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Q1. What do you mean by collision?

Solution

A collision between two bodies is said to have taken place when the two bodies physically touch each other. This short lived mutual interaction results into the change in energy and momentum of the interacting bodies.

Q2. What do you understand by plastic collision?

Solution

A collision between two bodies in which two bodies stick together after collision and then move as one body, is called perfectly inelastic collision or plastic collision. In this type of interaction, momentum of the system remains conserved and loss of kinetic energy is maximum.

Q3. 250 joules were spent in lifting a 10 kg weight to a height of 2 m. Calculate the acceleration with which it was raised. (Take g = 10 m/s²)

Solution

Work done in lifting the weight = 250 J Work done against the gravitational force = m(g + a)h 250 = 10 x (10 + a) x 2 10 + a = 12.5 a = 2.5 m/s²

Q4. What is the source of the kinetic energy of the falling rain drop?

Solution

The source of kinetic energy of a falling rain drop is the gravitational potential energy which it gains due to height from the ground and is converted into its kinetic energy.

Q5. Explain why the water at the foot of the waterfall is at different temperature from that at the top?

Solution

When water reaches the ground, its gravitational potential energy is converted into kinetic energy. A part of its kinetic energy is converted into heat on reaching the surface. This causes a rise in temperature of water. So, the water at the foot of the waterfall is at a higher temperature as compared to the water at the top of the waterfall.

Q6. Define coefficient of restitution or coefficient of resilience.

Solution

Coefficient of restitution is defined as the ratio of relative velocity of separation of the two bodies after collision to the relative velocity of approach before collision. It is denoted by 'e'.

Q7. An airplane with a mass of 2000 kg sinks 1000 metres moving forward a horizontal distance of 10,000 metres. What will be its new potential energy? (Take g = 10 m/s²)

Solution

Mass of the airplane is m = 2000 kg Decrease in height (sinks) is h = 1000 m Distance covered = 10,000 metres So by the potential energy relation, the change in the gravitational potential energy of the airplane = m x g x h. Thus, the new potential energy is 2000 x (10) x (1000) = 2 x 10⁷ J

Q8. A lift at a construction site moves 10 labourers every minute from the first floor of the building to the second. The second floor is 6.0 metres above the first floor. If the average mass of labourers is 56 kg, determine the power requirement of the lift in order to move this number of passengers in this amount of time.

Solution

Number of passengers is 10   Work done by one passenger = force x height                                           = F × d                                           = (56 kg × 9.8 m/s²) × 6.0 m                                           = 3292.8 J   Work done by ten passengers = 3292.8 J × 10                                            = 32928 J   P = Work done by ten passengers / time = (32928 J) / (60 s)   P = 548.8 W

Q9. What is an elastic and inelastic collision?

Solution

Elastic Collision- A collision between two bodies in which both the linear momentum and kinetic energy of the system (colliding bodies) remains conserved is called an elastic collision.   Inelastic Collision- A collision between two bodies in which linear momentum of the system remains conserved but the kinetic energy is not conserved, is called an inelastic collision.

Q10. Predict the sign of the work done in the following cases:-(a) work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket. (b) work done by gravitational force in the above case (a).(c) work done by friction on a body sliding down an inclined plane. (d) work done by the resistive force of air on a vibrating pendulum in bringing it to rest.

Solution

(a) Since tension in the rope is in the same direction as displacement, work done is positive.(b) Work done is negative since gravitational force acts downwards, while displacement is upwards.(c) Since friction opposes motion down the plane, it is directed opposite to displacement. Therefore work is negative.(d) Work done by resistive force of air is negative because this force opposes motion.

Q11. What are conservative and non-conservative forces. Give one example of each force.

Solution

Conservative force: If the work done by or against the force in moving a body depends only on the initial and final positions of the body and not on the nature of path followed between the initial and final positions, then the force is said to be a conservative force. Example - Gravitational force   Non-conservative force: If the work done by or against the force in moving a body depends on the path followed by the object from initial to final position, then the force is said to be a non-conservative.   Example - Frictional force.

Q12. What is h.p.? What is the relation between h.p. and watts?

Solution

Horse power is a unit to measure power.   1 h.p. = 746 watts

Q13. A metal ball rebounds better than a rubber ball. Why?

Solution

A metal ball rebounds better than a rubber ball because, when a rubber ball strikes a wall it is distorted and a huge amount of heat is produced due to rubbing of its molecules against each other. This type of effect does not take place in hard materials like a metal ball. There is a less loss of energy when the metal ball collide with the wall. Hence, a metal ball rebounds better than a rubber ball.

Q14. What are the conditions under which a force does no work?

Solution

A force does no work when

the displacement is zero,

the displacement is perpendicular to the direction of the force, and

the conservative force moves a body over a closed path.

Q15. Write four properties of conservative forces.

Solution

(1)   Work done by or against conservative force depends only on the initial and final positions of the body. (2)   Work done by or against a conservative force in a round trip is zero. (3)   Work done by a conservative force is completely recoverable. (4)   If only the conservative forces are acting on the body, then its total mechanical energy is conserved.

Q16. How motion of the pendulum conserves energy? Explain.

Solution

Energy is conserved when all the external forces are absent or not doing work and also the influence of friction and air resistance is negligible.   Let us consider a pendulum bob swinging to and fro. There are only two forces acting upon the pendulum bob, gravity which acts downward and the tension force (an external force) which tries to pull upwards. The tension force does not make much difference as nearly at all times it is directed at a 90^o angle to the motion.   As the bob swings to the right, it gains kinetic energy and reaches maximum height where all the kinetic energy is transformed to potential energy, then again it moves towards left and the potential energy is converted to kinetic energy and the bob reaches the extreme left where the kinetic energy is completely transformed to potential energy again.   Hence, at all the times the sum of the potential and kinetic energies of the pendulum remains constant.

Q17. What is difference between head-on and an oblique collision?

Solution

Head-on collision- A collision is said to be head-on collision when the colliding objects move along a straight line joining their centres.   Oblique Collision- When the colliding objects do not move along the straight line joining their centres, the collision is said to be oblique collision.

Q18. Explain how the energy is transformed in roller coaster?

Solution

A roller coaster operates on the principle of energy transformation. Work is initially done on a roller coaster to lift to its initial highest point. Once it is lifted to the top, the roller coaster gains a large quantity of potential energy. As the roller coaster descends on the track, its potential energy is transformed into kinetic energy due to which the roller coaster gains speed. As the roller coaster climbs the summit, its kinetic energy is transformed into potential energy and as it comes down its gained potential energy is converted to kinetic energy.   On a roller coaster ride the total amount of mechanical energy is the same at every location along the track. Mechanical Energy = Kinetic Energy + Potential Energy The amount of kinetic energy and the amount of potential energy is constantly changing.

Q19. State the law of conservation of energy.

Solution

The total energy of an isolated system remains constant, as long as all the forms of energy are accounted for. Energy may be transformed from one form to another but the total energy of the isolated system remains constant.

Q20. State the series of changes in energy when a pole vaulter runs towards the bar and crosses it.

Solution

As the pole vaulter runs towards the bar, he gains kinetic energy. As he bends the pole, he does work on the pole and stores elastic potential energy in the pole. That energy is converted into gravitational potential energy as he goes up and over the bar.