CBSE Class 9 Science Chapter 07 Work, Energy, And Simple Machines MCQs Set 02

Science Objective Questions and Answers: Chapter 07 Work, Energy, And Simple Machines

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Question: When a force is applied to an object but the object does not move, how much work is done according to the scientific definition?
A. The work equals the force multiplied by zero displacement, resulting in zero work
B. Work is done because effort and energy are expended by the force-applying agent
C. The work equals the force multiplied by the distance the force could have moved the object
D. Work cannot be determined without knowing the mass of the object
Show Answer & Explanation

Answer: (A) The work equals the force multiplied by zero displacement, resulting in zero work

Explanation:
According to the definition in Eq. (7.1), work equals force times displacement in the direction of force. If displacement is zero, the product is zero regardless of the magnitude of the force applied.

Question: A stretched elastic band is released, causing an object in contact with it to shoot forward. Before release, the band has the capacity to do work due to its shape. What is this stored energy called?
A. Kinetic energy of the band
B. Elastic potential energy
C. Thermal energy of the band
D. Chemical energy stored in the material
Show Answer & Explanation

Answer: (B) Elastic potential energy

Explanation:
The chapter explains that energy stored in a deformed object—such as a stretched band or compressed spring—due to its configuration is called potential energy. When the shape changes back, this stored energy is released and can do work on nearby objects.

Question: An object falling freely under gravity experiences a constant downward force. As it falls from height h to the ground, how does its kinetic energy change compared to its loss in potential energy?
A. Kinetic energy increases by less than the potential energy decreases
B. Kinetic energy increases by exactly the same amount as the potential energy decreases
C. Kinetic energy increases by more than the potential energy decreases
D. There is no relationship between the two changes
Show Answer & Explanation

Answer: (B) Kinetic energy increases by exactly the same amount as the potential energy decreases

Explanation:
• The object starts with potential energy mgh and zero kinetic energy at the top
• At the bottom, it has zero potential energy and kinetic energy equal to (1/2)mv²
• From conservation of mechanical energy shown in equations 7.9 and 7.10, the lost potential energy is entirely converted to gained kinetic energy
• This demonstrates the work-energy theorem: work done by gravity equals change in kinetic energy

Question: In a fixed pulley system used to raise a flag, the effort force needed to lift the flag equals the weight of the flag. What does this tell us about the mechanical advantage of a fixed pulley?
A. It is less than 1, making the task harder
B. It is exactly 1, so no force reduction occurs
C. It is greater than 1, providing significant mechanical advantage
D. It varies depending on the mass of the flag
Show Answer & Explanation

Answer: (B) It is exactly 1, so no force reduction occurs

Explanation:
The chapter states clearly that a fixed pulley only changes the direction of the force, not its magnitude. Since the effort required equals the load, the mechanical advantage calculated using Eq. (7.12) is load/effort = 1.

Question: A girl lifts a dumbbell upward against gravity and then slowly lowers it. When the girl moves the dumbbell downward, what is the sign of the work done by her hand on the dumbbell?
A. Positive, because she applies a force
B. Negative, because the force she applies opposes the downward displacement
C. Zero, because the dumbbell moves at constant velocity
D. Positive, because lowering requires energy from her muscles
Show Answer & Explanation

Answer: (B) Negative, because the force she applies opposes the downward displacement

Explanation:
As explained in section 7.1.2, when lowering the dumbbell, the girl applies an upward force to control its descent, but the displacement is downward. Since force and displacement are in opposite directions, the work done is negative. Example 7.1 illustrates this exact scenario.

Question: The SI unit of both work and energy is the joule. One joule is defined as the work done when what condition is met?
A. A force of 1 newton is applied to any object
B. A force of 1 newton displaces an object by 1 metre in the direction of the force
C. Any force causes a 1-metre displacement
D. An object gains kinetic energy equal to its mass in kg
Show Answer & Explanation

Answer: (B) A force of 1 newton displaces an object by 1 metre in the direction of the force

Explanation:
From Eq. (7.2) and the definition on page 118, 1 joule is specifically the product of 1 newton of force and 1 metre of displacement in the direction of that force.

Question: A ball thrown upward reaches its maximum height where its velocity momentarily becomes zero. At this highest point, which statement about the ball's energy is correct?
A. Both kinetic and potential energy are zero
B. Kinetic energy is zero and potential energy is at its maximum value
C. Potential energy is zero and kinetic energy is maximum
D. Both kinetic and potential energy are at their maximum values
Show Answer & Explanation

Answer: (B) Kinetic energy is zero and potential energy is at its maximum value

Explanation:
At the highest point, the ball's velocity is zero, so kinetic energy equals (1/2)m(0)² = 0. However, the ball is at maximum height h, so its potential energy mgh is maximum. This represents the point where all the initial kinetic energy has been converted to potential energy.

Question: Two workers push identical boxes up the same ramp to a platform at the same height. Worker A takes 2 minutes while Worker B takes 4 minutes. How do their power outputs compare, assuming they exert approximately the same effort?
A. Worker A outputs twice the power of Worker B
B. Worker B outputs twice the power of Worker A
C. Their power outputs are equal because the same work is done
D. The power cannot be compared without knowing the mass of the boxes
Show Answer & Explanation

Answer: (A) Worker A outputs twice the power of Worker B

Explanation:
Power is defined as work divided by time (Eq. 7.11). Both workers do the same work W in moving the boxes to the same height. Since Worker A completes the task in half the time, their power P = W/t is twice that of Worker B, whose power is W/(2t).

Question: A spring is compressed by pushing on it, then released. The spring rapidly returns to its original shape and propels a nearby object forward. What sequence of energy transformations occurs?
A. Mechanical energy to elastic potential energy to kinetic energy
B. Elastic potential energy to kinetic energy of the object
C. Kinetic energy to elastic potential energy to mechanical energy
D. Chemical energy to thermal energy to kinetic energy
Show Answer & Explanation

Answer: (B) Elastic potential energy to kinetic energy of the object

Explanation:
When the spring is compressed, the work done stores elastic potential energy (Eq. 7.8 concept applied to springs). Upon release, this potential energy is converted into kinetic energy of the object, as illustrated in Fig. 7.14 and discussed in section 7.4.2.

Question: A child on a slide reaches the bottom faster than expected if the slide is frictionless. Assuming no friction, at the bottom of a slide of height h, what determines the child's speed?
A. Only the height h and gravitational acceleration g
B. The height h, the mass of the child, and the angle of the slide
C. The total distance traveled along the slide
D. The mass of the child and the length of the slide
Show Answer & Explanation

Answer: (A) Only the height h and gravitational acceleration g

Explanation:
From Example 7.8, using conservation of mechanical energy, the potential energy mgh at the top converts entirely to kinetic energy (1/2)mv² at the bottom, giving v = √(2gh). This result depends only on h and g, not on mass or the shape of the path.

Question: A seesaw is balanced with a heavier child sitting closer to the fulcrum and a lighter child sitting farther away. What principle of levers explains why this arrangement achieves balance?
A. The heavier child exerts a larger gravitational force
B. Effort times effort arm equals load times load arm, as stated in Eq. (7.15)
C. The distance from the fulcrum determines the direction of motion
D. Greater mass automatically creates a mechanical advantage
Show Answer & Explanation

Answer: (B) Effort times effort arm equals load times load arm, as stated in Eq. (7.15)

Explanation:
The fundamental principle of lever balance, demonstrated in Activity 7.5 and stated in Eq. (7.15), is that n₁ × L₁ = n₂ × L₂. A heavier load at a shorter distance can balance a lighter load at a greater distance because the products of mass and distance are equal.

Question: A car's brakes apply a backward force opposite to the car's direction of motion. According to the work-energy theorem, what happens to the car's mechanical energy as the brakes slow it down?
A. Mechanical energy increases because brakes apply force
B. Mechanical energy decreases as the work done by brakes is negative
C. Mechanical energy remains constant unless friction acts
D. Mechanical energy transforms into the car's structure
Show Answer & Explanation

Answer: (B) Mechanical energy decreases as the work done by brakes is negative

Explanation:
The braking force opposes the displacement of the car, resulting in negative work. By the work-energy theorem (Eq. 7.3), negative work means the mechanical energy of the car decreases. This decrease corresponds to the loss of kinetic energy as the car slows, as illustrated in Example 7.6 and the escape ramp scenario in Example 7.9.

Question: A person uses a lever to pry open a stuck wooden crate. The lever arm on the effort side is 3 times longer than the lever arm on the load side. What mechanical advantage does this lever provide?
A. 1, making it no better than lifting directly
B. 2, doubling the lifting capacity
C. 3, tripling the lifting capacity
D. Cannot be determined without knowing the mass of the crate
Show Answer & Explanation

Answer: (C) 3, tripling the lifting capacity

Explanation:
From Eq. (7.16), mechanical advantage equals the ratio of effort arm to load arm. If the effort arm is 3 times the load arm, then MA = effort arm / load arm = 3. This means the lever multiplies the applied force by 3, allowing a much heavier load to be moved.

Question: A watermill (gharat) harnesses the energy of flowing water to grind grain. As water flows downhill, what energy conversion primarily enables the grinding action?
A. Thermal energy converts to mechanical energy
B. Kinetic energy of water drives the wheel's rotation
C. Potential energy of elevated water converts to kinetic energy
D. Chemical energy in water releases mechanical energy
Show Answer & Explanation

Answer: (C) Potential energy of elevated water converts to kinetic energy

Explanation:
As explained in the section "Bridging Science and Society" on page 136, water at the top of the hill possesses gravitational potential energy. As it flows down, this potential energy is converted to kinetic energy. The moving water's kinetic energy drives the wheel, which then rotates the grinding stone.

Question: When a moving cricket ball strikes stationary wickets, both the ball and the wickets experience forces as a result of Newton's third law. How do the amounts of work done on each object compare?
A. Equal amounts because the forces are equal and opposite
B. The ball does more work because it was moving before collision
C. The wickets do more work because they are heavier
D. They depend on the distances moved by each object after collision
Show Answer & Explanation

Answer: (D) They depend on the distances moved by each object after collision

Explanation:
Although the forces on the ball and wickets are equal and opposite (Newton's third law), work depends on both force and displacement. Example 7.3 illustrates that the ball may move a different distance than the wickets after collision, so the work done on each is W = F × d, which can be different. The work transferred depends on the actual displacements.

Question: A student pushes a heavy box along a horizontal floor with a constant force. The box moves in the direction of the push. After examining the motion, the student notices that doubling the applied force while maintaining the same displacement results in twice the work done. Which concept does this observation directly illustrate?
A. Work is inversely proportional to the displacement of the object
B. Work done on an object is directly proportional to the force applied when displacement is constant
C. The SI unit of work is the newton-metre
D. Work cannot be done unless the object accelerates
Show Answer & Explanation

Answer: (B) Work done on an object is directly proportional to the force applied when displacement is constant

Explanation:
The scenario demonstrates that when displacement remains fixed, doubling the force doubles the work output. This follows directly from the work equation W = F × s. Since displacement is constant, work scales linearly with applied force.

Question: A stretched rubber band stores energy in its deformed state. When released, the rubber band propels a small object forward, causing it to move with kinetic energy. What is the primary reason the stretched rubber band can transfer energy to the object?
A. The rubber band loses mass as it returns to its original shape
B. Internal forces in the rubber band do work on the object as the band returns to its unstretched state
C. The object's potential energy increases during the collision
D. Friction between the rubber band and the object converts thermal energy to mechanical energy
Show Answer & Explanation

Answer: (B) Internal forces in the rubber band do work on the object as the band returns to its unstretched state

Explanation:
A deformed object like a stretched rubber band stores elastic potential energy. When released, the internal restoring forces within the material perform work on the object in contact with it, converting the stored potential energy into the object's kinetic energy.

Question: At the exact moment a cricket ball reaches its highest point after being thrown upward, which of the following is true about the ball's state of motion?
A. Both velocity and acceleration are zero at the highest point
B. Velocity is zero but gravity still exerts a downward force causing acceleration
C. Acceleration is zero because the ball momentarily stops moving
D. The ball's kinetic energy equals its potential energy
Show Answer & Explanation

Answer: (B) Velocity is zero but gravity still exerts a downward force causing acceleration

Explanation:
At maximum height, the ball's velocity is instantaneously zero, but it is not in equilibrium. Gravitational force continues to act downward, producing a constant downward acceleration of approximately 10 m s⁻². The ball is always accelerating even when velocity becomes zero.

Question: A child reaches the bottom of a frictionless slide of height h with velocity v = √(2gh). If the same child were to descend a slide of height 2h (also frictionless), what velocity would be reached at the bottom, and what principle justifies this result?
A. v = √(4gh); the work-energy theorem shows kinetic energy change equals work done by gravity
B. v = √(2gh); mechanical energy remains constant regardless of height
C. v = 2√(2gh); kinetic energy increases linearly with height
D. v = √(gh); gravitational potential energy halves with doubled height
Show Answer & Explanation

Answer: (A) v = √(4gh); the work-energy theorem shows kinetic energy change equals work done by gravity

Explanation:
• Doubling height doubles the gravitational potential energy decrease
• By work-energy theorem, work done by gravity equals change in kinetic energy
• Solving (1/2)mv² = mg(2h) gives v = √(4gh) = 2√(gh)
• Result depends only on height and g, not on slide shape or mass

Question: A weightlifter holds a heavy barbell steady overhead without moving it vertically. During this static hold, the lifter's muscles are contracting and expending internal energy, yet the barbell gains no kinetic energy. According to the scientific definition of work, how much work does the lifter do on the barbell?
A. Positive work equal to the product of the barbell's weight and the height it was lifted
B. Zero joules, because there is no displacement in the direction of the applied force
C. Negative work equal to the gravitational potential energy of the barbell
D. The work cannot be determined without knowing the time the barbell is held
Show Answer & Explanation

Answer: (B) Zero joules, because there is no displacement in the direction of the applied force

Explanation:
The scientific definition of work requires both force and displacement in the direction of that force. Here, although the lifter applies an upward force balancing the barbell's weight, the barbell undergoes zero vertical displacement. Therefore W = F × 0 = 0 J. The lifter's internal energy expenditure is real but does not constitute mechanical work on the barbell.

Question: An inclined plane of length L is used to raise an object of mass m to a height h. The work done in pushing the object up the incline at constant velocity equals the work done in lifting it vertically to the same height. What does this outcome reveal about simple machines?
A. Simple machines reduce the total work required to complete a task
B. Simple machines change the force or direction needed but conserve total work done
C. Simple machines create energy by reducing friction
D. The mechanical advantage of an incline always exceeds 2
Show Answer & Explanation

Answer: (B) Simple machines change the force or direction needed but conserve total work done

Explanation:
Despite the mechanical advantage reducing the required force (F′ < mg), the total work remains constant because the displacement increases proportionally. This illustrates a fundamental principle: simple machines do not reduce total work input; they only redistribute it by trading force for distance, making tasks feel easier by reducing peak effort.

Question: A fielder throws a ball that strikes stationary wickets, causing them to topple. During the collision, which statement best describes the energy transformation and work done?
A. The ball does positive work on the wickets by transferring its kinetic energy to them
B. The wickets do zero work on the ball because they are initially at rest
C. Mechanical energy is created during the collision to lift the wickets
D. The ball's kinetic energy remains unchanged because it strikes an immovable object
Show Answer & Explanation

Answer: (A) The ball does positive work on the wickets by transferring its kinetic energy to them

Explanation:
The moving ball possesses kinetic energy and applies a force on the wickets during the collision. This force acts in the direction the wickets move, so the ball does positive work on them. The wickets' increase in kinetic energy equals (approximately) the work the ball does on them, demonstrating energy transfer through mechanical interaction.

Question: A crane lifts a mass m to the 10th floor in time t₁ and to the 20th floor in time t₂ = 2t₁. If each floor has the same height, what is the ratio of power required for the second lift compared to the first?
A. 1:1 (same power required)
B. 2:1 (twice the power)
C. 1:2 (half the power)
D. 4:1 (four times the power)
Show Answer & Explanation

Answer: (A) 1:1 (same power required)

Explanation:
Work done to reach 10th floor = mgh₁₀. Work done to reach 20th floor = mgh₂₀ = 2mgh₁₀. Power for 10th floor = mgh₁₀/t₁. Power for 20th floor = 2mgh₁₀/(2t₁) = mgh₁₀/t₁. The ratio is 1:1, meaning equal power is required in both cases because although the second lift involves twice the work, it takes twice the time.

Question: A person walks up a flight of stairs carrying a suitcase at constant velocity. The person's hand applies an upward force on the suitcase equal to its weight, and the suitcase moves upward and horizontally. Which force does work on the suitcase in the scientific sense?
A. Only the gravitational force does work because it opposes the motion
B. Only the hand's applied force does work because it supports the suitcase
C. Both the gravitational force and the applied force do work, but with opposite signs
D. The person's hand does positive work while the gravitational force does negative work
Show Answer & Explanation

Answer: (D) The person's hand does positive work while the gravitational force does negative work

Explanation:
The applied force by the hand and the gravitational force are both present. The applied upward force has an upward component matching the upward displacement, resulting in positive work. Gravity points downward while displacement is partly upward, so gravity does negative work. The net effect—from the work-energy theorem—shows the object gains potential energy.

Question: A slingshot is pulled back to stretch the elastic band and then released. Just before release, the stretched band has the capacity to do work on a projectile. What term most accurately describes the energy stored in the stretched band at this moment?
A. Kinetic energy, because the band is about to move
B. Elastic potential energy, resulting from the band's deformation
C. Thermal energy, due to friction in the stretching process
D. Gravitational potential energy, relative to the ground
Show Answer & Explanation

Answer: (B) Elastic potential energy, resulting from the band's deformation

Explanation:
Energy stored in a deformed object due to its shape or configuration is called potential energy. In this case, because the deformation is elastic (the band returns to its original shape), it is specifically elastic potential energy. This stored energy is converted to kinetic energy of the projectile upon release.

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