Ans. If some force is acting on a body but there is no displacement, then no work is done. For example, a man is pushing hard against a wall but the wall remains fixed in its place. In this case, the work done will be zero. Mathematically: W = F × S. In this case S = 0, so W = F × 0 = 0.
Ans. A slow car can have more kinetic energy than a fast motorcycle because kinetic energy depends on both mass and speed. A car's large mass can make up for its slower speed, resulting in more energy compared to a lighter motorcycle moving faster. For example, a car of mass 2000 kg moving with speed of 10 m/s: EK = 1/2 × 2000 × (10)² = 100,000 J. A motorcycle of mass 200 kg with speed of 40 m/s: EK = 1/2 × 200 × (40)² = 160,000 J.
Ans. Power is the rate at which work is done, calculated using P = Work/Time. For force F1: P1 = 5/10 = 0.5 W. For force F2: P2 = 3/5 = 0.6 W. Therefore, force F2 delivers greater power than force F1.
Ans. The gravitational potential energy is given as P.E = mgh. The gain in potential energy depends only on height, mass, and gravity, not speed. Therefore, the gain remains 4500 J regardless of her speed.
Ans. Work is defined as the product of the magnitude of force and the distance covered in the direction of force. The SI unit of work is joule (J). One joule of work is done when a force of one newton acting on a body moves it through a distance of one meter in its direction: 1 J = 1 N × 1 m.
Ans. The potential energy of a body of mass m raised through a height h is given by: P.E = mgh, where m is the mass of the body, g is the acceleration due to gravity, and h is the height raised.
Ans. Suppose a body of mass m is moving with velocity v. An opposing force F acting on the body through a distance s brings it to rest. Then, K.E = Work done = F × s. As F = ma and s = average velocity × time = (v + 0)/2 × t = vt/2. Hence, K.E = ma × vt/2 = 1/2 m × a × v × t. Since a = v/t, thus K.E = 1/2 m × (v/t) × v × t = 1/2 mv².
Ans. The ratio of useful output energy to the total input energy is called the efficiency of a working system: Efficiency = Useful output energy / Total input energy. A system cannot have 100% efficiency because during energy conversion, some energy is always wasted, typically as heat. No device can convert all input energy into useful output due to inevitable energy losses, making an ideal perpetual machine impossible.
Ans. Power is defined as the time rate of doing work. Mathematically, Power = Work / Time. If W is work done in time t, then P = W/t. The SI unit of power is watt (W). One watt is the work done at the rate of one joule per second.
Ans. Renewable sources are energy resources that are replaced by new ones after use, such as hydroelectricity, solar energy, wind energy, and tidal energy. Non-renewable sources are energy resources that are not replaced after use, such as fossil fuels and nuclear energy.
Ans. Kinetic energy of a body cannot be negative. It has either a positive value or zero. Since EK = 1/2 mv², kinetic energy depends on mass (always positive) and the square of velocity (always positive). Therefore, kinetic energy cannot be negative.
Ans. Kinetic energy is given by K.E = 1/2 mv². For the object with velocity v: K.E = 1/2 mv². For the object with mass 2m and velocity 1/2v: K.E' = 1/2 × 2m × (v/2)² = 1/4 mv². Comparing both, K.E > K.E', so the object moving with velocity v has greater kinetic energy.
Ans. No, the kinetic energy does not change. Since K.E = 1/2 mv², kinetic energy depends only on mass and speed, not direction. With constant speed, the car's kinetic energy remains the same even on a curved path.
Ans. An object having one joule of potential energy means it has the capacity to do one joule of work. This implies a force of one newton is required to raise it through a height of 1 m: 1 J = 1 N × 1 m.
Ans. A tyre bursts on a motorway because at high speed, significant friction is produced, generating heat. This heat increases air pressure inside the tyre. Excessive heat and pressure weaken the tyre, causing it to burst.
Ans. When the cricket ball hits the window: (i) Kinetic energy of the ball is transferred to the window. (ii) Part of the energy deforms or breaks the window. (iii) Some energy converts into sound (the smashing noise). (iv) A small amount converts to heat due to friction.
Ans. No, he is doing no work. Work requires a force to cause displacement in its direction. Although the man exerts force on the water, he remains at rest relative to the shore, so no work is done relative to the shore.
Ans. The cyclist starts with maximum potential energy and minimum kinetic energy at the top of the first hill. As he descends, potential energy converts to kinetic energy, reaching maximum speed (and kinetic energy) at the bottom. While climbing the next hill, kinetic energy converts back to potential energy, slowing him down until he stops at the top of the next hill.
Ans. Yes, timber is a renewable heat energy source if forests are sustainably managed, allowing trees to regrow. When wood is burned, it releases heat energy. Sustainable management ensures continuous supply, making it renewable.
Ans. Work is the product of force and displacement in the direction of force. If W is work, F is force, and S is displacement, then W = F × S. The SI unit of work is joule (J).
Ans. The work done by a single beat of the human heart is approximately 0.5 J.
Ans. Work is done when a force acts on a body and the body moves in the direction of the force. Two conditions must be met: a force must act on the body, and the body must undergo displacement due to that force.
Ans. Energy is the ability of a body to do work. It is a scalar quantity measured in joules. Two types of mechanical energy are kinetic energy (energy due to motion) and potential energy (energy due to position).
Ans. We need energy to perform work and various activities, including movement, growth, and maintaining bodily functions.
Ans. Potential energy is the ability of a body to do work due to its position. Gravitational potential energy is specifically the energy due to height above the Earth's surface, given by EP = mgh.
Ans. Elastic potential energy is the energy stored in a compressed or stretched spring. Chemical potential energy is the energy stored in chemicals, such as in a battery, which can be converted to electrical energy through chemical reactions.
Ans. Kinetic energy is the energy a body possesses due to its motion, given by EK = 1/2 mv², where m is mass and v is velocity.
Ans. The energy content of the nuclear bomb dropped on Hiroshima was approximately 8.0 × 10¹³ J.
Ans. The energy output of a power station in one year is typically around 10¹⁶ J.
Ans. A stretched bow can store enough energy to shoot an arrow up to 1 km away.
Ans. Yes, the train's potential energy continuously changes as it moves along the roller coaster track, converting between potential and kinetic energy.
Ans. Both a raised hammer and a stretched bow possess potential energy.
Ans. Our body gets energy from the chemical potential energy stored in food, which is released through metabolic processes.
Ans. The law of conservation of energy states that energy cannot be created or destroyed. It can only be transformed from one form to another, and the total amount of energy remains constant.
Ans. Einstein's theory of relativity states that mass and energy are interchangeable. Mass can be converted into energy (as in nuclear reactions) and vice versa, leading to the conservation of mass-energy rather than separate conservation of each.
Ans. Before electricity, one primary function of fossil fuels was to provide light.
Ans. A solar-powered car won the World Solar Challenge race in Darwin, Australia, in 1990.
Ans. Geothermal energy is used in Japan, Russia, Iceland, Italy, New Zealand, and the USA. Over 85% of Icelanders use geothermal energy to heat their homes.
Ans. Heating with geothermal energy costs about one-third of the cost of burning oil for electric heaters.
Ans. Thermal energy from burning coal was the major energy source for steam engines in the past.
Ans. The radioactive fallout from the 1986 Chernobyl nuclear accident in Russia affected people, livestock, and crops. While 31 people died from direct exposure, about 600,000 were significantly exposed to the fallout.
Ans. Burning fossil fuels releases about five billion tonnes of carbon dioxide into the atmosphere annually.
Ans. A nuclear power plant uses energy released from nuclear fission reactions to generate electricity.
Ans. Fossil fuels take millions of years to form and cannot be replenished once depleted, making them non-renewable.
Ans. Solar energy is most preferred because it does not pollute the environment.
Ans. Washing machine, electric motor, juicer, ceiling fan, and drill machine.
Ans. A generator converts mechanical energy into electrical energy.
Ans. Power is the rate of doing work. If P is power, W is work, and t is time, then P = W/t.
Ans. One watt is the power when one joule of work is done in one second: 1 W = 1 J/s.
Ans. The watt is named after James Watt (1736–1819), a Scottish engineer who perfected the steam engine.
Ans. Efficiency is the ratio of useful output energy to total input energy: Efficiency = (Useful output energy / Total input energy) × 100%.
Ans. An ideal machine is one with 100% efficiency, where output energy equals input energy.
Ans. Diesel engine: 35%, Petrol engine: 25%, Electric motor: 80%, Bicycle: 15%.
