Physics [2 Mark Question Answer]

E.g. of class I lever with M.A. = 1: A physical balance has both arms equal (i.e. effort arm = load arm), thus its M.A. = 1
E.g. of class I lever with M.A. = 1: A pair of scissors used to cut a piece of cloth has blades longer than the handle (i.e. effort arm is shorter than the load arm), thus its M.A. = 1.
E.g. of class I lever with M.A. = 1: Shears used for cutting thin metal sheets have much longer handles as compared to the blades (i.e. effort arm is longer than the load arm), thus its M.A. = 1 and it serves as a force multiplier.

Efficiency: Efficiency of a machine is the ratio of the useful work done by the machine to the work put into the machine by the effort. In other words, it is the ratio of the work output to the work input.

Simple machine: A machine is a device by which we can either overcome a large resistive force at some point by applying a small force at a convenient point and in a desired direction or by which we can obtain a gain in speed.

(a) given force = 500N

Vertical displacement = Final position - initial position = BC = 4m

work done = force x displacement = 500 x 4 = 2000J

(b) P.E gained = mgh = (mg) h = 500 x 4 = 2000J

The energy of a body is its capacity to do work.
The SI unit of work is 'joules' and the CGS unit is 'erg'.

Given, mass = 20kg

displacement = 40m

(a) In vertical direction work is done against gravity

Work done = mgh = 20 x 9.8 x 40 = 7840J

(b) In horizontal direction

Work done = Fs cos0 = (mg)s cos 0° = 20 x 9.8 x 40 = 7840J

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.
The SI unit of work is 'joules' and the CGS unit is 'erg'.

Let m be the mass of the metre scale

In balancedcondition, sum of clockwise moments = sum of anticlockwise moments

400 x 10 = m x 90

or 4000 = 90m

or m = 44.4g

the mass of the scale is 44.4g

Passengers are usually advised not to stand in the upper deck of the double deck bus. When the passengers are standing, the C.G. rises. This decreases the stability of the bus. When the passengers are sitting, the C.G. gets lowered and stability of the bus increases.

We bend forward in order to keep ourselves in a stable equilibrium while climbing up a hill. By bending forward we increases the base of the support, so that the vertical line passing through our centre of gravity still falls within the base.

Leaning tower of Pisa is stable because a line through the centre of gravity falls within the structure's base. If the line falls outside the structure's base then there is a possibility that overturning will occur. This structure could be classified as unstable.

(i) Two equal and opposite parallel forces acting along different lines on a body constitute a couple.

(ii) Moment of couple = either force x perpendicular distance

= 4 x 10

= 40 Nm

This is due to the fact that the body of the passenger is in the state of rest as long as the bus is at rest. When the bus starts, his feet acquire the velocity of the bus and come to motion with the moving bus, while the upper portion of his body due to inertia of rest tends to remain in the state of rest, resulting in his tendency to fall backwards.

When a man gets down from a moving train, his feet come to rest immediately, while the upper part of his body due to inertia of motion still remains in motion and consequently he leans in forward direction. The person while getting down of a train should run forward in the direction of the moving train to avoid fall.

When a truck is not fully loaded, its COG is at a high point and hence the turning moment of the weight is much greater, thus, the truck will be quite unstable and there are chances of toppling, when a truck takes a sharp turn.

A man bends forward in order to keep himself in a stable equilibrium while climbing up a slope. By bending forward he increases the base of the support, so that the vertical line passing through his centre of gravity may still fall within the base.

(i) We keep our body balanced on two feet by keeping the center of gravity of our body between our feet. It acts normal to the sea level vertically downwards. If COG goes out we fall or we get unbalanced.
A boy standing on both legs has his COG in balanced position and is thus in stable equilibrium but a boy standing on one leg has his COG in unbalanced position which makes him quite unstable and hence it is easier to push him.

Diagram showing the relative position of vertices of the triangle when it is suspended by a pin from the hole A:

The position of vertices changes the triangle is in equilibrium and the centre of gravity lies on the vertical line through the point of suspension as the weight acts along this same line.

The center of gravity is at intersection of lines BE and AD. The distance a can be calculated as a = h/3
 

(i) Moment of force = applied force x perpendicular distance from the line of action

Moment of force at O = 8 x 1.5 = 12 Nm

(ii) Momemnt of force = applied force x perpendicular distance from the line of line

Moment of force at P = 8 x 3 = 24 Nm

(a) If both the forces act at the same point of the body, they have the same line of action, and then the moment becomes zero.
(b) If both the forces act at two different points of the body at a separation d then they constitute a torque whose value is given F x d.

The physical quantity is 'torque'.
Torque may be defines as the turning effect produced by a force on a rigid body about a point, pivot or fulcrum. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.

Principle of moments: If a body is in equilibrium under the action of number of force, then the sum of clockwise moments is equal to the sum of anticlockwise moments.

A body is said to be in equilibrium under the action of a number of forces, if the forces are not able to produce any change in the state of rest or of uniform motion or uniform rotation.

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.

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).

The centre of gravity of a body depends upon:
(i) Body's weight
(ii) Body's shape

Moment of force = applied force x perpendicular distance from the line of action

10 = 20 x perpendicular distance from the line of action

or perpendicular distance from the line of action = 0.5m

Kinetic energy of a body depends upon:
(i) Mass of the body
(ii) Speed of the body

(a) As the height above the ground increases, the potential energy also increases.
(b) At the highest point, the height of the cricket ball is maximum and hence the potential energy is also maximum.

P.E. of a body of mass M and at a height H above the earth's surface is:
P.E. = MgH ; here, g = acceleration due to gravity.

The parameters that can change the kinetic energy are:
(i) Mass
(ii) Speed

Its kinetic energy changes with the change in velocity.
Velocity becomes zero at the highest point.

The law of conservation of energy states that energy cannot be created or destroyed; the sum total of energy in a closed system remains unchanged. Energy only changes from one form to another.

Whenever one form of energy dissipates or disappears, another form of equivalent amount of energy is produced; this is referred to as transformation of energy.
E.g. when a particular switch is pressed electric lamps light up owing to the heat produced in the filament. This is the transformation of electrical energy to heat and light energy.