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The power of a motor pump is 2 kW. How much water per minute the pump can raise to a height of 10 m? (Given g = 10 m s–2)

Soln: Power of pump = 2kW =2000W Time (t)= 60sec Height (h) = 10m g = 10m/s2 Power =work done per unit time. Work done = mgh = m ×10×10 =100m 2000W = 10𝑚 60𝑠 Therefore, m = 1200kg So, the pump can raise 1200kg of water in one minute.

ScienceClass 9CBSE

Can any object have momentum even if its mechanical energy is zero? Explain.

Soln: No, Because its potential and kinetic energy are zero.

ScienceClass 9CBSE

Can any object have mechanical energy even if its momentum is zero? Explain.

Soln: Momentum is the product of mass and velocity. If the body is at rest its velocity will be zero. If it is reast at height it possess gravitational potential energy hence mechanical energy.

ScienceClass 9CBSE

Avinash can run with a speed of 8 m s–1 against the frictional force of 10 N, and Kapil can move with a speed of 3 m s–1 against the frictional force of 25 N. Who is more powerful and why?

Soln: P=f×v P1= 10× 8 = 80W P2=25×3=75W Hence Avinash is more powerful than Kapil

ScienceClass 9CBSE

A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?

Soln: Initial velocity=v=3v Initial kinetic energy= 1v2 2 Kinetic energy= 1v2 2 1m(3v)2= 9(1 mv2) 2 2 Ratio between Initial kinetic energy and final kinetic energy is 1:9

ScienceClass 9CBSE

1. A body is falling from a height h. After it has fallen a height h 2 , it will possess (a) only potential energy (b) only kinetic energy (c) half potential and half kinetic energy more kinetic and less potential energy

Soln: Answer is (c) half potential and half kinetic energy Explanation: When body is at height h, its potential will be hundred percent and kinetic energy will be zero. In the same way when body reached ground its potential energy will be zero and kinetic energy will be maximum. At height h/2 both potential energy and kinetic energy of the body will be half.

ScienceClass 9CBSE

Water stored in a dam possesses (a) no energy (b) electrical energy (c) kinetic energy potential energy

Soln: Answer is (d) potential energy Explanation: Potential energy is the energy stored in an object because of the position. Water in the dam possess energy because of position hence it is potential energy.

ScienceClass 9CBSE

The work done on an object does not depend upon the (a) displacement (b) force applied (c) angle between force and displacement initial velocity of the object

Soln: Answer is (d) initial velocity of the object Explanation: Workdone is the product of force and displacement hence force and displacement are the required attributes for the work to be done. But workdone is not dependent of initial velocity.

ScienceClass 9CBSE

Which one of the following is not the unit of energy? (a) joule (b) newton metre (c) kilowatt kilowatt hour

Answer is (c) kilowatt Explanation: Kilowatt is SI Unit of electrical power

ScienceClass 9CBSE

A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. The work done against the gravitational force will be (g =10 m s–2) (a) 6 ×103 J (b) 6 J (c) 0.6 J zero

Here Direction of workdone is perpendicular to gravitational force direction.Hence the workdone against the gravity is zero.

ScienceClass 9CBSE

An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass is 3.5 kg. Both spheres are dropped simultaneously from a tower. When they are 10 m above the ground, they have the same (a) acceleration (b) momenta (c) potential energy kinetic energy

Soln: Momentum, potential energy and kinetic energy varies with weight. But in this case acceleration is because of acceleration due to gravity which is independent of mass hence acceleration is the right answer.

ScienceClass 9CBSE

In case of negative work the angle between the force and displacement is (a) 00 (b) 450 (c) 900 (d) 1800

Soln: Answer is (d ) 1800 Explanation: workdone =FS cosɵ Cos180= -1 Hence the answer is 1800

ScienceClass 9CBSE

A car is accelerated on a levelled road and attains a velocity 4 times of its initial velocity. In this process the potential energy of the car (a) does not change (b) becomes twice to that of initial (c) becomes 4 times that of initial becomes 16 times that of initial

Answer is (a) does not change Explanation: Potential energy is the product of height, mass and gravity. Hence height is a factor in determining potential energy. Here the height does not change hence potential energy of the car does not change.

ScienceClass 9CBSE

When a body falls freely towards the earth, then its total energy (a) increases (b) decreases (c) remains constant first increases and then decreases

Answer is (c) remains constant Explanation Body falling freely towards the earth possess same kinetic and potential energy and it follows Law of conservation of energy hence its total energy remains constant.

ScienceClass 9CBSE

Why is AIDS considered to be a ‘Syndrome’ and not a disease?

HIV AIDS is caused by HIV Virus which enter our body through body fluids such as blood and breast milk or by sexual means. HIV virus spread to lymphnodes and it block our immune system. This creates a condition where our body cannot fight minor diseases like common cold. A person with HIV can get pneumonia from common cold, minor gut infection may turn into sever diarrhoea. In HIV a person will not have specific symptoms rather he will have a series of infections due hence HIV is called as a syndrome not a disease.

ScienceClass 9CBSE

Give any four factors necessary for a healthy person.

Hygienic environment: Clean water and a hygienic surrounding can keep us away from water and air borne diseases. Personal Hygiene: We should keep our body clean to be healthy. Food: Hygienic and nutritious food help to boost immune system hence balanced diet is essential for being healthy. Vaccination: Vaccination against severe disease is very essential to be healthy.

ScienceClass 9CBSE

Becoming exposed to or infected with an infectious microbe does not necessarily mean developing noticeable disease. Explain

Our immune system fight disease causing pathogens and clears them by killing to gives us protection from disease. When we get exposed to certain pathogen our immune system sends active cells that kill and clear the pathogen. Because of the immune system we don’t often get disease despite of getting exposed to infectious microbe.

ScienceClass 9CBSE

Why are antibiotics not effective for viral disease?

Antibiotics act on bacteria and fungi by blocking certain biochemical activities necessary for the living of bacteria. On the other hand virus has very few biochemical processes which are hard to target for antibiotic. Hence antibiotics are not effective against viruses.

ScienceClass 9CBSE

Why do some children fall ill more frequently than others living in the same locality?

There may be many reasons behind fall ill of few children . Some of them are listed below. Lack of balanced diet leading to malnutrition Weak immunity Living in congested unhygienic conditions Lack of clean drinking water and hygienic food Missing vaccinations.

ScienceClass 9CBSE

What precautions will you take to justify “prevention is better than cure”.

Precautions to take to avoid diseases are as follows Maintaining personal as well as public hygiene is very essential Awareness about causes, symptoms and source of diseases Using clean water and avoid storage of water to avoid growth of Balanced diet Regular exercise will boost our immunity Vaccination

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