Physics [2 Mark Question Answer]

When the head of a nail is struck with a hammer, the mechanical energy of the hammer pushes the nail into the plank of wood and in this process a part of energy is converted into heat energy.

P.E. is the energy possessed by a body by virtue of its position, shape or configuration but K.E. is the energy possessed by a body by virtue of its velocity or motion.
E.g. A ball kept on a cliff possesses P.E. but as soon as it is kicked it possesses K.E.

If 1 joule of work is done in 1 second, the power is said to be 1 watt.
1 H.P. = 746 watt

Horse power is the unit of power. The horsepower used for electrical machines is defined as exactly 746 watt.
Classically, a horse exerting 1 H.P. can raise 330 pounds of coal 100 feet in a minutes, or 33 pounds of coal 1,000 feet in one minute, or, 1,000 pounds 33 feet in one minute.

Four different forms of energy:
(i) Solar energy
(ii) Mechanical energy
(iii) Wind energy
(iv) Nuclear energy

'Work' is said to be done when the applied force makes the body move i.e., there is a displacement of body. It is equal to the product of force and the displacement of the point of application of the force in the direction of force.

When the displacement of the body is in the direction opposite to that of the applied force,the work done is negative.
W = Fd cos 0 = Fd cos 180° = -Fd

Work is said to be done only when the applied force on a body makes the body move but power is the rate of doing work.
The SI unit of work is 'joules' and that of power is 'watt'.

The work done is zero because:
Work done = force x displacement x cos 0
Now, since 0 = 90°; cos 0 = and hence work done is equal to zero.

Work done depends upon:
(i) the magnitude and direction of the applied force, and
(ii) the displacement it produces.

Examples of work done:
(i) In free fall of a body of mass m, under gravity from a height h, the force of gravity (F=mg) is in the direction of displacement (=h) and the work done by the gravity is mgh.
(ii) A coolie lifting a load does work against gravity.

The SI unit of work is joule.
1 joule of work is said to be done when a force of 1 newton displaces a body through 1 metre in its own direction.

The displacement of the man and suitcase is along the horizontal direction. Thus, the angle between the displacement and the force of gravity is 90°;
Thus, W = Fd cos 90° = 0
Hence, no work is done against gravity in this case.

When the angle between the direction of motion and that of the force is 90°;
W = Fd cos 90° = 0
When the angle between the direction of motion and that of the force is 0°;
W = Fd cos 0° = Fd
Hence in the second case, when the angle is 0°; the work done is more.

Work done depends upon:
(i) the magnitude and direction of the applied force, and
(ii) the displacement it produces.

A block and tackle system of pulleys consists of two blocks of pulleys, each block having one or more than one pulley. The upper block of pulleys is fixed to a rigid support and the lower block of pulleys is movable. The number of pulleys in the movable block is either equal to or one less than the number of pulleys in the fixed block.

The purpose of a machine is:
(i) To multiply force
(ii) To obtain a gain or decrease in speed
(iii) To change to direction of application of force in a convenient direction

No, a machine cannot be 100% efficient because mechanical advantage is always less than theoretical mechanical advantage due to friction and the weight of the moving parts.

The two reasons are:
(a) Some effort is wasted and overcoming the friction between the strings and the grooves of the pulley.
(b) Some effort is wasted in lifting up the movable block along with the load.

In a single fixed pulley, some effort is wasted in overcoming friction between the strings and the grooves of the pulley; so the effort needed is greater than the load and hence the mechanical advantage is less than the velocity ratio.

single movable pulley acts as a force multiplier

A block and tackle system of pulleys consists of two blocks of pulleys, each block having one or more than one pulley. The upper block of pulleys is fixed to a rigid support and the lower block of pulleys is movable. The number of pulleys in the movable block is either equal to or one less than the number of pulleys in the fixed block.

Two uses of pulley:
(i) To change the direction of application of force to a convenient direction.
(ii) To multiply force.
Yes, a pulley is a force multiplier

The force needed for circular motion is 'centripetal force'.
This force is always directed towards the centre of circle at each point of its path.

In uniform linear motion, the velocity is constant and acceleration is zero i.e., the uniform linear motion is an un-accelerated motion, while in a uniform circular motion the velocity is variable (although speed is uniform), so it is an accelerated motion. 

Centrifugal force is not a real force, but it is considered to describe (understand) a certain motion. It should not be considered as the force of reaction of centripetal force, although its magnitude is same as that of centripetal force.

In uniform circular motion, a particle travels equal distances along the circular path in equal intervals of time, so the speed of particle is uniform. But the direction of motion of the particle changes at each point of circular path. Due to continuous change in direction of motion, the velocity of the particle is not uniform (or velocity is variable) i.e., the motion is accelerated.

(i) If a rigid body is free to move, the applied force will cause translational motion.
(ii) If a rigid body moves around a center or is pivoted at a point, the applied force will cause rotational motion.

If the turning effect on the body is clockwise, moment of force is called the clockwise moment and is taken as negative.
If the turning effect on the body is anticlockwise, moment of force is called the anticlockwise moment and is taken as positive.

The factors on which the moment of force about a point depends are :
1. The magnitude of force applied.
2. The distance of the line of action of the force from the axis of rotation.

If a rigid body is pivoted at a point, then the applied force will rotate the body about the fixed point or about the axis passing through the fixed point. This motion is called rotational motion.
e.g. Earth's rotation about its axis, wheels of car in motion.

When some force is applied on a rigid body free to move and the body starts moving along a straight path in the direction of force. This is known as linear or translational motion. Points on the rigid body, undergo displacements forming parallel lines and magnitude of displacement is the same for individual point.
e.g. the motion of a bullet fired from a gun, a ball thrown straight up and falling back straight down.

The turning effect produced by a force on a rigid body about a point, pivot or fulcrum is called the moment of force or torque. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.
Moment of a force = Force x perpendicular distance of the pivot from the force.
Its SI unit is newton-metre (Nm).

(i) Force 'R' has the least moment about 'O' because its perpendicular distance is least from 'O'.
(ii) Force 'P' has the maximum moment about 'O' because its perpendicular distance is maximum from 'O'.