AP 9th Class Physical Science Chapter 9 Solution – Work and Energy
Andhra Pradesh SCERT 9th Class Physical Science Chapter 9 Work and Energy Solution for AP 9th Class Physics/Chemistry Exam. Lots of Students of Andhra Pradesh Board will search on internet for Andhra Pradesh Class 9 Physical Science Textbook Solution or Study Material for AP 9th exam. Here you search will end! Here in this page we have provided for all question answer for AP SCERT Chapter 9 Work and Energy.
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AP 9th Class Physics Chapter 9 Work and Energy Solution
1) Define work and write its units. (AS1)
Ans: –Work is the multiple of force and displacement (w=fs). Its unite is N.
2) Give few examples where displacement of an object is in the direction opposite to the force acting on the object. (AS1)
Ans: –If we through a ball to the sky in this case the direction is opposite to force acting on it. In another example is when we rolls a ball in floor the friction of the floor restricted the speed in opposite direction.
3.)Identify the wrong statement among the following. Rewrite them by making necessarycorrections. (AS1)
a) Work and energy have different units.
b) When an aeroplane takes off, the work done by its weight is positive.
c) The potential energy of spring increases when it is extended and decreases when it is compressed.
d) If the work done by external forces on a system is negative then the energy of the system decreases.
e) When a body is falling freely from a height, its kinetic energy remains constant.
f) The unit of power is watt.
Ans: –a) work and energy have same unit.
b) when an aeroplane takes off the work done by its weight is negative.
C) when we extend or compress the spring potential energy increases and when spring is release potential energy decrease.
d) External negative force decrease the energy of a system.
e) When a body is failing freely from a height its kinetic energy increases.
f)The unit of power is j/s or watt.
4.) What is mechanical energy? (AS1)
Ans: –Mechanical energy = potential energy + kinetic energy.
5) State the principle of conservation of energy. (AS1)
Ans: – conservation of energy States that energy never created or destroyed it just only change it’s from.
6) When you lift a box from the floor and put it on an almirah the potential energy of the box increases but there is no change in its kinetic energy. Is it violation of conservation of energy? Explain. (AS7)
Ans: – When we put the box on an almirah the potential energy increase and the fore we applied to do this work which is equal to the gravitational energy so there is no change in velocity. Then the kinetic energy remain constant.
7) One person says that potential energy of a particular book kept in an almirah is 20 J and other says it is 30J. Is one of them necessarily wrong? Give reasons. (AS2 ,AS1)
Ans: –We know that potential energy of any object is mgh where m is mass, g is gravitational constant and h is height from the ground. So in case 20j or 30j both are potential energy only difference is the height from the ground.
8.) In which of the following cases is the work done positive or zero or negative? (AS1)
a) Work done by the porter on a suitcase in lifting it from the platform on tohis head. (Work done is positive)
b) Work done by the force of gravity on suitcase as the suitcase falls from porter’shead. (Work done is positive)
c) Work done by the porter standing on platform with suitcase on his head. (Work done is zero)
d) Work done by force of gravity on a ball thrown up vertically up into the sky. (Work done is negative)
e) Work done by force applied by hands of a man swimming in a pond. (Negative work done)
9.) What is potential energy? Derive an equation for gravitational potential energy of a body of mass ‘m’ at a height ‘h’. (AS1)
Ans: –The ability of a body to do work is the potential energy of that body.
The mass of the body is m. When it kept at a height h from the ground then the force of gravity is mg. As we know that the work done is (force × displacement) or (f×h) or mgh.
10.) When an apple falls from a tree what happens to its gravitational potential energy just asit reaches the ground? After it strikes the ground? (AS7)
Ans: –When the apple just reaches the ground the gravitational potential decrease as the kinetic energy increase while falling from tree.
And after touching the ground kinetic energy decrease.
11.) Let us assume that you have lifted a suitcase and kept it on a table. On which of thefollowing does the work done by you depend or not depend? Why? (AS2, AS1)
a) The path taken by the suitcase. b) The time taken by you in doing so.
c) The weight of the suitcase. d) Your weight.
Ans: –The work done by any object is equal to the multiplication force and displacement. So we see that there is no dependency on the path taken by the suitcase or time taken by us or our weight.
12.) When you push your bicycle up an incline, the potential energy of the bicycle and yourself increase. Where does this energy come from? (AS7)
Ans: –The potential energy of the bicycle is came from the muscle of our body. The energy of our body muscle transform to potential energy of the bicycle.
13) Why does a person standing for a long time get tired when he does not appear to be doing any work? (AS7)
Ans:-The muscle of our all time work while standing it works more so more energy required by this the body muscle get tired . And for that standing long time create tiredness in our body.
14.) What is kinetic energy? Derive an equation for the kinetic energy of a body of mass ‘m’ moving at a speed ‘v’. (AS1)
Ans: – The kinetic energy of an object is create when it start movement.
Let the acceleration due to gravity of a body of mass m is ‘a’. As the body start from rest so ins initial velocity is zero. Displacement is s.
According to newton equation of motion, v^2=u^2 + 2as. Or, s=v^2/2a.
As we know that work done=f ×s = ma × (v^2/2a) = 1/2mv^2.
Then the kinetic energy of the body is 1/2mv^2.
15.) A free-fall object eventually stops on reaching the ground. What happens to its kinetic energy? (AS1)
Ans: –When the object eventually stops on reaching the ground the kinetic energy will become zero. This energy become in various from after hitting the ground.
16.) A man carrying a bag of total mass 25kg climbs up to a height of 10m in 50 seconds. Calculate the power delivered by him on the bag. (AS1)
Ans: – Power = (work / time) = (F×s /t); where f is force, s is displacement and t is time.
From the question we get mass is 25 kg and we know gravitational constant g is 10m/s^2. So force or mg =25×10=250N.
Then power =250×10 /50 = 50 watt.
17) A 10 kg ball is dropped from a height of 10m. Find (a) the initial potential energy of theball, (b) the kinetic energy just before it reaches the ground, and (c) the speed justbefore it reaches the ground. (AS1)
Ans: – a) The initial potential energy = mgh.
Given that mass m is 10kg, height h is 10m. So the potential energy is (10×10×10 = 1000) 1000(j).
b) When the ball reaches the ground the potential energy of the ball convert into kinetic energy, so the kinetic energy will be 1000j.
c) The kinetic energy = 1/2mv^2,
or, v =√ (2×kinetic energy)/mass
So the speed will be =√ (2×1000)/10 = 14.14 m/s.
18) Calculate the work done by a person in lifting a load of 20 kg from the ground and placing it 1m high on a table. (AS1)
Ans: – Thework done = force × displacement = f × s.
From questions it is given that mass is 20kg, displacement is 1 m and we know g is 10. Then work done by the person in lifting is (20×1×10 =200) 200 j.
19) Find the mass of a body which has 5J of kinetic energy while moving at a speed of 2m/s. (AS1)
Ans: – let the mass of a body is m, we know that kinetic energy =1/2mv^2.
It is given that v=2m/s, so 5 = ½×m×2^2.
Or, m = 5/2 =2.5 kg.
20) A cycle together with its rider weighs 100kg. How much work is needed to set it moving at 3 m/s. (AS1)
Ans: – In moving the kinetic energy required is ½×mv^2.
It is given that m=100kg, v=3m/s, so energy is (1/2×100×3^2 = 450)450N.
The work needed to set it moving is 450N.
21.) When the speed of a ball is doubled its kinetic energy (AS1)
(a) Remains same (b) gets doubled
(c) Becomes half (d) becomes 4 times.
Ans: – d) becomes 4 times.
22.) Two bodies of unequal masses are dropped from the top of building. Which of thefollowing is equal for both bodies at any instance? (AS1, AS2)
(a) Speed (b) force of gravity
(c) Potential energy (d) kinetic energy
Ans: –a) speed.
23.) A man with a box on his head is climbing up a ladder. The work done by the man on the box is (AS1)
(a) Positive (b) Negative
(c) Zero (d) Undefined.
Ans: –a) positive.
24.) A porter with a suitcase on his head is climbing up steps with uniform speed. The work done by the “weight of the suitcase” on the suitcase is (AS1)
(a) Positive (b) Negative
(c) Zero (d) Undefined
Ans: – b) negative.
26.) How do you appreciates role of energy conversion occurring naturally in maintaining ecological balance of nature? (AS6)
Ans: –The hydroelectric power plant totally depends on the flow of river water which came from the melting of ice of glacier or the rain. Which is a gift of nature to us. The daily use petroleum also extracted from the dumped dead plant, animal million year ago. The food grain is the source of energy in our body. So in every stage we depends on the nature and by this ecological balance of nature maintain.
27.) Collect pictures showing various situation where potential energy possessed by object depends on its shape and position. Prepare a scrap book. (AS4)
Ans.
28) Draw a diagram to show conservation of mechanical energy in cafe of free falling body. (AS5) Ans:-
Ans:-