CBSE Class 9 Science Chapter 07 Work, Energy, And Simple Machines MCQs Set 01

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Chapter-wise Objective Questions: Chapter 07 Work, Energy, And Simple Machines

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Question: Which of the following statements correctly defines work in the scientific sense?
A. Work is done whenever a force is applied to an object, regardless of whether it moves.
B. Work occurs when a force displaces an object in the direction of the force.
C. Work is the total effort exerted by a person performing a task.
D. Work is done whenever an object is in motion.
Show Answer & Explanation

Answer: (B) Work occurs when a force displaces an object in the direction of the force.

Explanation:
According to the chapter, work is done by a force on an object when the force displaces the object in the direction of that force. A force applied without displacement (like pushing a wall) does no work, and a force perpendicular to motion also results in zero work.

Question: A girl carries a box horizontally while walking at constant velocity. Why does she do no work on the box even though she feels tired?
A. The box is not accelerating, so no work is needed.
B. The force she applies is perpendicular to the displacement of the box.
C. Her muscles are not strong enough to do work.
D. The box is moving at constant velocity, which means no force acts on it.
Show Answer & Explanation

Answer: (B) The force she applies is perpendicular to the displacement of the box.

Explanation:
The upward force applied by the girl to support the box is perpendicular to the horizontal displacement of the box. Since work requires force and displacement to be in the same direction, no work is done on the box. Her fatigue comes from internal muscle contraction and energy use, not from work done on the box itself.

Question: What happens to the kinetic energy of an object when the magnitude of its velocity is doubled?
A. The kinetic energy doubles.
B. The kinetic energy triples.
C. The kinetic energy quadruples.
D. The kinetic energy remains unchanged.
Show Answer & Explanation

Answer: (C) The kinetic energy quadruples.

Explanation:
Kinetic energy is given by K = ½mv². When velocity v is doubled to 2v, the new kinetic energy becomes ½m(2v)² = ½m(4v²) = 4 × ½mv², which is four times the original kinetic energy. This relationship is shown in Example 7.4 of the chapter.

Question: A pendulum bob swings from point P to point Q (the lowest point) and then to point R. At which point is the mechanical energy of the bob minimum?
A. At point P only.
B. At point Q only.
C. At point R only.
D. The mechanical energy is the same at all three points.
Show Answer & Explanation

Answer: (D) The mechanical energy is the same at all three points.

Explanation:
• Mechanical energy = kinetic energy + potential energy
• At P and R: velocity is zero (zero kinetic energy), but potential energy is maximum
• At Q: potential energy is zero, but kinetic energy is maximum
• The sum remains constant throughout the motion, demonstrating conservation of mechanical energy.

Question: An object is lifted to a height h₁ on Earth and another identical object is lifted to the same height h₁ on the Moon, where gravitational acceleration is one-sixth that of Earth. Which object has greater potential energy?
A. The object on Earth has greater potential energy.
B. The object on the Moon has greater potential energy.
C. Both have equal potential energy.
D. The comparison cannot be made without knowing the mass.
Show Answer & Explanation

Answer: (A) The object on Earth has greater potential energy.

Explanation:
Potential energy is U = mgh. Since both objects have the same mass m and height h, but Earth's gravitational acceleration (g) is six times larger than the Moon's, the object lifted on Earth will have six times greater potential energy. The chapter shows that gravitational potential energy depends directly on g.

Question: A goalkeeper stops a ball by pushing backward against its motion. Which of the following correctly describes the work done in this situation?
A. The work done by the goalkeeper on the ball is positive because force is applied.
B. The work done by the goalkeeper on the ball is negative because the force opposes the displacement.
C. The work done by the goalkeeper is zero because the ball eventually stops moving.
D. The work done cannot be determined without knowing the ball's mass.
Show Answer & Explanation

Answer: (B) The work done by the goalkeeper on the ball is negative because the force opposes the displacement.

Explanation:
When the goalkeeper applies a force opposite to the direction of the ball's motion, the work done is negative. This is demonstrated in Example 7.2, where the goalkeeper's hand moving back by 15 cm against the ball's forward motion results in negative work: W = 200 N × (−0.15 m) = −30 J.

Question: According to the work-energy theorem, when positive work is done on an object, what must happen to its total mechanical energy?
A. It must decrease.
B. It must increase.
C. It must remain constant.
D. It depends on the direction of the force.
Show Answer & Explanation

Answer: (B) It must increase.

Explanation:
The work-energy theorem states that work done on an object equals the change in its energy. Positive work means energy is being added to the object, so its mechanical energy increases. This is why objects gain kinetic energy when forces push them in the direction of motion.

Question: Two objects, A with mass m and B with mass 4m, possess the same kinetic energy. What is the ratio of their velocities?
A. vₐ : vᵦ = 1 : 2
B. vₐ : vᵦ = 2 : 1
C. vₐ : vᵦ = 1 : 4
D. vₐ : vᵦ = 4 : 1
Show Answer & Explanation

Answer: (B) vₐ : vᵦ = 2 : 1

Explanation:
Since kinetic energy K = ½mv², and both objects have equal kinetic energy: ½m(vₐ)² = ½(4m)(vᵦ)². Simplifying: (vₐ)² = 4(vᵦ)², so vₐ = 2vᵦ, giving the ratio vₐ : vᵦ = 2 : 1. This is consistent with the Pause and Ponder question in the chapter.

Question: Why does an inclined plane reduce the effort needed to lift a heavy object compared to lifting it vertically?
A. The mass of the object decreases on an incline.
B. The force required is spread over a larger distance, reducing the effort needed at any point.
C. Gravity acts differently on an incline.
D. The object becomes lighter due to the angle of the incline.
Show Answer & Explanation

Answer: (B) The force required is spread over a larger distance, reducing the effort needed at any point.

Explanation:
An inclined plane increases the distance over which the force acts while keeping the total work constant. The mechanical advantage is L/h (length divided by height), so a longer, gentler ramp requires less force. However, the force must be applied over a greater distance, so the total work remains the same, as shown in Equation 7.13 and Example 7.12.

Question: What is the mechanical advantage of a fixed pulley?
A. Greater than 1, allowing heavier loads to be lifted with less effort.
B. Exactly 1, providing no reduction in force but changing its direction.
C. Less than 1, requiring more force than the load.
D. Zero, since it does not help lift objects.
Show Answer & Explanation

Answer: (B) Exactly 1, providing no reduction in force but changing its direction.

Explanation:
A fixed pulley changes the direction of the applied force but does not reduce its magnitude. The effort required equals the load, so the mechanical advantage is load/effort = 1. This makes it convenient to pull downward rather than upward, but it does not make the task physically easier in terms of force reduction.

Question: A student climbs stairs to reach the top of a building, while another is lifted by an elevator to the same height. Both have the same mass. How do the gains in their potential energies compare?
A. The climbing student gains more potential energy.
B. The elevator student gains more potential energy.
C. Both gain the same potential energy.
D. The comparison depends on the time taken.
Show Answer & Explanation

Answer: (C) Both gain the same potential energy.

Explanation:
Potential energy U = mgh depends only on mass m, height h, and gravitational acceleration g. Since both students have the same mass and reach the same height, they gain the same potential energy regardless of the path taken or time spent. This is shown in Example 5 of the "Revise, Reflect, Refine" section, demonstrating that potential energy is path-independent.

Question: In a lever system, if the effort arm is doubled while the load arm remains the same, what happens to the mechanical advantage?
A. It is halved.
B. It remains the same.
C. It doubles.
D. It becomes zero.
Show Answer & Explanation

Answer: (C) It doubles.

Explanation:
Mechanical advantage of a lever = effort arm / load arm. If the effort arm is doubled and the load arm stays constant, the mechanical advantage doubles. This relationship is shown in Equation 7.16, where increasing the effort arm directly increases the mechanical advantage of the lever.

Question: A ball is thrown vertically upward. At the highest point of its trajectory, which quantity is zero?
A. The gravitational force acting on the ball.
B. The kinetic energy of the ball.
C. The acceleration of the ball.
D. The potential energy of the ball.
Show Answer & Explanation

Answer: (B) The kinetic energy of the ball.

Explanation:
• At the highest point, velocity is zero, so kinetic energy K = ½mv² = 0
• Gravitational force (mg) still acts downward
• Acceleration due to gravity (g) is still present, pointing downward
• Potential energy depends on height and is maximum at the highest point
This is confirmed in the multiple-choice question in the "Revise, Reflect, Refine" section.

Question: Which of the following is an example of elastic potential energy being converted to kinetic energy?
A. A ball rolling down a slope.
B. A stretched rubber band shooting a projectile when released.
C. Water falling from a dam.
D. A car accelerating on a horizontal road.
Show Answer & Explanation

Answer: (B) A stretched rubber band shooting a projectile when released.

Explanation:
A stretched rubber band stores elastic potential energy due to its deformation. When released, this stored energy is converted into kinetic energy of the projectile, as described in the slingshot example in Section 7.4.2. The other options involve gravitational potential energy or energy from fuel or motors, not elastic deformation.

Question: Power is defined as the rate of doing work. If two machines do the same amount of work but one completes it in half the time, how do their power outputs compare?
A. The faster machine has half the power.
B. The faster machine has twice the power.
C. Both machines have equal power.
D. The comparison depends on the mass of the objects involved.
Show Answer & Explanation

Answer: (B) The faster machine has twice the power.

Explanation:
Power P = W/t. If work W is constant but time t is halved, then power doubles. The chapter emphasizes that doing the same work in less time requires more power. Example 7.10 illustrates this: the same lifting task done in less time requires more power in watts.

Question: A fielder throws a cricket ball towards the wickets with a certain velocity. When the ball strikes the wickets, it transfers energy and causes them to fall. Which statement best explains the energy transformation that occurs?
A. The kinetic energy of the ball is completely destroyed upon impact.
B. The kinetic energy of the ball is transferred to the wickets, giving them the ability to move.
C. The ball loses all its potential energy just before hitting the wickets.
D. The gravitational potential energy of the ball increases after the collision.
Show Answer & Explanation

Answer: (B) The kinetic energy of the ball is transferred to the wickets, giving them the ability to move.

Explanation:
When the moving ball collides with the stationary wickets, the kinetic energy it possesses is transferred to the wickets through the collision, causing them to acquire kinetic energy and move. This demonstrates the concept that a moving object can do work on another object by transferring its energy.

Question: An archer pulls back the string of a bow, bending the arms of the bow in the process. Before the arrow is released, the bent bow stores energy due to its deformed shape. What type of energy does the stretched bow possess?
A. Kinetic energy related to the bow's motion.
B. Gravitational potential energy based on the bow's height.
C. Elastic potential energy stored in the deformed material.
D. Chemical energy released during the bending process.
Show Answer & Explanation

Answer: (C) Elastic potential energy stored in the deformed material.

Explanation:
A deformed object like a stretched bow stores energy in its shape or configuration. This stored energy, which can do work when the object returns to its original form, is called potential energy—specifically elastic potential energy in this case.

Question: A cart is pulled up a smooth inclined plane at constant speed using a spring balance. Compared to lifting the cart vertically through the same height, the spring balance reading is:
A. exactly the same as the weight of the cart.
B. greater than the weight of the cart.
C. less than the weight of the cart.
D. zero because the cart moves at constant velocity.
Show Answer & Explanation

Answer: (C) less than the weight of the cart.

Explanation:
• An inclined plane reduces the force needed to move an object to a higher level
• The force required is reduced because it is distributed over a longer distance along the ramp
• The mechanical advantage of the inclined plane is the ratio L/h, which is greater than 1
• Therefore, the required force (effort) is less than the weight, though work done remains the same

Question: A ball is released from rest at the top of a smooth slide. As it reaches the bottom, which of the following occurs?
A. Only kinetic energy increases; potential energy remains zero throughout.
B. Potential energy decreases and is entirely converted into kinetic energy.
C. Both kinetic and potential energy remain constant.
D. Kinetic energy decreases while potential energy increases.
Show Answer & Explanation

Answer: (B) Potential energy decreases and is entirely converted into kinetic energy.

Explanation:
As the ball slides down, it loses height, so its gravitational potential energy decreases. By conservation of mechanical energy (assuming no friction), this lost potential energy is converted into kinetic energy. At the bottom, the ball has maximum kinetic energy and minimum potential energy.

Question: A machine applies a force F to move an object over a distance d in the direction of the force. The machine then performs the same task but in half the time. How does the power output of the machine change in the second case?
A. It decreases to half its original value.
B. It increases to twice its original value.
C. It remains exactly the same.
D. It increases by a factor of four.
Show Answer & Explanation

Answer: (B) It increases to twice its original value.

Explanation:
Power is the rate of doing work, expressed as P = W/t. If the work done (W = F × d) remains constant but the time is halved, the power output must double.

Question: A spring is compressed and then released, propelling a small object forward. Which energy transformation sequence correctly describes this process?
A. Kinetic energy → elastic potential energy → kinetic energy.
B. Gravitational potential energy → elastic potential energy → kinetic energy.
C. Elastic potential energy → kinetic energy → gravitational potential energy.
D. Kinetic energy → gravitational potential energy → elastic potential energy.
Show Answer & Explanation

Answer: (C) Elastic potential energy → kinetic energy → gravitational potential energy.

Explanation:
Initially, elastic potential energy is stored in the compressed spring. Upon release, this energy is converted into kinetic energy of the object as it moves forward. If the object rises, some kinetic energy may later convert to gravitational potential energy.

Question: Two identical balls are dropped from the same height. Ball A falls on soft sand and comes to rest by making a depression. Ball B falls on concrete and bounces back. Which statement about the work done by the impact surface is correct?
A. Both surfaces do zero work because the ball's final height is less than its initial height.
B. The sand does negative work on the ball, bringing it to rest; the concrete does positive work by bouncing it back.
C. The sand does positive work equal to the ball's initial gravitational potential energy.
D. The concrete surface does more negative work than the sand because the ball bounces higher.
Show Answer & Explanation

Answer: (B) The sand does negative work on the ball, bringing it to rest; the concrete does positive work by bouncing it back.

Explanation:
When the sand stops the ball, it exerts a force opposite to the ball's motion (downward displacement), so work done is negative. The concrete, by bouncing the ball upward, exerts an upward force during the rebound (opposite to the downward displacement during impact), also doing negative work. However, the sand removes more energy as heat and deformation, while the concrete returns energy to the ball through bouncing.

Question: In Activity 7.2, a pendulum bob at point P (highest position) has only potential energy. At point Q (lowest position), what is the relationship between the bob's total mechanical energy at P and its total mechanical energy at Q?
A. Mechanical energy at Q is half that at P.
B. Mechanical energy is equal at both points (ignoring friction).
C. Mechanical energy at Q is greater because the bob is moving.
D. Mechanical energy at P is greater because the bob is higher.
Show Answer & Explanation

Answer: (B) Mechanical energy is equal at both points (ignoring friction).

Explanation:
The law of conservation of mechanical energy states that when only gravitational forces act on an object, its total mechanical energy (kinetic plus potential) remains constant. At P, all energy is potential; at Q, all is kinetic—but the sum is the same at both points.

Question: A person uses a seesaw to lift a heavy rock. The rock is placed close to the fulcrum, while the person applies effort far from the fulcrum. Why does this arrangement allow the person to lift a heavier rock than by direct lifting?
A. The rock becomes lighter when placed near the fulcrum.
B. The longer effort arm produces a larger force on the rock through the principle of moments.
C. The fulcrum eliminates the effect of gravity on the rock.
D. The mechanical advantage equals the distance from fulcrum to the person.
Show Answer & Explanation

Answer: (B) The longer effort arm produces a larger force on the rock through the principle of moments.

Explanation:
In a lever, the relationship effort × effort arm = load × load arm governs equilibrium. With a longer effort arm and shorter load arm, the mechanical advantage (load/effort) increases, allowing a smaller applied force to overcome a larger load.

Question: A truck on an escape ramp (filled with sand) slows down as it climbs. According to the work-energy theorem, what happens to the truck's kinetic energy?
A. It increases because the truck is moving upward against gravity.
B. It decreases because the sand and gravity both do negative work on it.
C. It remains constant because the truck maintains some forward velocity.
D. It is converted entirely into thermal energy by friction.
Show Answer & Explanation

Answer: (B) It decreases because the sand and gravity both do negative work on it.

Explanation:
• Negative work by gravity reduces kinetic energy (truck climbs against gravitational force)
• Negative work by sand friction also reduces kinetic energy (opposes motion)
• The work-energy theorem states that net work equals change in kinetic energy
• Since both forces oppose motion, net work is negative and kinetic energy decreases until the truck stops

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