Class 9 Science Notes Chapter 7: Work, Energy, and Simple Machines
Table of Contents
- Work
- Energy
- The Work Energy Theorem
- James Prescott Joule
- Forms of Energy
- Mechanical Energy
- Conservation of Mechanical Energy
- Power
- Simple Machines
2. Work
Work is done by a force on an object, when the force displaces the object in the direction of the force.
3. Energy
An object having capacity to do work is said to possess energy.
4. Work Energy Theorem
Work done on an object or a system is equal to the change in its energy.
work done on an object = change in its energy
5. Forms of Energy
Mechanical Energy: Mechanical energy Energy due to motion or position of objects
Thermal Energy: Thermal energy Energy that makes things warm or hot.
Light Energy: Light energy Energy that allows us to see.
Sound Energy: Sound energy Energy of vibrations of air or other molecules.
Electrical Energy: Electrical energy Energy related to position or motion of charges.
Nuclear Energy: Nuclear energy Energy stored in the nuclei of atoms.
Chemical Energy: Chemical energy Energy stored in fuels and food in the form of chemical bonds between atoms.
6. Mechanical Energy
Mechanical energy is the energy that an object possesses due to its motion or position.
Kinetic Energy :The energy possessed by an object due to its motion is called kinetic energy.
Potential Energy: 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.
10. Simple Machines
Simple machines are devices that make work easier by changing the magnitude or direction of the force that needs to be applied, though they do not reduce total work.
Pause and Ponder
1. In the previous chapter, a weightlifter is shown holding a barbell steady in her hands (Fig. 6.8). Is she doing any work on the barbell while holding it steady?
2. Is the work done by friction on the stack of coins that travels on a rough surface (Fig. 6.13c) — positive, negative or zero?
3. When you pedal a bicycle on a flat road, your muscles supply energy. In what forms does this muscular energy appear as you ride?
4. Two objects A and B of mass m and 4 m have the same kinetic energy. What is the ratio of the magnitude of velocities of A and B?
5. Does the kinetic energy of an object which moves with constant velocity change with its position?
6. Does the potential energy of an object near the surface of the Earth change if it moves with constant velocity in the horizontal direction?
What if the object is gradually raised in the vertical direction?
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12. Why do you push an object closer to scissors (fulcrum) when you want to cut an object which is hard?
13. Throughout history, many designs of perpetual machines (using wheels, weights or magnets) have been proposed but none actually work. Why do all real machines eventually slow down and stop? Explain in terms of work and energy.