WORK, ENERGY AND POWER-QnA

Q1: What is the SI unit of work?

The SI unit of work is joule (J), which is equal to the work done when a force of one newton displaces a body by one metre in the direction of the force.

Q2: When is work said to be zero even if a force acts on a body?

Work is zero when there is no displacement in the direction of the applied force, even if the force is non-zero.

Q3: Write the formula for kinetic energy.

The kinetic energy of a body is given by \(K = \frac{1}{2}mv^2\).

Q4: Is kinetic energy a scalar or vector quantity?

Kinetic energy is a scalar quantity because it has magnitude only and no direction.

Q5: What does positive work indicate?

Positive work indicates that the force has a component in the direction of displacement.

Q6: Define power.

Power is the rate at which work is done or energy is transferred per unit time.

Q7: Write the SI unit of power.

The SI unit of power is watt (W).

Q8: What kind of force is gravity?

Gravity is a conservative force.

Q9: Can work done be negative?

Yes, work done is negative when the force acts opposite to the direction of displacement.

Q10: What is mechanical energy?

Mechanical energy is the sum of kinetic energy and potential energy of a body.

Q11: What happens to the speed if net work done is zero?

The speed of the body remains unchanged.

Q12: What is elastic potential energy?

Elastic potential energy is the energy stored in a body due to elastic deformation.

Q13: Is friction a conservative force?

No, friction is a non-conservative force.

Q14: What does the area under an F–x graph represent?

It represents the work done by the force.

Q15: State Hooke’s law.

Hooke’s law states that the restoring force of a spring is directly proportional to its extension within elastic limits.

Q1: Define work with its mathematical expression.

Work is said to be done when a force causes displacement. Mathematically, work is the dot product of force and displacement: \(W = Fs\cos\theta\).

Q2: Differentiate between positive and negative work.

Positive work occurs when force and displacement are in the same direction, while negative work occurs when they are in opposite directions.

Q3: Why does a porter carrying a load on a level road do no work against gravity?

The displacement is horizontal while gravitational force acts vertically downward, making the angle between them 90°, hence work done is zero.

Q4: State the work–energy theorem.

The work–energy theorem states that the net work done on a body equals the change in its kinetic energy.

Q5: Why is centripetal force said to do no work?

Centripetal force is always perpendicular to the displacement in circular motion, so the work done is zero.

Q6: What happens to mechanical energy in the presence of friction?

Mechanical energy decreases because part of it is converted into heat and other non-mechanical forms.

Q7: Write the expression for elastic potential energy of a spring.

Elastic potential energy is given by \(U = \frac{1}{2}kx^2\).

Q8: What is instantaneous power?

Instantaneous power is the rate of doing work at a particular instant of time.

Q9: State one condition for conservation of mechanical energy.

Mechanical energy is conserved only when no non-conservative forces act on the system.

Q10: What remains conserved in an inelastic collision?

Linear momentum of the system remains conserved in an inelastic collision.

Q1: Explain positive, negative and zero work with examples.

Work is positive when force and displacement are in the same direction, such as gravity acting on a falling object. Work is negative when force opposes motion, like friction on a moving block. Work is zero when force is perpendicular to displacement, as in circular motion.

Q2: Derive the expression for kinetic energy.

Consider a body of mass m accelerated from rest to velocity v under force F. Using equations of motion and work done, the work done equals \(\frac{1}{2}mv^2\), which is the kinetic energy gained.

Q3: Explain conservative forces with examples.

Conservative forces are those whose work depends only on initial and final positions. Examples include gravitational force and spring force. Their work over a closed path is zero.

Q4: Explain why speed is maximum at equilibrium position in spring motion.

At equilibrium position, potential energy is minimum and kinetic energy is maximum, resulting in maximum speed of the body.

Q5: Distinguish between elastic and inelastic collisions.

In elastic collisions, both momentum and kinetic energy are conserved, whereas in inelastic collisions only momentum is conserved and kinetic energy decreases.

Q1: Explain the concept of work in physics and discuss situations where work is zero.

Work in physics refers to the transfer of energy that occurs when a force causes displacement of an object. Mathematically, it is defined as the dot product of force and displacement, which means both magnitude and direction are important. However, work can be zero even when a force is applied. If there is no displacement, such as pushing a rigid wall, no work is done. Similarly, if the force acts perpendicular to the displacement, like centripetal force in circular motion, the work done is zero. These cases highlight that force alone is not sufficient for work; displacement in the direction of force is essential.

Q2: Describe kinetic energy and discuss the factors affecting it.

Kinetic energy is the energy possessed by a body due to its motion. It depends on two factors: mass and speed. Since kinetic energy is proportional to the square of velocity, even a small increase in speed leads to a large increase in kinetic energy. This explains why fast-moving vehicles cause greater damage during collisions. Kinetic energy is independent of direction, making it a scalar quantity. Understanding kinetic energy is crucial for analyzing motion, collisions, and energy transfer in mechanical systems.

Q3: Explain the law of conservation of mechanical energy with an example.

The law of conservation of mechanical energy states that the total mechanical energy of a system remains constant if only conservative forces act. Mechanical energy is the sum of kinetic and potential energy. For example, in the free fall of a body in vacuum, gravitational potential energy continuously converts into kinetic energy, but their sum remains constant at every point. This principle is widely used in solving problems related to motion under gravity, springs, and orbital motion.

Q4: Discuss elastic and inelastic collisions in detail.

Collisions are interactions between bodies during a short interval of time. In elastic collisions, both momentum and kinetic energy are conserved, meaning no energy is lost. These collisions are ideal and rare in real life. In inelastic collisions, momentum is conserved but kinetic energy is not; part of it converts into heat, sound, or deformation. Most real-life collisions are inelastic. Understanding collisions is essential in mechanics, safety engineering, and particle physics.

Q5: Explain power and distinguish between average power and instantaneous power.

Power measures how fast work is done or energy is transferred. Average power is calculated over a time interval, while instantaneous power refers to power at a specific moment. In real processes, power often varies with time, making instantaneous power more useful for analysis. For example, the power output of a vehicle engine changes continuously with speed. Power plays a vital role in machines, engines, and energy systems.