Law of momentum conservation for class 9th (lec 5)

 video lectures for the Law of momentum conservation

Question 1: A body of mass `5 kg` moving with a speed of `3ms^{-1}` collides head-on with a body of mass `3 kg` moving in the opposite direction at a speed of `2ms^{-1}`.The first body stops after the collision. Find the final velocity of the second body. a) `-9ms^{-1}` b) `6ms^{-1}` c) `3ms^{-1}` d) `9 ms^{-1}`

Question 2: A body `‘A’` of mass `m_{1}` on collision exerts a force on another body `B` of mass `m_{2}`. If the acceleration produced in `B` is `a_{2}`, then the acceleration (in magnitude) of `A` is ______. a) `\frac{m_{1}}{m_{2}}a_{2}` b) `\frac{m_{2}}{m_{1}}a_{2}` c) `(m_{1}+m_{2})a_{2}`

d) `(m_{1}-m_{2})a_{2}`

Question 3: State the law of conservation of momentum.

Question 4: If no external ______ acts, the total momentum of the colliding bodies is conserved. a) force b) momentum c) work d) non of above

Question 5: State and verify the law of conservation of momentum.

Question 6: Two bodies `A` and `B`, moving in the same direction collide and after collision, move with the common velocity in the direction of `A`.

(a) The magnitude of force exerted by A on B is greater than the magnitude of force exerted by B on A.

(b) Both of them exert an equal but opposite force on each other.

(c) The change in momentum of A and B are equal but opposite in direction.

(d) Both (b) and (c).

Question 7: Explain why a car does not move when the force is applied on it by a person present inside the car?

Question 8: A person jumps onto a cement floor from a height of `1 m` and comes to rest in `0.1 sec`. The same person jumping from a height of `9 m` into a sand pit, comes to rest in `1 sec`. Compare the forces exerted on him by the cement floor and sand pit.

(a) `\frac{10\sqrt{5}}{3\sqrt{5}}`

(b) `\frac{3\sqrt{5}}{10\sqrt{5}}`

(c) `\frac{10\sqrt{5}}{3}`

(d) `\frac{10}{3\sqrt{5}}`

Question 9: Raju was selected for the long jump in athletics at his school. He could do a ‘long jump’ well on a sand bed, but when he demonstrated this on the road, he felt pain in the joints. Explain.

Answer Key

Q1. (c), Q2. (b)

Q3. The conservation of momentum states that, within some problem domain, the amount of momentum remains constant; momentum is neither created nor destroyed, but only changed through the action of forces as described by Newton's laws of motion.

Q4. (a)

Q5. For two or more bodies in an isolated system acting upon each other, their total momentum remains constant unless an external force is applied.

Q6. (d),

Q7. (1)  The force applied by a person from inside a vehicle constitutes internal force. (2)  Newton’s first law: A body remains in a state of rest unless acted upon by a net external force.

Q8. (a)

Q9. We know, F × t = m(v – u) The change in momentum is constant when he jumps either on the road or on the sands bed because the initial and final velocities are the same Then, F × t = constant

If the impulsive time is increased, then the impact of impulse force decreases and vice versa. When Raju jumped on the road, the impulsive time is less and the impulsive force is more and this is the reason for the pain in the joint.