9th Class Physics New Book Chapter 5 Short Questions
9th Class Physics New Book Chapter 5 Short Questions Notes
5.1 what is the work done on an object that remains at rest when a force is applied on it?
Work done is the product of force and displacement in the direction of the force. Because there is no movement, so there is no work
W = Fd = F(o) = 0 J
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5.2 A slow moving car may have more kinetic energy than a fast moving motorcycle .How is it possible ?
Due to heavy mass of car, its energy will be greater than the energy of motorcycle.
$$K.E\;=\;\frac12mv^2$$
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5.3 A force F1 does 5J of work in 10s. Another force F2 does 3J of work in 5s. Which force delivers greater power ?
We know that Power = Work/time.
Power of F1 = 5/10 =0.5 watts
Power of F2=3/5 = 0.6 watts
Hence F2 delivers greater power than F1.
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5.4 A women runs up a flight of stairs . The gain in her gravitational potential energy is 4500J. If she runs up the same stairs with twice the speed, what will be her gain in potential energy ?
Running faster means she does the same work in less time. Hence her power increases but not the potential energy because potential energy depends on mass, gravitational acceleration and height not the time. Her potential energy be again 4500 J.
P.E = mgh
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5.5 Define work and its SI units .
Work is defined as the product of magnitude of force and the distance covered in the direction of the force. SI unit of work is Joule. it is denoted by J. One joule work is done when a force of 1 Newton acting on a body moves it through a distance of 1 meter in its own direction.
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5.6 what is the potential energy of a body of mass m when it is raised through a height h?
Potential energy = mgh. Unit of Potential energy is Joule.
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5.7 Find an expression for the kinetic energy of a moving body?
$$K.E\;=\;\frac12mv^2$$
Where m is mass of the body and v is the speed of the body.
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5.8 Define efficiency of a working system . Why a system cannot have 100% efficiency?
Efficiency of a working system is the ratio of useful output energy and the total input energy.
Energy output is always less then the input energy. We cannot convert all the input energy into required output, hence we can obtain 100% efficiency.
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5.9 What is power? Define the unit used for it.
Power is defined as the time rate of doing work. Power = Work/Time
Watt is SI unit of Power. It is denoted by W. Watt is defined as the work done at the rate of 1 Joule per second.
1KW = 1000 watts
One Horse Power ( hp ) = 746 watts
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5.10 Differentiate between renewable and non-renewable energy source.
We can naturally reproduce renewable energy sources in short time. For example solar energy and wind energy. They are unlimited, environment friendly and cheaper. They do not have harmful effects.
We cannot naturally reproduce non-renewable energy sources. For example Natural gas and nuclear fuels. Non-renewable energy sources are limited. They can cause pollution and costlier. They can cause global warming.
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