CBSE Class 8 Physics Force MCQs Set 02

Answer: (c) vector quantity

Explanation:
1. A force has both magnitude and direction, so it is a vector.

Teacher's Note:
1. A force is completely described by its magnitude and direction.
2. Speed is a scalar, but velocity is a vector.

Answer: (b) a force always acts on it

Explanation:
1. By Newton's second law, F = ma, so an accelerating body must have a net force on it. Speed may decrease or only the direction may change.

Teacher's Note:
1. Acceleration means change in velocity.
2. Change in velocity requires a net force.

Answer: (a) zero

Explanation:
1. Uniform velocity means zero acceleration, so by F = ma the net force is zero.

Teacher's Note:
1. A body moving with constant velocity is in equilibrium.
2. Do not confuse velocity with acceleration.

Answer: (d) 10⁵

Explanation:
1. 1 N = 1 kg m/s² = 1000 g × 100 cm/s² = 10⁵ g cm/s² = 10⁵ dyne.

Teacher's Note:
1. Dyne is the CGS unit of force.
2. 1 dyne = 10⁻⁵ N.

Answer: (c) its weight changes but its mass does not

Explanation:
1. Mass is the amount of matter in the rock and stays the same. Weight depends on gravity, which is different on the earth and the moon.

Teacher's Note:
1. Weight = mass × g.
2. g on the moon is about one-sixth of g on the earth.

Answer: (b) the speed of the body decreases

Explanation:
1. The net force is 6 − 5 = 1 N towards the left, opposite to the motion, so the body slows down.

Teacher's Note:
1. Add forces taking direction into account.
2. A net force opposite to the motion reduces speed.

Answer: (a) 1 N

Explanation:
1. The force needed equals the weight: W = mg = 0.1 kg × 10 m/s² = 1 N.

Teacher's Note:
1. Convert grams to kilograms first.
2. Weight is the force of gravity on the mass.

Answer: (d) its magnitude and direction

Explanation:
1. A force is a vector quantity and needs both magnitude and direction to be fully described.

Teacher's Note:
1. The point of application is also useful.
2. Vector quantities need both.

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

Explanation:
1. A stationary body has zero net force; forces such as weight and the normal reaction cancel.

Teacher's Note:
1. Zero net force does not mean zero forces.
2. Balanced forces do not change the state of motion.

Answer: (b) remains unchanged

Explanation:
1. The coefficient of friction depends on the nature of the surfaces and not on the normal reaction. Friction force doubles, but the coefficient stays the same.

Teacher's Note:
1. Friction force = coefficient × normal reaction.
2. The coefficient depends on the surfaces in contact.

Answer: (a) 10 N

Explanation:
1. The spring is stretched by a tension of 10 N throughout, so the balance reads 10 N (not the sum).

Teacher's Note:
1. The scale reads the tension in the spring.
2. Each end pulls with 10 N and the spring is in equilibrium.

Answer: (d) a body moving in a circular path at constant speed

Explanation:
1. Constant speed in a circle means the direction of velocity keeps changing, so there is acceleration and hence a net (centripetal) force. In the other three cases velocity is constant or zero, so net force is zero.

Teacher's Note:
1. Velocity changes if direction changes.
2. A net force is needed for change in direction.

Answer: (c) Frictional force

Explanation:
1. Friction acts only when surfaces touch, so it is a contact force.

Teacher's Note:
1. Gravity, electric and magnetic forces are non-contact forces.
2. Contact forces need touch.

Answer: (b) 10⁻⁵ N

Explanation:
1. 1 N = 10⁵ dyne, so 1 dyne = 10⁻⁵ N.

Teacher's Note:
1. This conversion is useful in CGS problems.
2. Dyne is a very small force.