Class 9 – Science Extra Questions – Ch. 10 – Work and Energy

Ans: D

Explanation: Water held back by a dam is at a higher elevation than it would be if it were flowing freely. This stored energy due to its position is called potential energy. When the dam is opened, this potential energy is converted into kinetic energy as the water flows downwards.
Correct Option: D

Ans: C

Explanation: Work is defined as the force applied over a distance. The SI unit of force is the Newton (N), and the SI unit of distance is the meter (m). Therefore, the SI unit of work is the product of these two units, which is Newton-meter (N·m). The Newton-meter is also known as the Joule (J). Option A is the unit of force, and option B is the unit of distance. Option C, newton-meter, correctly represents the unit of work.
Correct Option: C

Ans: A

Explanation: Energy consumed is calculated by multiplying power by time. In this case, the power is 2 kW and the time is 5 hours. Therefore, the total energy consumed is 2 kW * 5 hours = 10 kWh.
Correct Option: A

Ans: A

Explanation: Power is defined as the rate at which work is done, and it can also be calculated as the product of force and velocity when the force is constant and in the same direction as the velocity. The formula for power (P) is P = Force (F) × Velocity (v). In this case, the force is 150 N and the velocity is 5 m/s. Therefore, the power output of the runner is 150 N × 5 m/s = 750 Watts (W).
Correct Option: A

Ans: D

Explanation: Power is defined as the rate at which energy is transferred or converted. It is calculated by dividing the energy consumed by the time taken to consume it. The formula for power is P = E/t, where P is power, E is energy, and t is time. In this case, the energy consumed (E) is 10 joules, and the time taken (t) is 5 seconds. Therefore, the power consumption (P) is 10 J / 5 s = 2 watts.
Correct Option: D

Ans: C

Explanation: In physics, work is defined as the force applied to an object multiplied by the distance the object moves in the direction of the force. The formula for work is W = F * d * cos(theta), where F is the force, d is the displacement, and theta is the angle between the force and displacement. In this scenario, the scientist exerts a force on the wall, but the wall does not move. Therefore, the displacement (d) is zero. Since the displacement is zero, the work done on the wall is also zero, regardless of the force exerted or how tired the scientist becomes.
Correct Option: C

Ans: C

Explanation: To solve this problem, we first need to calculate the kinetic energy of the truck at both speeds.
The formula for kinetic energy (KE) is KE = 0.5 * m * v^2, where m is mass and v is velocity.

Initial kinetic energy (KEi) when v = 20 m/s:
KEi = 0.5 * 800 kg * (20 m/s)^2
KEi = 0.5 * 800 * 400
KEi = 160000 Joules

Final kinetic energy (KEf) when v = 30 m/s:
KEf = 0.5 * 800 kg * (30 m/s)^2
KEf = 0.5 * 800 * 900
KEf = 360000 Joules

The work done (W) by the truck to increase its speed is the change in kinetic energy:
W = KEf – KEi
W = 360000 J – 160000 J
W = 200000 Joules

Power (P) is the rate at which work is done, and is given by the formula P = W / t, where t is time. We are given the power output of the truck as 20000 W. We can rearrange this formula to solve for time: t = W / P.

t = 200000 J / 20000 W
t = 10 seconds

Therefore, the duration required is 10 seconds.

Correct Option: C

Ans: A

Explanation: When a cyclist pushes down on a pedal, the muscles are contracting to generate force. This contraction is powered by the conversion of chemical energy stored in molecules like ATP into mechanical energy (kinetic energy) that moves the pedals. At the microscopic level, this involves a series of chemical reactions that release energy, which is then used to cause muscle fibers to slide past each other, resulting in movement. While some thermal energy is also produced as a byproduct, the primary transformation is chemical to kinetic.
Correct Option: A

Ans: C

Explanation: The potential energy (PE) gained by an object is given by the formula PE = mgh, where m is the mass of the object, g is the acceleration due to gravity, and h is the height it is lifted. We are given PE = 9800 J, m = 200 kg, and g = 9.8 m/s². We need to find h.
Rearranging the formula to solve for h, we get h = PE / (mg).
Substituting the given values:
h = 9800 J / (200 kg * 9.8 m/s²)
h = 9800 J / (1960 kg⋅m/s²)
h = 5 m
Correct Option: C

Ans: A

Explanation: Work is defined as the product of the force applied to an object and the distance over which that force is applied, provided the force has a component in the direction of motion. In this scenario, a constant force is applied, causing the stationary object to move a certain distance. This directly fits the definition of work being done, as force is applied over a distance. Option B is incorrect because the initial state of the object (stationary) does not preclude work from being done on it. Option C is incorrect; work is done when the force is applied in the direction of motion or has a component in that direction, not necessarily perpendicular. Option D is irrelevant; mass is not required to calculate work done when force and distance are known.
Correct Option: A