9th Class Physics Chapter 5 Constructed Response Questions
9th Class Physics Chapter 5 Constructed Response Questions
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.
- Draw a diagram of what happened.
- 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. It can be regrown by planting trees. But unsustainable cutting can make it non-renewable.
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