CBSE Class 10 Physics Simple Machines MCQs Set 15

Answer: (a) mechanical advantage

Explanation:
1. Mechanical advantage is defined as M.A. = load / effort.
2. Efficiency is the ratio of work output to work input, and the ratio of load arm to effort arm is the inverse of the mechanical advantage of a lever.

Teacher's Note:
1. Mechanical advantage has no unit because it is a ratio of two forces.
2. For a lever, M.A. = effort arm / load arm.

Answer: (b) always greater than one

Explanation:
1. For an inclined plane M.A. = length of the plane / height = 1/sin(angle).
2. For any angle below 90 degrees, sin(angle) is less than 1, so M.A. is greater than 1.
3. A smaller angle gives a larger mechanical advantage, so the last option is wrong.

Teacher's Note:
1. At exactly 90 degrees the plane is vertical and M.A. = 1.
2. This is why a gentle ramp makes loading easier than a steep one.

Answer: (a) greater than one

Explanation:
1. In a second order lever the load lies between the fulcrum and the effort.
2. So the effort arm is longer than the load arm and M.A. = effort arm / load arm is always greater than 1.

Teacher's Note:
1. A nut cracker and a wheelbarrow are second order levers.
2. A third order lever, such as charcoal tongs, always has M.A. less than 1.

Answer: (c) 8

Explanation:
1. In the first system each movable pulley doubles the mechanical advantage.
2. With n movable pulleys M.A. = 2^n.
3. For n = 3, M.A. = 2^3 = 8.

Teacher's Note:
1. Do not confuse the number of pulleys with the number of movable pulleys.
2. Four movable pulleys would give 16.

Answer: (c) \(30^\circ\)

Explanation:
1. M.A. = 1/sin(angle) = 2, so sin(angle) = 1/2.
2. Therefore the angle is 30 degrees.

Teacher's Note:
1. sin 30 degrees = 1/2, so the effort along the plane is half the load.
2. The price paid is that the effort moves twice as far as the load.

Answer: (d) both the circumference of the head of the screw and its pitch

Explanation:
1. For a screw M.A. = 2 π r / d, where 2 π r is the circumference of the head (or of the path of the effort) and d is the pitch.
2. So both quantities decide the mechanical advantage.

Teacher's Note:
1. A large head circumference and a small pitch give a large M.A.
2. This is why a screw jack can lift a car with a small effort.

Answer: (b) a percentage

Explanation:
1. Efficiency = work output / work input is a pure ratio.
2. It is therefore commonly written as a percentage.

Teacher's Note:
1. Since it is a ratio of two energies it has no unit.
2. A value such as 0.6 is the same as 60 per cent.

Answer: (d) 100 per cent

Explanation:
1. Work output can never exceed work input.
2. An ideal machine without friction has output equal to input, so efficiency is 100 per cent.
3. Real machines have friction and are always less than 100 per cent efficient.

Teacher's Note:
1. Efficiency above 100 per cent would mean creating energy, which is impossible.
2. Friction is the main reason for the loss.

Answer: (d) velocity ratio

Explanation:
1. Velocity ratio V.R. = distance moved by effort / distance moved by load.
2. For one turn of a wheel and axle this equals R / r, the ratio of the radii.

Teacher's Note:
1. M.A. = load / effort is measured by forces, V.R. by distances.
2. Efficiency = M.A. / V.R.

Answer: (b) stable, unstable and neutral

Explanation:
1. When a body is slightly displaced, its centre of gravity may rise, fall or stay at the same height.
2. These cases give stable, unstable and neutral equilibrium respectively.

Teacher's Note:
1. A cone resting on its base is stable, balanced on its tip is unstable, and lying on its side is neutral.
2. Lower the centre of gravity, greater the stability.

Answer: (a) the product of the force and the perpendicular distance of the point from the line of action of the force

Explanation:
1. The turning effect of a force about a point is measured by its moment.
2. Moment = force × perpendicular distance from the point to the line of action of the force.

Teacher's Note:
1. The unit of moment is newton metre.
2. Moments are called clockwise or anticlockwise by the sense of turning.

Answer: (b) equal to the sum of the anticlockwise moments

Explanation:
1. This is the principle of moments.
2. For a body in rotational equilibrium the total clockwise moment equals the total anticlockwise moment about any point.

Teacher's Note:
1. A physical balance is a first order lever using this principle.
2. The weight of the rod acts at its centre of gravity.

Answer: (b) barometer

Explanation:
1. A barometer measures atmospheric pressure.
2. A thermometer measures temperature, a hygrometer measures humidity and an ammeter measures current.

Teacher's Note:
1. Torricelli made the first mercury barometer.
2. An aneroid barometer, graduated in height, is called an altimeter.

Answer: (d) area

Explanation:
1. Pressure = thrust / area, where thrust is the total force acting normally on the surface.

Teacher's Note:
1. The SI unit is newton per square metre, called pascal.
2. The same thrust gives a larger pressure on a smaller area.

Answer: (b) the pascal (N/m\(^2\))

Explanation:
1. Pressure = force / area, so its unit is N/m\(^2\).
2. This unit is named the pascal (Pa).

Teacher's Note:
1. In the CGS system the unit of pressure is dyne per cm\(^2\).
2. 1 atmosphere is about \(1.013 \times 10^5\) Pa.

Answer: (b) pressure

Explanation:
1. One pascal is a pressure of one newton acting on each square metre, 1 Pa = 1 N/m\(^2\).
2. Force is measured in newtons, work in joules and power in watts.

Teacher's Note:
1. The unit is named after Blaise Pascal.
2. Pressure is a scalar even though it arises from a force.

Answer: (c) 10.34 m

Explanation:
1. The same atmospheric pressure supports both columns, so h(water) × 1 = h(mercury) × 13.6.
2. h(water) = 0.76 × 13.6 = 10.34 m.

Teacher's Note:
1. This is why a water barometer is not practical, as it would be over 10 m tall.
2. Mercury is used because it is very dense.

Answer: (d) fall well within the base of support

Explanation:
1. A body is stable if, on slight tilting, the weight produces a turning effect that brings it back.
2. This happens when the vertical through the centre of gravity falls inside the base.

Teacher's Note:
1. Once the vertical line moves outside the base, the body topples.
2. A wide base and low centre of gravity both add to stability.

Answer: (b) efficiency

Explanation:
1. Work output is the work done on the load and work input is the work done by the effort.
2. Their ratio is the efficiency of the machine.

Teacher's Note:
1. Efficiency is usually less than 1 because of friction.
2. M.A. = V.R. only for an ideal machine.

Answer: (d) increase the circumference traced by the effort and so increase the mechanical advantage

Explanation:
1. M.A. of a screw = circumference traced by the effort / pitch.
2. A long handle makes the circumference traced by the effort large, so the M.A. is large and a small effort can raise a heavy load.

Teacher's Note:
1. The same idea explains why a long spanner loosens a nut easily.
2. The handle does not change the pitch.