Physics [3 Mark Question Answer]

Man having a box on his head who climbs up a slope does more work against the force of gravity because he has more potential energy by virtue of his position i.e., height.
As, P.E. = W.D. = F × S
= mg × h

Submarines are provided with ballast tanks. When required to dive, the water is allowed to enter the tanks. And when required to sail, the water is forced out by compressed air.
Work is a scalar quantity.

Solar cell.
Its two uses are:
(i) All artificial satellites and space probes mainly depend upon electricity generated by solar cells.
(ii) Solar cells are used for lighting, operating water pumps, running radio and television sets in remote areas and traffic signals, etc.

It is not advisable to use wood as fuel even though it can be replenished because the burning of wood produces smoke that causes pollution which results in environmental imbalance.

(i) An object kept at a height above the ground.
(ii) A compressed spring, and
(iii) A stretched catapult.

Force due to gravity on the body F = mg
Work done in lifting the body against the force of gravity to a height h above the ground
W = F × d = mg × h = mgh
Thus, the potential energy of the body = mgh.

(i) Mechanical energy
(ii) Chemical energy
(iii) Solar energy
(iv) Heat energy
(v) Light energy.

Energy is defined as the capacity of a body to do work. Energy is a scalar quantity. The energy possessed by a body is measured by the amount of work done by a body while changing its state of rest or state of motion.

The power becomes 10 times its initial value. This is because of power = work/time. Thus, power α 1/time for a given work. Reducing the time by a factor of 10 thus increases the power 10 times.

The S.I. unit of work is newton-metre or joule (J).
1 Joule = 1 N × 1 m.
1 J of work is said to be done by a force of 1 N if it displaces a body by 1 m in the direction of the force.

Amount of work done is equal to zero.
Work is said to be done only when there is displacement produced. In the case of a body moving in a circular path, the body comes to its original place, therefore, there is no displacement, and hence work is zero.

Work is said to be done when a force displaces a body through some distance, in its own direction, i.e.,
Work = Force × Displacement
W = F × d
Work is a scalar quantity.

A ship loaded with light goods is more liable to be overturned than the one loaded with heavy goods. This is because the CG in the former case is high and so a slight displacement due to water current may cause the vertical line passing through the CG to fall outside the base, thereby overturning the ship.

It is easier to knock down a boy who is standing on one foot than one who is standing on two because boy standing on both feet has a larger base area and hence it has a more stable equilibrium than a boy standing on one foot.

The position of CG of a body depends on the distribution of mass in the body. It can be situated outside or inside the material of the body.
For example, The CG of wire is at its mid-point but CG of a ring lies at the centre where there is no material.

It is equal to the product of the magnitude of the force and the perpendicular distance of the line of action of a force from the axis of rotation. If the moment of force is assigned a negative value, it means the clockwise tendency of force.

A large force is required to stop the loaded vehicle. It is because the loaded vehicle has greater momentum than the empty vehicle as the mass of the loaded vehicle is more than that of the empty one. So, it requires more force to stop.

When the bullet strikes the glass, the part of the glass coming in contact with the bullet immediately shares the large velocity of the bullet and makes a hole in it while the rest of the glass pane, due to the inertia of rest, remains at rest and so is not smashed. But when a stone strikes the same glass pan, the surrounding part of the glass gets time to share the comparatively low velocity of stone and so it is smashed.

Earth exerts a force of attraction on each object. This force is due to gravity and its magnitude is directly proportional to the mass of the object. This force is known as gravitational force and its units are gravitational units.
In the MKS system, the gravitational unit of force is kilogramme force i.e., kgf. In CGS system it is gramme force, gf.

The potential energy of stored water changes to kinetic energy of flowing water, The kinetic energy of the flowing water rotates turbine. The turbine, in turn, rotates the coil of the generator and changes into electric energy.

Initial potential energy = mgh
Final potential energy = mg × 2h = 2mgh
Increase in potential energy = Final potential energy – Initial potential energy
= 2mgh – mgh = mgh.

When a number of forces acting on a body produce no change in its state of rest or of uniform motion the body is said to be in equilibrium.
The two conditions of equilibrium of a body:
(i) The resultant of all the forces acting on the body should be equal to zero.
(ii) The resultant moment of all the forces acting on the body about a point should be zero.

A force can produce the following three effects:
(i) It can bring changes in dimensions. For example, in stretching a spring its length increases.
(ii) It can start or stop motion. For example, a ball moves when kicked.
(iii) It can change the speed or direction of motion. For example, a hockey player changes the speed and direction of the motion of the ball by his stick.

Moment of force = force x perpendicular distance of from point O
Moment of force = F x r
5Nm = 10 x r
R = 5/10 = 0.5 m

(i) F’ in the fig. shows the direction of restoring force.
(ii) Spring balance.