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NCERT Class 9 Science Important Chapter 7 Work, Energy, and Simple Machines
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Work, Energy, and Simple Machines
Chapter – 7
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. How is the work done by a constant force defined in science?
Ans: The work done by a constant force acting on an object is defined as the force applied multiplied by the displacement in the direction of the force.
2. What is the SI unit of both work and energy?
Ans: The SI unit of both work and energy is the joule (J).
3. How is 1 joule of work defined?
Ans: 1 joule of work is done on an object when a constant force of 1 newton is applied to it and it is displaced by 1 metre in the direction of the force.
4. If you push hard against a rigid wall but it does not move, how much work have you done on the wall?
Ans: The work done on the wall is zero because there is no displacement of the object.
5. Why is the work done zero when a force acts in a direction perpendicular to the displacement?
Ans: The work done is zero because there is no displacement in the direction of the applied force.
6. When is the work done by a force considered positive?
Ans: The work done is considered positive when the displacement is in the same direction as the applied force.
7. Give an example of a situation where negative work is done.
Ans: A goalkeeper stopping a moving football does negative work because the force applied is in the direction opposite to the displacement of the ball.
8. State the work-energy theorem.
Ans: The work-energy theorem states that the work done on an object is equal to the change in its energy.
9. What is kinetic energy?
Ans: The energy possessed by an object due to its motion is called kinetic energy.
10. What is the mathematical expression for the kinetic energy of an object?
Ans: The kinetic energy of an object of mass m moving with a velocity v is given by the expression 1/2 mv².
11. What happens to the kinetic energy of an object if its velocity is doubled?
Ans: If the velocity of an object doubles in magnitude, its kinetic energy becomes 4 times its original value.
12. What is potential energy?
Ans: The energy stored by an object as a result of its deformation or in a system of objects due to their relative positions is called potential energy.
13. What is the formula for the gravitational potential energy of an object at a certain height?
Ans: The potential energy of an object of mass m at a height h from the ground is mgh, where g is the acceleration due to gravity.
14. What is the principle of conservation of mechanical energy state?
Ans: It states that as an object moves due to gravitational force, its mechanical energy (sum of kinetic and potential energy) remains constant if no other external forces act on it.
15. How is power defined in physics?
Ans: Power is defined as the rate at which work is done, calculated as the work done divided by the time taken.
16. What is the SI unit of power, and how is it related to joules?
Ans: The SI unit of power is the watt (W), where 1 watt is equal to 1 joule of work done per second.
17. How many watts are there in one horsepower (hp)?
Ans: One horsepower is equal to 746 watts.
18. What are simple machines?
Ans: Simple machines are devices that make work easier by changing the magnitude or direction of the force that needs to be applied, although they do not reduce the total work required.
19. How is the mechanical advantage of a simple machine defined?
Ans: Mechanical advantage is defined as the ratio of the load (the force to be overcome) to the effort (the force applied).
20. What mathematical principle balances a lever?
Ans: A lever is balanced when the effort multiplied by the effort arm is equal to the load multiplied by the load arm.
Fill in the Blanks:
1. The SI unit of both work and energy is the __________.
Ans: Joule (J).
2. The energy possessed by an object due to its motion is called __________ energy.
Ans: Kinetic.
3. The gravitational potential energy of an object is given by the formula __________.
Ans: mgh.
4. The SI unit of power is the __________.
Ans: Watt (W).
5. The fixed point about which a lever rotates is called the __________.
Ans: Fulcrum.
True or False:
1. Work done is zero if there is no displacement of the object.
Ans: True.
2. If the velocity of an object doubles, its kinetic energy becomes twice its original value.
Ans: False.
3. One horsepower is equal to 746 watts.
Ans: True.
4. Simple machines reduce the total amount of work required.
Ans: False.
5. Mechanical advantage is the ratio of load to effort.
Ans: True.
Long Questions & Answers:
1. Explain the scientific concept of work, detailing the specific conditions under which the work done by a force is considered positive, negative, and zero, giving relevant examples for each.
Ans: In science, work is defined as the product of the force applied to an object and its displacement in the direction of that force. The work done is considered positive when the displacement aligns completely with the direction of the applied force, such as when you push a wheelchair forward and it moves in that exact direction. Conversely, work is negative when the force opposes the object’s displacement, like a goalkeeper applying a stopping force backward against a football that is moving forward. Furthermore, work is definitively zero if there is no displacement at all, like when pushing exhaustingly against a rigid wall, or if the applied force is perpendicular to the displacement, such as the upward force exerted by a person carrying a box while walking horizontally across a room.
2. State the work-energy theorem and explain how the kinetic energy of an object is derived from the work done on it by a constant force, including the effect of changing the object’s velocity.
Ans: The work-energy theorem states that the work done on an object results in a direct change in its energy, which provides a profound way to understand kinetic energy, defined as the energy possessed by an object strictly due to its motion. When a constant force acts on an object starting from rest, the work done translates entirely into its kinetic energy, mathematically expressed as half of its mass multiplied by the square of its velocity. Because this kinetic energy depends on the square of the velocity, if a vehicle’s speed doubles, its kinetic energy exponentially increases to become four times its original value, demonstrating how doing mechanical work is a fundamental method of transferring energy and altering an object’s state of motion.
3. Describe what gravitational potential energy is and explain the principle of conservation of mechanical energy using the example of an object falling freely from a certain height.
Ans: Gravitational potential energy is the energy stored by an object due to its vertical position or height relative to the Earth’s surface, mathematically defined as the product of its mass, the acceleration due to gravity, and its height. According to the principle of conservation of mechanical energy, the total mechanical energy (which is the sum of kinetic and potential energy) of an object remains absolutely constant as long as no external forces act upon it. For instance, when an object falls freely from a height, its initially maximum potential energy gradually decreases while its kinetic energy simultaneously increases as its velocity builds, meaning the lost potential energy is perfectly and continuously converted into kinetic energy, keeping the total mechanical energy conserved throughout the entire fall
4. Define power in physics, explain its relationship with work and time, and discuss its standard unit along with how it applies to real-life situations like lifting objects or accelerating vehicles.
Ans: Power is fundamentally defined as the rate at which work is done, illustrating the critical difference between performing a task slowly versus quickly, even if the total amount of work remains identical. The standard SI unit of power is the watt, which equates to one joule of work completed per second, though other units like horsepower are frequently used for measuring car engines or water pumps, where one horsepower equals exactly 746 watts. To accomplish the same amount of work in a much shorter time interval, or to perform more work within the same timeframe, a proportionately higher power output is required, which explains why a car engine accelerating rapidly to reach a high speed or a weightlifter lifting a heavy mass very quickly demands a substantial amount of power.
5. What are simple machines, and how does a lever function to provide a mechanical advantage, including an explanation of its different classes based on the positions of the fulcrum, load, and effort?
Ans: Simple machines, such as pulleys, inclined planes, and levers, are ingenious devices that make tasks significantly easier by altering the magnitude or direction of the applied force, even though they cannot actually reduce the total amount of work required. A lever, specifically, is a rigid bar that rotates around a fixed point called a fulcrum, and it provides a mechanical advantage by balancing the product of the effort and the effort arm with the product of the load and the load arm, thereby allowing a smaller effort applied over a larger distance to effectively move a much heavier load. These levers are categorized into three distinct classes depending on the relative arrangement of these components: Class I levers have the fulcrum placed in the middle (like a seesaw), Class II levers place the load in the middle (like a lemon squeezer), and Class III levers apply the effort in the middle (like a pair of tongs).
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