CBSE Class 8 Science Advanced Physics Force MCQs Set 01

Answer: (a) Vector quantity.

Explanation:
1. A force has a size (magnitude) and also a direction in which it acts.
2. A quantity with both magnitude and direction is a vector.

Teacher's Note:
1. Pushing a door towards you or away from you gives different results, so direction matters.
2. Mass and time are scalars; force is not.

Answer: (c) The combination of forces acting on it balances each other

Explanation:
1. A body at rest does not change its state of motion.
2. This happens when the forces on it are balanced, so the net force is zero.
3. So the correct statement is (C).

Teacher's Note:
1. A book on a table has weight and the table pushes up equally.
2. Forces are present, but they cancel.

Answer: (b) 5.25×10⁵

Explanation:
1. 1 N = 10⁵ dyne.
2. 5.25 N = 5.25 × 10⁵ dyne.
3. This is option (B).

Teacher's Note:
1. Newton is the bigger unit, so the number of dynes must be larger.
2. Multiply by 10⁵ to go from N to dyne.

Answer: (c) net force acting on ball is non zero.

Explanation:
1. A ball sliding down an incline speeds up, so it is accelerating.
2. Acceleration needs a net force, so the net force is not zero.
3. The correct option is (C).

Teacher's Note:
1. The component of weight along the slope is not balanced.
2. Constant speed in a straight line would need zero net force.

Answer: (c) 0 N.

Explanation:
1. You push one way with 80 N.
2. Your two friends push the opposite way with 55 + 25 = 80 N.
3. Net force = 80 − 80 = 0 N.

Teacher's Note:
1. Add forces in the same direction first, then subtract the opposite groups.
2. Equal and opposite forces cancel.

Answer: (d) Speed of body decreases.

Explanation:
1. Net force = 6 − 5 = 1 N towards the left.
2. The body moves right, so the net force is opposite to its motion.
3. Therefore its speed decreases.

Teacher's Note:
1. Find the net force and compare its direction with the motion.
2. If the force is opposite to the motion, the body slows down.

Answer: (d) All of the above.

Explanation:
1. A force can change direction of motion, as when a bat hits a ball.
2. It can change the state of rest or uniform motion.
3. It can change shape, as when a spring is stretched.
4. So all of the above are correct.

Teacher's Note:
1. The effects of force are four: start, stop or slow, change direction, change shape.
2. Each option is one of these effects.

Answer: (d) A force always acts on it.

Explanation:
1. Acceleration can mean speeding up, slowing down or turning, so (A) and (B) are not always true.
2. A body need not fall to the earth, so (C) is false.
3. By F = ma, an acceleration needs a net force, so (D) is correct.

Teacher's Note:
1. No force means no acceleration.
2. Remember F = ma: a is zero only when the net force is zero.

Answer: (c) May or may not be balanced.

Explanation:
1. Forces on a book resting on a table are balanced.
2. Forces on a falling stone are unbalanced.
3. So external forces may or may not be balanced.

Teacher's Note:
1. Give one example of each case to decide.
2. 'Always' and 'never' statements are usually wrong in physics.

Answer: (c) 10⁵

Explanation:
1. 1 N = 1 kg × 1 m/s² = 1000 g × 100 cm/s² = 100000 g cm/s².
2. = 10⁵ dyne, option (C).

Teacher's Note:
1. Kilogram to gram gives 10³ and metre to centimetre gives 10².
2. Together 10³ × 10² = 10⁵.

Answer: (b) newton.

Explanation:
1. The dyne is the CGS unit and gf is a gravitational unit.
2. The SI unit of force is the newton (N).

Teacher's Note:
1. 1 N = 1 kg m s⁻².
2. Newton is named after Sir Isaac Newton.

Answer: (c) 0

Explanation:
1. Uniform velocity means the acceleration is zero.
2. F = ma = 1 × 0 = 0.

Teacher's Note:
1. Moving at constant velocity does not need a net force.
2. The 5 m/s is only a distraction.

Answer: (b) 1 N.

Explanation:
1. Mass = 100 g = 0.1 kg.
2. Force needed = weight = mg = 0.1 × 10 = 1 N (taking g = 10 m/s²).

Teacher's Note:
1. Change grams into kilograms first.
2. Using g = 9.8 gives 0.98 N, which is close to 1 N.

Answer: (b) 5.00

Explanation:
1. Weight = mg, so m = 9.8 ÷ 9.8 = 1 kg.
2. a = F ÷ m = 5 ÷ 1 = 5.00 m/s².

Teacher's Note:
1. Convert weight into mass before using F = ma.
2. Do not use the weight itself as the mass.

Answer: (b) Its weight will change, but not mass.

Explanation:
1. Mass is the amount of matter and does not change with place.
2. Weight = mg and g differs on the moon and the earth, so the weight changes.
3. Option (B) is correct.

Teacher's Note:
1. The moon's g is about one sixth of the earth's.
2. Mass stays the same everywhere.

Answer: (c) Its magnitude and direction.

Explanation:
1. Magnitude says how strong the force is.
2. Direction says where it acts.
3. Both are needed, option (C).

Teacher's Note:
1. A vector needs both numbers and direction.
2. '5 N' alone is incomplete.

Answer: (c) The combination of forces acting on it balance each other.

Explanation:
1. A body at rest keeps its state when the forces on it cancel.
2. So the correct option is (C).

Teacher's Note:
1. Forces may act, but the net force is zero.
2. Vacuum has nothing to do with rest.

Answer: (d) poundal

Explanation:
1. FPS means foot-pound-second.
2. The unit of force in this system is the poundal.
3. kgf and gf are metric gravitational units; the pound is a unit of mass.

Teacher's Note:
1. Unit of force = mass × acceleration.
2. poundal = 1 pound × 1 ft/s².

Answer: (b) 1350 N

Explanation:
1. F = ma = 45 × 30.
2. F = 1350 N.

Teacher's Note:
1. Multiply mass and acceleration directly.
2. Both are already in SI units.

Answer: (d) Frictional.

Explanation:
1. Gravitational, electrostatic and magnetic forces act from a distance.
2. Friction acts only when surfaces touch, so it is a contact force: (D).

Teacher's Note:
1. Ask: does it need touching?
2. Friction does, the other three do not.

Answer: (c) 10⁻⁵ N.

Explanation:
1. 1 N = 10⁵ dyne.
2. So 1 dyne = 1 ÷ 10⁵ N = 10⁻⁵ N.

Teacher's Note:
1. Dyne is the smaller unit.
2. Do not mix it up with 1 N = 10⁵ dyne.

Answer: (b) \( F \propto M \)

Explanation:
1. F = Ma.
2. If a is the same, F is directly proportional to M, F ∝ M.

Teacher's Note:
1. Keep the quantity that is the same fixed.
2. Here a is fixed, so force grows with mass.

Answer: (b) An acceleration of 1 ms⁻² in a body of mass 1 kg.

Explanation:
1. By definition 1 N = 1 kg × 1 m/s².
2. So a mass of 1 kg gets an acceleration of 1 m/s²: (B).

Teacher's Note:
1. Check both mass unit and acceleration unit.
2. Option (A) with 1 g would need only 0.001 N.

Answer: (a) 1500 N

Explanation:
1. a = (v − u) ÷ t = (80 − 30) ÷ 50 = 1 m/s².
2. F = ma = 1500 × 1 = 1500 N.

Teacher's Note:
1. First find the acceleration from the change in velocity.
2. Then multiply by the mass.

Answer: (a) Force

Explanation:
1. A body changes its state of rest or motion only when a force acts.
2. So we need a force.

Teacher's Note:
1. Force is the cause of change of motion.
2. Mass and time alone cannot start or stop a body.

Answer: (a) (a)-(iv), (b)-(iii), (c)-(i), (d)-(ii)

Explanation:
1. (a) The running child speeds up: force makes an object move faster, (iv).
2. (b) Blowing a balloon changes its shape, (iii).
3. (c) Pushing a table makes a stationary object move, (i).
4. (d) Catching a ball stops a moving object, (ii).
5. The pairs are (a)-(iv), (b)-(iii), (c)-(i), (d)-(ii), option (A).

Teacher's Note:
1. Name the effect of force in each situation first.
2. Then match it with the statement.

Answer: (b) Circle

Explanation:
1. A force that always points to a fixed point and varies as 1/(distance)² is the kind of force that keeps a planet or satellite going round its centre.
2. Of the given options, the closed path is a circle (in general such orbits are conic curves, and the circle is the simplest closed one).

Teacher's Note:
1. Think of the earth going around the sun.
2. Always being pulled towards a point makes the path bend round it.

Answer: (a) It will continue to move with the same speed along the original orbit of spacecraft.

Explanation:
1. When released, the ball already has the same orbital speed as the spacecraft.
2. The same gravitational pull keeps bending its path in the same way.
3. So it keeps moving along the same orbit beside the spacecraft.

Teacher's Note:
1. Objects released inside or near a satellite seem to float beside it.
2. Both fall around the earth together.

Answer: (d) 7

Explanation:
1. The earth pulls the plank down: 1 force (its weight).
2. The plank touches three step edges. At each edge there is a normal force from the step: 3 forces.
3. At each of these edges there is also friction along the plank: 3 forces.
4. Total = 1 + 3 + 3 = 7.

Teacher's Note:
1. Count weight, normal forces and friction forces separately.
2. Each contact point gives two forces.

Answer: (b) 13 N

Explanation:
1. The windmill stands still when the turning effects (force × distance from the centre) in the two directions are equal.
2. Clockwise: 10 × 2 + 8 × 1 = 28 N m.
3. Anticlockwise: F × 2 + 1 × 2 = 2F + 2.
4. 2F + 2 = 28, so F = 13 N.

Teacher's Note:
1. Multiply each force by its distance from the centre.
2. Forces turning it the same way add up.

Answer: (c) A trolley moving at a constant velocity down an inclined plane.

Explanation:
1. Balanced forces mean zero acceleration, that is constant velocity.
2. (A) and (B) speed up, and in (D) the direction keeps changing.
3. Only the trolley in (C) has constant velocity.

Teacher's Note:
1. Constant velocity means constant speed and constant direction.
2. Moving in a circle changes direction.

Answer: (d) Force is same for all surfaces.

Explanation:
1. Friction depends on the weight of the block and the nature of the surfaces.
2. It does not depend on the area of contact.
3. So the force is the same for every face.

Teacher's Note:
1. A bigger area spreads the weight over more points, so the effect cancels.
2. Weight and the surface material decide friction here.