9th Class Physics New Book 2025 Chapter 5 Answer Question
9th Class Physics New Book 2025 Chapter 5 Answer Question
5.1 Can the kinetic energy of a body ever be negative?
No, Kinetic energy will never be negative. Because mass and square of speed will always be positive.
$$K.E=\frac12mv^2$$
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5.2 Which one has the greater kinetic energy; an object travelling with a velocity v or an object object twice as heavy travelling with a velocity of 1/2 v?
The object of mass m moving with velocity v has greater kinetic energy.
Kinetic energy formula:-
$$K.E=\frac12mv^2$$
Object 1 mass =m and velocity = v
So its kinetic energy will be
$$K.E=\frac12mv^2$$
Object 2 mass= 2m and velocity = v/2
$$K.E=\frac12(2m){(\frac v2)}^2\\=\frac12(2m)\frac{v^2}4\\=\frac12\frac12mv^2\\=\frac12(K.E\;of\;1st\;object)$$
Hence kinetic energy of the 1st object will be double the kinetic energy of the 2nd object.
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5.3 A car is moving along a curved road at constant speed. Does its kinetic energy change?
$$K.E=\frac12mv^2$$
kinetic energy depends on mass and speed. Due to constant speed its kinetic energy will not change.
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5.4 Comment on the statement. ” An object has one joule of potential energy.”
An object has one joule of potential energy is incomplete sentence because no reference level is given. Sentence is complete If we say an object has one joule of gravitational potential energy with respect to earth when it is raised to a certain height above the earth’s surface. One joule of potential energy means one joule of work is done to raise an object against the gravity when reference level is earth.
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5.5 While driving on a motorway, tyre of a vehicle sometimes bursts. What may be its cause
Friction between tyre and rode causes the tyre to heat up. The air inside the tyre expand and increases the internal pressure. If the pressure exceeds the tyre’s strength, the tyre may burst.
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5.6 While playing cricket on a street, the ball smashes a window pane. Describe the energy changes in this event.
The moving ball has kinetic energy. When it hits its energy changes into sound energy and heat energy and some of the energy is used to break the glass. After break pieces of glass move with kinetic energy.
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5.7 A man rowing boat upstream is at rest with respect to the shore. Is he doing work?
Yes, he is doing work. Boat is at rest but the man is using energy to balance the opposing force of water. So, he is doing the work.
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5.8 A cyclist goes downhill from the top of a steep hill without pedalling and takes it to the top of the next hill.
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Draw a diagram of what happened.
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Analyse this event in terms of potential and kinetic energy. Label your diagram using these terms.
At the top of the hill the cyclist maximum potential energy and zero kinetic energy. As he goes down the hill, gravitational potential energy converts into kinetic energy. At the bottom its kinetic energy will be maximum and potential will be minimum. Going up the next hill its kinetic energy again starts to convert into the potential energy. At the top of the next hill its energy will be completely potential energy.
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5.9 Is timber or wood renewable source of heat energy? Comment.
Yes timber and wood are renewable sources of heat energy. By planting trees, we can regrow it. But unsustainable cutting can make it non-renewable.
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9th Class Physics New Book 2025 Chapter 5 Answer Question
5.1 What is meant by kinetic energy? State its unit. Describe how it is determined.
The kinetic energy of a body is the energy that a body possesses by virtue if its motion. A object which is moving has the ability to do work due to its motion and this ability is called kinetic energy.
Kinetic energy formula:-
$$K.E=\frac12mv^2$$
Unit of kinetic energy is joule. We denote Joule by J. One joule of kinetic energy is the energy possessed by a body when a force of 1 newton moves it 1 meter in the direction of the force.
The work done on a body by a net force is equal to the change in its kinetic energy. If a force F moves a body through a distance d along the line of force, the work done w is
w= F.d
This work increases the body’s kinetic energy. K.E=w
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5.2 State the law of conservation of energy. Explain it with the help of an example of a body falling from certain height in terms of its potential energy and kinetic energy.
Law of conservation of energy states that energy can never be destroyed nor created. It may be transformed from one form to another, but the total amount of energy never changes.
Let a body of m be at rest at point “a” above the height h from the ground. At that point its kinetic energy will be zero and its potential energy will be mgh so its total energy will be mgh.
$$K.E_a=0\\P.E_a=mgh\\T.E_a=K.E_a\;+\;P.E_a\\=0+mgh=mgh$$
Now the body is allowed to drop to point “b” at a height x from the ground. This body will gain its kinetic energy and loss its potential energy. The loss of potential energy will appear as the gain in its kinetic energy.
$$K.E_b=mgx\\P.E_b=mg(h-x)\\T.E_b=K.E_b\;+\;P.E_b\\=mgx+mg(h-x)\\=mgx+mgh-mgx\\=mgh\\$$
Just before hitting the ground at point “c” all of its potential energy will be changed into kinetic energy.
$$K.E_c=mgh\\P.E_c=0\\T.E_c=K.E_c\;+\;P.E_c\\=mgh+0=mgh$$
Thus the total energy remains the same.
On hitting the ground, this energy is dissipated as heat energy and sound energy.
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5.3 Differentiate between renewable and non renewable sources of energy. Give three examples for each.
The sources of energy which are replaced by new ones after their use are called renewable energy sources. Non-renewable sources are the sources which are depleted with the continuous use.
Examples of renewable energy sources are solar energy, wind energy and wave energy.
Examples of non-renewable energy sources are coal, natural gas and nuclear energy.
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5.4 Explain what is meant by efficiency of a machine. How is it calculated? Why there is a limit for the efficiency of a machine?
Efficiency of a machine is the ratio of useful output energy and the total input energy.
We can calculate the efficiency by dividing the useful output energy with the total input energy.
There is a limit for the efficiency of machine due to friction, air resistance, sound, deformation and heat loss.
Efficiency of a machine can never be 100 percent.
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5.5 Describe the process of electricity generation by drawing a block diagram of the process in the following cases.
Hydroelectric power generations
Fossil fuels
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