All questions of Speed and Energy for Grade 4 Exam

D

Work is done when an object moves or stops

Work is a concept in physics that refers to the transfer of energy from one object to another. It is done when a force is applied to an object and the object moves in the direction of the force. However, work is also considered to be done when an object stops moving.

Definition of work

In physics, work is defined as the product of the force applied to an object and the displacement of the object in the direction of the force. Mathematically, work (W) is calculated using the formula:

W = F * d * cos(theta)

where F is the applied force, d is the displacement of the object, and theta is the angle between the force and the displacement vectors.

Explanation of the options

a) When an object moves: When a force is applied to an object and the object moves in the direction of the force, work is done. This is because the force is causing a displacement in the object.

b) When an object is stationary: When an object is stationary, there is no displacement. Therefore, work is not done on the object in this case.

c) When an object stops: When an object is in motion and comes to a stop, work is considered to be done. This is because a force is applied to the object to bring it to a stop, causing a displacement in the opposite direction of the force.

d) A and C both: Both options A and C are correct. Work is done when an object moves and when an object stops. In both cases, there is a force applied to the object that results in a displacement.

Conclusion

In conclusion, work is done when an object moves in the direction of a force and when an object stops due to an applied force. Both scenarios involve a force causing a displacement, which is the key requirement for work to be done.

Explanation:
When we talk about work, we are talking about the energy transferred when a force is applied over a distance. In other words, work is only done when a force causes an object to move. With that in mind, let's look at the given situations:

a) A spaceship moves at constant velocity:
- If a spaceship is moving at a constant velocity, then there is no acceleration, and no net force acting upon it. Therefore, no work is being done.

b) You push on a heavy box but cannot move it:
- In this situation, you are applying a force to the box, but the box is not moving. Since there is no displacement, no work is being done.

c) A child slides down a playground slide:
- In this situation, the force of gravity is causing the child to slide down the slide. Work is being done by gravity as it is causing the child to move a certain distance.

d) You slam on the brakes and your car stops quickly:
- When you slam on the brakes of your car, the brakes are applying a force to the wheels, which in turn apply a force to the car. This force causes the car to slow down and stop. Work is being done by the brakes because they are causing the car to move a certain distance.

Therefore, the correct answer is option 'B', where no work is done when you push on a heavy box but cannot move it.

Explanation:

The correct answer is option 'D' - Biofuel. Biofuel is a type of fuel that is derived from living and dead biological organisms. It is a renewable source of energy as it is made from organic matter which can be replenished.

Here is a detailed explanation of biofuel:

Definition of Biofuel:
Biofuel is a type of fuel that is produced from organic matter, such as plants, algae, and even animal waste. It is considered a renewable source of energy because the organic matter used to make biofuel can be grown and harvested repeatedly.

Types of Biofuel:
There are several types of biofuels, including:

1. Ethanol: Ethanol is a biofuel that is made from crops such as corn, sugarcane, and wheat. These crops undergo a fermentation process to convert their sugars into ethanol. Ethanol can be used as a fuel additive or as a complete replacement for gasoline in vehicles.

2. Biodiesel: Biodiesel is a biofuel that is made from vegetable oils, animal fats, or recycled cooking grease. These feedstocks undergo a chemical process called transesterification to convert them into biodiesel. Biodiesel can be used as a replacement for diesel fuel in vehicles.

3. Biogas: Biogas is a biofuel that is produced through the anaerobic digestion of organic matter, such as agricultural waste, food waste, and sewage. The organic matter is broken down by bacteria in the absence of oxygen, producing a mixture of methane and carbon dioxide. Biogas can be used as a fuel for heating, electricity generation, and even as a vehicle fuel.

Advantages of Biofuel:
- Renewable: Biofuel is derived from organic matter, which can be grown and harvested repeatedly.
- Reduced greenhouse gas emissions: Biofuel produces fewer greenhouse gas emissions compared to fossil fuels, helping to mitigate climate change.
- Energy independence: Biofuel can be produced locally, reducing dependence on foreign oil imports.
- Job creation: The biofuel industry can create jobs in agriculture, manufacturing, and research.

Conclusion:
Biofuel is an important source of renewable energy that is derived from living and dead biological organisms. It can be produced from a variety of feedstocks and has several advantages over fossil fuels. By using biofuel, we can reduce greenhouse gas emissions, promote energy independence, and create jobs in the biofuel industry.

Understanding the Energy and Work Involved in Walking
Walking involves various forms of energy and work, primarily linked to the motion and position of the body. The correct answer is option 'D' because it encompasses the essential energies and work done by the body's weight.
1. Kinetic Energy
- Kinetic energy is the energy of motion.
- When walking, the legs and body are in constant movement, which means kinetic energy is being generated.
2. Potential Energy
- Potential energy is the energy stored due to an object's position.
- When walking uphill, for example, the body gains potential energy, which is related to its height above the ground.
3. Work Done by the Body's Weight
- The weight of the body (mass multiplied by the gravitational force) plays a significant role in walking.
- As you walk, gravitational forces act on your body, contributing to the work done against gravity when moving up or down slopes.
Why Other Options Are Incorrect
- Option A mentions mechanical energy but lacks specificity on work done by gravity.
- Option B incorrectly includes work done by external forces, which isn’t solely dependent on the body.
- Option C excludes the work done by the body's weight, which is crucial in walking dynamics.
Conclusion
In summary, option 'D' accurately captures the essential energy types and work involved in walking: kinetic energy from movement, potential energy from height changes, and work done by the body's weight against gravity. Understanding these concepts is vital for comprehending the physics of movement.

Since the acceleration is 0, the velocity must be constant. therefore the momentum of the body will also be constant.

Wheel, axle and gears is the correct answer.

When an object is lifted to a higher position, its potential energy increases. This is because it has the potential to do more work due to its elevated position.

Understanding Energy in a Moving Car
Energy plays a crucial role in the operation of a moving car. Here’s how it works:
1. Energy Types in a Car
- Cars primarily use chemical energy stored in fuel (like gasoline or diesel).
- This energy is released through combustion in the engine, converting it into other forms of energy.
2. Conversion to Kinetic Energy
- The chemical energy from the fuel is transformed into mechanical energy.
- This mechanical energy is used to turn the car’s wheels, which results in movement.
3. Kinetic Energy Explained
- When a car is in motion, it possesses kinetic energy.
- Kinetic energy is the energy of an object due to its motion, which increases with the speed of the car.
4. Why Option C is Correct
- Option C states: "It is converted into kinetic energy to move the car."
- This is correct because the energy from the fuel is not just there; it actively transforms to create motion.
5. Other Options Explained
- Option A: Energy does not keep the car stationary.
- Option B: Energy does not allow the car to remain in a fixed position.
- Option D: Energy is essential for movement, not for prevention.
Conclusion
In summary, energy is vital for a moving car as it is converted into kinetic energy, enabling the car to travel from one place to another efficiently. Understanding this process highlights the importance of energy in everyday vehicles.

Friction helps in gripping surfaces and moving effectively. It provides the necessary resistance to prevent slipping and enables control during movement.

Gravitational force is responsible for making objects fall to the ground. Gravity pulls everything towards the center of the Earth. Fun fact: On the Moon, gravity is much weaker, so you would weigh less there!

Barometer , Anemometer can't be because it is used to calculate wind speed.

Introduction:
In a nuclear power plant, thermal pollution refers to the excessive heating of water bodies like lakes or rivers due to the release of heated water from the plant. This increase in water temperature can have detrimental effects on aquatic life. Therefore, it is crucial to prevent thermal pollution to maintain the ecological balance of the surrounding environment.

Cooling Tower:
The part of a nuclear power plant that prevents thermal pollution of lakes or rivers is the cooling tower. The cooling tower is responsible for removing excess heat from the power plant's cooling system by transferring it to the atmosphere. It accomplishes this through the process of evaporation.

How Cooling Towers Work:
1. Heat Exchange: The cooling tower receives hot water from the power plant's cooling system, which contains excess heat.
2. Water Distribution: The hot water is distributed over the fill surfaces of the cooling tower. The fill surfaces provide a large surface area for the water to come into contact with air.
3. Airflow: Air is drawn into the cooling tower through the lower part and flows upward. As the hot water trickles down the fill surfaces, it comes into contact with the incoming air.
4. Evaporation: As the hot water is exposed to the air, a portion of it evaporates, taking away heat in the process. This evaporation causes a cooling effect on the remaining water.
5. Exhaust: The heated air, now saturated with moisture, is expelled from the cooling tower through the top, while the cooled water collects at the bottom and is returned to its source.

Benefits of Cooling Towers:
- Thermal Pollution Prevention: By continuously removing heat from the power plant's cooling system, cooling towers help regulate the temperature of the water bodies where the heated water is discharged. This prevents thermal pollution and minimizes the impact on aquatic ecosystems.
- Energy Efficiency: Cooling towers facilitate the reuse of water within the power plant's cooling system, reducing the need for fresh water intake. This conserves water resources and improves the overall energy efficiency of the plant.
- Cost-effectiveness: Cooling towers are a cost-effective solution for managing the excess heat generated by power plants. They provide an efficient means of heat dissipation without relying on large quantities of water or other resources.

Conclusion:
In a nuclear power plant, the cooling tower plays a vital role in preventing thermal pollution of lakes or rivers. By removing excess heat from the plant's cooling system through evaporation, cooling towers ensure that the discharged water does not excessively raise the temperature of surrounding water bodies. This helps maintain the ecological balance and protect aquatic life in the vicinity of the power plant.

What is Force?
Force is a fundamental concept in physics that describes an interaction that can change the motion of an object. It is primarily defined as:

• A push or pull applied to an object

Understanding Force
Force can be understood through its various characteristics:

• Push or Pull: When you push a door to open it, you apply a force. Similarly, pulling a drawer involves exerting a force in the opposite direction.
• Effect on Motion: Force can cause an object to start moving, stop moving, or change direction. For instance, kicking a soccer ball applies a force that moves the ball forward.
• Measured in Newtons: The standard unit of force is called a Newton (N). This helps quantify the amount of force applied.

Types of Force
There are various types of forces, which can be categorized as:

• Contact Forces: Forces that occur when objects physically touch each other, such as friction and tension.
• Non-contact Forces: Forces that act at a distance, such as gravitational force and magnetic force.

Real-Life Examples
To grasp the concept of force better, consider these examples:

• Gravity: The force that pulls objects toward the Earth.
• Friction: The force that opposes the motion of objects sliding against each other.

In summary, understanding force is essential as it lays the foundation for studying motion and mechanics in everyday life.

Understanding the Wheel and Axle
The wheel and axle is a fundamental simple machine that plays a crucial role in various mechanical systems. Here’s a detailed explanation of how it works:
Definition
- The wheel and axle consists of two circular objects: a larger wheel and a smaller cylindrical rod called the axle.
- These two components are connected so that they rotate together.
How It Works
- When force is applied to the wheel, it turns the axle, allowing for movement.
- This mechanism reduces friction, making it easier to move heavy loads.
- The wheel's larger radius allows for a greater distance to be covered with less effort compared to directly lifting an object.
Applications
- The wheel and axle is found in various everyday items, such as:
- Cars (where wheels rotate around axles)
- Bicycles (wheels attached to axles)
- Rolling carts (helping in transportation)
Advantages
- Enhances efficiency in moving objects.
- Reduces the amount of force needed to lift or move a load.
- Allows for smoother and easier transportation.
Conclusion
- The wheel and axle is a simple yet powerful machine that serves as the foundation for many technologies we use today.
- Its design demonstrates how basic principles of physics can be applied to improve our daily lives.
Understanding this machine's mechanics is essential for grasping the concepts of simple machines in physics.

"Nice shot!"

Frictional force opposes the motion of objects and slows them down when they are in contact with a surface. Fun fact: Without friction, we wouldn’t be able to walk properly because our feet would keep slipping!

Scissors are an example of a class 1 lever, where the fulcrum is located between the load and the effort. The same principle applies to seesaws. Fun fact: Class 1 levers can increase force or speed, depending on the design.

A moving car possesses mechanical energy, as it has both potential and kinetic energy. Fun fact: The faster the car moves, the more kinetic energy it has!

Understanding the Difference Between Work and Energy
The concepts of work and energy are fundamental in physics and are closely related but distinct.
Definition of Work
- Work is defined as the transfer of energy that occurs when a force is applied to an object, causing it to move.
- The formula for work is: Work = Force × Distance × cos(θ), where θ is the angle between the force and the direction of movement.
Definition of Energy
- Energy is the ability or capacity to do work. It exists in various forms, such as kinetic, potential, thermal, and more.
- Energy can be stored or transferred, and it is conserved in a closed system.
Relationship Between Work and Energy
- When work is done on an object, energy is transferred to that object, allowing it to change its state or position. For example, lifting a book off the ground requires work, which transfers energy to the book as gravitational potential energy.
- Conversely, when an object does work (like a falling object hitting the ground), it transfers energy away.
Why Option A is Correct
- Option A clearly states that "Work is the transfer of energy, while energy is the ability to do work." This statement accurately captures the essence of both concepts.
- The other options incorrectly define or confuse the terms, making option A the only accurate choice.
Understanding these differences helps in grasping how forces and movements interact in our physical world.

To stay safe when using sharp tools like scissors, always carry them with the pointed edge downward to avoid accidents. Fun fact: Sharp objects should always be stored in a tool box to prevent injury!

Light energy helps us to see things. It comes from sources like the Sun, bulbs, and candles. Fun fact: Light from the Sun takes about 8 minutes to reach the Earth!

The fulcrum is the fixed point around which a lever moves. Depending on the position of the fulcrum, levers can be classified into different types. Fun fact: Your elbow acts as a fulcrum when you bend your arm!

The current from the battery going around the iron nail turns iron nail into a magnet.
More coil around nail more current around nail results in more magnetism which means more powerful magnet.

Electricity traveling through a wire is an example of electricity flowing through a conductor.

A ramp is an inclined plane, which helps to move heavy objects with less effort. It reduces the amount of force needed to lift an object. Fun fact: The ancient Egyptians used inclined planes to build the pyramids!

Understanding Total Energy and Applied Forces
When discussing the total energy of a moving object, it is essential to understand the role of applied forces and how they relate to energy conservation.
What is Total Energy?
- Total energy of an object includes its kinetic energy (energy of motion) and potential energy (stored energy based on position).
Conservation of Energy Principle
- The principle of conservation of energy states that in a closed system, energy cannot be created or destroyed; it can only change forms.
Applied Forces and Energy
- If all applied forces are conservative, they do not dissipate energy through non-conservative forces like friction or air resistance.
- Examples of conservative forces include gravitational force and spring force.
Implications of Conservative Forces
- When only conservative forces are acting, the total mechanical energy (kinetic + potential) of the object remains constant.
- The work done by these forces merely converts energy from one form to another (e.g., kinetic energy to potential energy and vice versa) without changing the total energy quantity.
Conclusion
- Since the total energy remains unchanged when all applied forces are conservative, the correct answer is option 'C': it remains constant.
- Understanding this concept helps in analyzing motion in physics, especially in fields like mechanics and dynamics.

Understanding Work in Physics
In physics, work is defined as the transfer of energy that occurs when an object is moved over a distance by an external force. To determine if work is being done, we need to consider two essential factors:
- Force: There must be a force acting on the object.
- Displacement: The object must move in the direction of the force applied.
Analysis of Each Option
- Carrying a rucksack to school
- When you carry a rucksack, you exert an upward force against gravity.
- As you walk, the rucksack moves in the direction of the force you apply.
- Therefore, work is being done here.
- A vehicle warming up its motor
- While the engine generates heat and runs, there is no displacement of the vehicle.
- The key point is that the vehicle is not moving in the direction of any applied force, so no work is done.
- A computer running a program
- The computer processes data, but it does not move any object in a physical sense.
- Since there is no displacement of a physical object, work is not being done in the physics context.
- A man trying to open the door
- Although the man applies force to the door, if the door does not move, then no work is done.
- Work requires movement as a result of the applied force.
Conclusion
In summary, the only action in which work is clearly being done is option A: carrying a rucksack to school. This involves both a force and a displacement, meeting the criteria for work in physics.