Matric Notes Physics 9th Ch 6 Work and Energy Short Questions

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Q1) Can a centripetal force ever do work on an object? Explain.

Answer: No, centripetal force does not do work because the force acting on the object and instantaneous displacement of the object are always perpendicular to each other as shown in the figure. So;

                     W = FS cosϴ = FS cos90° = FS(0) = 0 J

The centripetal force allows the object to follow the circular trajectory. The centripetal force is directed towards the center while the tangential velocity of the object is perpendicular to the direction of action of force at every point. Hence no work can be done by it.

Q2) What happens to the kinetic energy of a bullet when it penetrates into a sandbag?

Answer: When a bullet of mass ‘m’ is moving with velocity ‘v’ it possesses kinetic energy given by,

                    KE = 1/2 mv2

When this bullet hits the sandbag, it penetrates into the sandbag and comes to rest after penetrating into it for some distance. When it comes to rest, the final velocity is 0 m/s.

The kinetic energy possessed by the bullet as given in the above equation is exhausted mostly in the work done against the resistance offered by the sandbag while penetrating into it. Some part of the kinetic energy may also have lost as heat and sound.

Q3) A meteor enters into earth’s atmosphere and burns. What happens to its kinetic energy?

Answer: A meteor has both potential and kinetic energy before entering the atmosphere of the Earth. As it enters and falls towards the surface of the Earth, frictional heating occurs, caused by the collision with the molecules in the atmosphere which reduces its velocity. Hence the kinetic energy decreases, which is converted to heat energy. Potential energy is continuously decreasing, being converted to kinetic energy. There is also mass loss due to ablation.

Q4) Two bullets are fired at the same time with the same kinetic energy. If one bullet has twice the mass of the other, which has the greater speed and by what factor? Which can do most work?

Answer: The bullet that has less mass will have greater speed than the bullet of greater mass.It can be proved as follow;

Let mass of first bullet be m1 then the mass of the other bullet will be m2= 2m1. Thus

Thus it can be seen from the above equation that velocity v1 of the bullet with mass m1 is √2v2 i.e. √2 times greater than the velocity v2 of the bullet of mass m2.

Since the kinetic energy of both the bullets is the same so, the work done by each of them will be equal.

Q.5) Can an object have different amounts of gravitational potential energy if it remains at the same elevation?

Answer: No, an object can not have different amounts of gravitational potential energy as long as it stays on the earth. Gravitational potential energy is given by the formula;

 G.P.E = mgh

As given the elevation remains the same it means ‘h’ is constant. The value of ‘g’ would be constant for the same elevation. Also the mass of the object is constant. Hence the gravitational potential energy remains the same.

Q.6) Why do roads leading to the top of a mountain wind back and forth?

Answer: Roads leading to the top of mountain wind back and forth because in this way less power is required is to move to the top than that of a single steep path.

According to the definition of power:

It is clear from the equation (i) that the greater the height, the greater would be the power required. If roads are made steeper, like in case of the inclined plane, the vertical height ‘h’ will be much larger due to this more power is required to reach to the top of mountains.

While for the roads that wind back and forth the steepness of the path is decreased by dividing it into the number of curved paths i.e now the vertical height ‘h’ will be less for each path and so less power is required to reach to the top.

Q7. What type of energy conservation can be observed in water dams?

Ans. The stored water in dame’s possessing potential energy when these water are allowed to run turbines. The P.E of water converted to K.E when turbine runs the K.E is converted into mechanical energy. Turbines run generators which converts mechanical energy to electrical energy.

Q8. What happens to K.E of a bullet when it penetrates into a sand bag?

Ans. When a bullet penetrates into a sand bag. The sand opposes it motion by a retarding force so K.E o air molecule. And some is transferred to the K.E of bullet is used against the regarding force of sand.

Q9. A meteor enters into earth’s atmosphere and burns. What happens to its kinetic energy?

Ans. When a meteor enters into earth’s atmosphere it burns and its K.E is converted into heat an light energy. And some is transferred to the K.E of air molecule.

Q10. What do you understand by term energy transformation support your answer by two examples?

Ans. Energy transformation is a process which converts energy from one form to another form it example are given.

1. When electrical energy is given to an electric motor it converts it to mechanical energy.

2. When fuel are burnt in an engine it converts fuel energy to mechanical energy.

Q11.  what kind of energy is possessed in the following cases? a) A stone laying on the top of a root b) Flying airplane. c) A wound up spring of a toy car.

Ans. a) A stone laying on the top of a root posses gravitational potential energy

b) Flying airplane posses K.E energy due to motion and P.E due to its position.

c) A wound up spring of a boy car posses elastic P.E.

Q12. in each of the following situation the system consists of a ball and earth describe the work done and changes in energy forms. A) you throw a ball horizontally B) horizontally  thrown by a fielder c)a ball is thrown vertically and t comes to rest at top of its flight (d) the ball back to earth where you catch?

Ans. (a) When a ball is thrown vertically then there is no work done by gravity and no change in energy b) Same as (a)