not lost but changes form due to work done. This explains why brakes get warm after use.
Question 9
Explain why more work is done when lifting a heavier object the same height.
Answer:
Work done is the product of force and distance moved in the direction of that force. For lifting, the force equals the weight of the object, which depends on its mass. A heavier object has a greater weight force. Lifting both objects the same height means the distance moved is constant. Therefore, work done on the heavier object is greater because the force is larger. More energy is transferred to increase the potential energy of the heavier object. This means the person lifting must supply more energy to move it. The relationship between work done and weight shows why heavier objects are harder to lift. Calculation example: work = weight × height. Greater force means more work and energy transfer.
Question 10
How can the concept of work done explain the operation of a moving elevator powered by an electric motor?
Answer:
An electric motor applies a force to lift the elevator shaft. Work done by the motor transfers electrical energy into gravitational potential energy of the elevator. The motor must apply a force equal to or greater than the elevator’s weight. As the elevator moves upwards, it gains height, so work done = force × distance. This increases the elevator’s gravitational potential energy. Some energy is lost as heat in the motor and cables due to friction. The motor needs electrical energy input to do the work lifting the elevator. The concept of work done explains how energy transfers occur in lifting systems. It shows the relationship between applied force, distance, and energy changes. This understanding helps design efficient elevator systems.
