Third law of motion for class 9th(lec 4)

 video lectures for the Third law of motion

Question 1: A body `‘A’` of mass `4kg` on collision exerts a force on another body `B` of mass `10kg`. If the acceleration produced in `B` is `10 ms^{-2}`, then the acceleration (in magnitude) of `A` is ______.

a) `25ms^{-2}`

b) `10 ms^{-2}`

c) `52 ms^{-2}`

d) `5 ms^{-2}`

Question 2: Explain Newton’s third law of motion with an example.

Question 3: Action and reaction

(a) always exist in pairs.

(b) are equal in magnitude.

(c) always act in opposite directions.

(d) All the above are true

Question 4: A Pakistani player Shoab Aktar bowls a cricket ball of mass `250 g` with a speed of `120 kmh^{-1}` towards the batsman Dhoni. (Assume that the ball strikes the bat with the same speed). The time of impact of the bat with the ball is `0.1s`. After striking, if the ball bounces back with half its speed before striking, find the force which Dhoni exerts on the ball.

(a) `125 N`

(b) `100 N`

(c) `150 N`

(d) `75 N`

Question 5: Two groups of students `A` and `B`, with

two students in each group, challenge to accelerate

two bodies `P` and `Q`, respectively, placed on a

frictionless surface as shown in the figure. Which

group is successful in imparting more acceleration?

Justify. Find the acceleration of both bodies.

(a) `1.5 ms^{-2}`

(b) `5 ms^{-2}`

(c) `1 ms`

(d) `1ms^{-2}`

Answer Key

Q1. (a)

Q2. Newton's third law states that for every action there is an equal and opposite reaction. As you stand on the ground, your body push on the earth with a force, and the earth reacts on your body with the same force in opposite direction. This is an example of Newton's third law.

Q3. (d), Q4. (a), Q5. (d)