MCQ : Work And Energy - 1 - Class 9 MCQ

Option A: Work done when force is zero - Positive work
This is incorrect. When the force applied is zero, no work is done, regardless of any displacement.

Option B: Work done when force and displacement are in opposite directions - Negative work
This is correct. When the force acts in the opposite direction to the displacement, the work done is negative.

Option C: Work done when force is applied but no displacement occurs - Work is done
This is incorrect. For work to be done according to the scientific definition, there must be both force and displacement. If there's no displacement, no work is done.

Option D: Work done when force and displacement are perpendicular - Maximum work
This is incorrect. When force and displacement are perpendicular, no work is done. This is because the cosine of 90 degrees is zero, resulting in zero work.

• Statement (i) is correct as work done by a force is indeed the product of the force and the displacement in the direction of the force.
• Statement (ii) is incorrect. Work is a scalar quantity and has only magnitude, not direction.
• Statement (iii) is correct because an object possessing energy can exert a force on another object, transferring energy and potentially causing displacement.
• Statement (iv) is correct; the unit of energy is joule (J), the same as the unit of work.

• Option A: Power - Rate of doing work
• This is the correct match. Power is defined as the rate at which work is done. The SI unit of power is watt (W), where 1 W = 1 J/s.
• Option B: Kinetic Energy - Potential energy of a raised object
• This is incorrectly matched. Kinetic energy is the energy possessed by an object due to its motion, not its position. Potential energy is the energy possessed by an object due to its position or height.
• Option C: Mechanical Energy - Only potential energy
• This is incorrect. Mechanical energy is the sum of kinetic and potential energy of an object, not just potential energy.
• Option D: Work - Force divided by distance
• This is incorrect. Work is defined as the force applied on an object multiplied by the distance moved by the object in the direction of the force, not divided by distance.

Explanation:

When a flying airplane is in motion, it possesses both potential and kinetic energy. Here's a detailed explanation of why it has both types of energy:
Potential Energy:
- Potential energy is the energy an object possesses due to its position or state.
- In the case of a flying airplane, it has potential energy due to its altitude or height from the ground.
- The higher the airplane is above the ground, the greater its potential energy.
Kinetic Energy:
- Kinetic energy is the energy an object possesses due to its motion.
- In the case of a flying airplane, it has kinetic energy due to its forward motion.
- The faster the airplane is moving, the greater its kinetic energy.
Combination of Potential and Kinetic Energy:
- As the airplane flies, it converts potential energy into kinetic energy and vice versa.
- When the airplane gains altitude, it increases its potential energy while decreasing its kinetic energy.
- When the airplane descends, it decreases its potential energy while increasing its kinetic energy.
- At any given moment during flight, the airplane has a combination of both potential and kinetic energy.
Conclusion:
- A flying airplane possesses both potential and kinetic energy simultaneously.
- Therefore, the correct answer is option C: both potential and kinetic energy.

• In this scenario, the Assertion is true because work in the scientific sense involves both force and displacement. If there is no displacement, according to the scientific definition, work is not done.
• The Reason is also correct, as it accurately explains why work is not done when there is no displacement.
• Therefore, the correct answer is Option A, where both the Assertion and the Reason are true and the Reason correctly explains the Assertion.

• Assertion's Correctness: The assertion that an object follows a curved path and falls back to the ground with the initial and final points on the same horizontal line is correct for a projectile motion scenario.
• Reason's Correctness: The reason given that the work done by the force of gravity in this situation is zero is incorrect. Gravity does work on the object during its projectile motion, constantly changing its speed and direction.
• Explanation: In projectile motion, gravity constantly acts on the object, causing it to accelerate towards the ground. This acceleration due to gravity affects the object's path, making it follow a curved trajectory. The work done by gravity is not zero, as it continuously affects the object's motion.
• Conclusion: While the assertion is true regarding the path of the object, the reason provided is false since gravity does work on the object during its motion. Therefore, Assertion is true, but Reason is false.

One joule equals the work done ( or energy expended) by a force of One Newton (N) acting over a distance of one meter (m). One Newton equals a force that produces an acceleration of one meter per second (s) per second on a one Kilogram (Kg) mass. Therefore, one joule equals one Newton meter.

We know, Power :- 
=  force * displacement/Time 
Force is 10 N, displacement = 6 m 
Time = 3 s 
Then, power = 10*6/3 => 10 * 2 ⇒ 20W

In science, work is defined as the product of force and displacement. This relationship is expressed as Work = Force × Displacement.

The law of conservation of energy states that energy can neither be created nor destroyed, only transformed from one form to another. The total energy in a closed system remains constant before and after any transformation.