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Solution of Work and Energy (Page No - 148 & 149) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Page 148
Solution 71
Work done is more on the bicycle because the truck does not move at all and the bicycle moves through a certain distance. And work is said to be done only when applied force produces motion in the body.


Page 149
Solution 72
The work done will decrease as the angle between the direction of force and direction of motion is increased gradually because 

W= F cos θ x s

and cos θ decreases as the angle θ is increased.

Solution 73
The work done will be zero when angle between the direction of force and direction of motion is 90obecause 

Solution of Work and Energy (Page No - 148 & 149) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9

Solution 75
The work done is zero because the gravitational force acts along the radius of the circular path, at right angles i.e. 90o to the motion of satellite.
Solution 76
Weight of man = Mg = 800 N
Weight of package = mg = 200 N
Total weight of man and package = Mg + mg = (M+m)g = 1000 N
Height of the summit, h = 1200 m
i) Work done = (M+m) x g x h = 1000 x 1200 = 12 x 105 J
ii) Potential energy of the package = m x g x h = 200 x 1200 = 2.4 x 105 J
Solution 77
Potential energy becomes maximum.
Solution 78
The work done by both X and Y are equal because irrespective of whether they reach the top of building by using a spiral or slanted ladder, the vertical distance moved by them against the gravity is same.
Solution 79
Yes, the kinetic energy of the ball thrown inside a moving bus depends on the
speed of the bus because the speed of the bus adds up to the speed with which
the ball is thrown inside the moving bus.
Solution 80 

Solution of Work and Energy (Page No - 148 & 149) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9

distance covered before stopping , s = 20 cm = 0.2 m

Here, initial velocity u = 400m/s

Final velocity = 0

Solution of Work and Energy (Page No - 148 & 149) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9

Kinetic energy of the bullet is mainly converted into hear energy (by friction).


 

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FAQs on Solution of Work and Energy (Page No - 148 & 149) - Physics by Lakhmir Singh, Class 9 - Extra Documents & Tests for Class 9

1. What is work in the context of physics?
Ans. In physics, work is defined as the measure of energy transfer that occurs when an object is moved against an external force. It is calculated by multiplying the force applied to an object with the displacement of the object in the direction of the force.
2. How is work related to energy?
Ans. Work and energy are closely related concepts in physics. The work done on an object is equal to the change in its energy. When work is done on an object, energy is transferred to it, increasing its energy. Similarly, when work is done by an object, energy is transferred from it, decreasing its energy.
3. What is the unit of work and energy?
Ans. The unit of work and energy in the SI system is the joule (J). One joule is equal to the work done when a force of one newton is applied to move an object one meter in the direction of the force.
4. How can one calculate the work done in lifting an object vertically?
Ans. The work done in lifting an object vertically can be calculated by multiplying the force applied to lift the object with the vertical distance it is lifted. Mathematically, the formula is given as: Work = Force × Distance × cosθ, where θ is the angle between the direction of the force and the direction of the displacement.
5. Can the work done on an object be negative?
Ans. Yes, the work done on an object can be negative. This happens when the force and displacement are in opposite directions. In such cases, the work done is considered as negative, indicating that energy is being transferred away from the object.
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