CBSE Class 8 Science Advanced Physics Force Long Answer Questions Set 01

Long Answer Type

Question 1. It is required to increase the velocity of a scooter of mass 80 kg from 5 ms⁻¹ to 25 ms⁻¹ in 2 second. Calculate the force required.
Answer:
u = 5 m/s, v = 25 m/s, t = 2 s, m = 80 kg.
a = (v − u) ÷ t = (25 − 5) ÷ 2 = 10 m/s².
F = ma = 80 × 10 = 800 N.
In simple words: The scooter must gain 10 m/s every second, which needs 800 N.

Teacher's Note:
1. Use a = (v − u)/t, then F = ma.
2. Quote the unit newton.

Question 2. A car of mass 1000 kg is moving with a velocity of 10 m/s and is acted upon by a forward force of 1000 N due to engine and retarding force of 500 N. Calculate the velocity after 10 seconds.
Answer:
Net force = 1000 − 500 = 500 N forward.
a = F ÷ m = 500 ÷ 1000 = 0.5 m/s².
v = u + at = 10 + 0.5 × 10 = 15 m/s.
In simple words: A net 500 N gives 0.5 m/s² for 10 s, so the speed rises by 5 m/s to 15 m/s.

Teacher's Note:
1. The net force is the engine force minus the retarding force.
2. Use v = u + at.

Question 3. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
Answer:
1. Thrust: the upward force produced by the burning fuel's gases pushing out of the rocket.
2. Weight (force of gravity): the downward pull of the earth, mg.
Air resistance also opposes the motion, but the two main forces are thrust (upwards) and weight (downwards). The rocket rises because the thrust is greater than the weight.
In simple words: The rocket is pushed up by thrust and pulled down by gravity.

Teacher's Note:
1. The book's key lists air resistance and a gravitational reaction; thrust and weight are the standard pair.
2. A rocket lifts off when thrust exceeds weight.

Question 4. State the effect of the force : (i) Squeezing a lemon. (ii) Taking out paste from a toothpaste tube. (iii) A lump of dough on a plate. (iv) Hit a ball by a cricket player with bat. (v) Brakes is applied to a moving car.
Answer:
(i) Squeezing a lemon: the force changes its shape (and juice comes out).
(ii) Toothpaste tube: pressing changes the shape of the tube and pushes the paste out, setting it in motion.
(iii) Lump of dough on a plate: pressing or pushing changes its shape.
(iv) Bat hitting a ball: the speed and the direction of the ball change (a stationary or moving ball is set in fast motion).
(v) Brakes on a moving car: the car slows down and stops.
In simple words: Each case changes the shape of the object or its motion.

Teacher's Note:
1. Name which of the four effects applies each time.
2. Shape: (i), (ii), (iii). Motion: (iv), (v).

Question 5. A car of mass 500 kg starts from rest and is acted upon by a forward force of 200 N due to engine and retarding force of 50 N. Calculate the velocity after 5 seconds.
Answer:
Net force = 200 − 50 = 150 N.
a = 150 ÷ 500 = 0.3 m/s².
v = u + at = 0 + 0.3 × 5 = 1.5 m/s.
In simple words: A net 150 N gives 0.3 m/s², so after 5 s the car moves at 1.5 m/s.

Teacher's Note:
1. Starting from rest means u = 0.
2. Net force first, acceleration next, velocity last.