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Page No 126: - Chapter 9 Force & Laws Of Motion class 9 ncert solutions Science - SaraNextGen [2024]


Question 1:

If action is always equal to the reaction, explain how a horse can pull a cart.

Answer:

A horse pushes the ground in the backward direction. According to Newton’s third law of motion, a reaction force is exerted by the Earth on the horse in the forward direction. As a result, the cart moves forward.

 

 

Question 2:

Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.

Answer:

Due to the backward reaction of the water being ejected

When a fireman holds a hose, which is ejecting large amounts of water at a high velocity, then a reaction force is exerted on him by the ejecting water in the backward direction. This is because of Newton’s third law of motion. As a result of the backward force, the stability of the fireman decreases. Hence, it is difficult for him to remain stable while holding the hose.

 

 

Question 3:

From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s−1. Calculate the initial recoil velocity of the rifle.

Answer:

Mass of the rifle, https://img-nm.mnimgs.com/img/study_content/curr/1/9/8/121/606/Chapter%209_html_7da100f7.gif

Mass of the bullet, https://img-nm.mnimgs.com/img/study_content/curr/1/9/8/121/606/Chapter%209_html_5e2d5a41.gif

Recoil velocity of the rifle = https://img-nm.mnimgs.com/img/study_content/curr/1/9/8/121/606/Chapter%209_html_680b2290.gif

Bullet is fired with an initial velocity,https://img-nm.mnimgs.com/img/study_content/curr/1/9/8/121/606/Chapter%209_html_617d97c2.gif

Initially, the rifle is at rest.

Thus, its initial velocity, v = 0

Total initial momentum of the rifle and bullet system https://img-nm.mnimgs.com/img/study_content/curr/1/9/8/121/606/Chapter%209_html_m35750d48.gif

Total momentum of the rifle and bullet system after firing:

https://img-nm.mnimgs.com/img/study_content/curr/1/9/8/121/606/Chapter%209_html_m6a18a558.gif

According to the law of conservation of momentum:

Total momentum after the firing = Total momentum before the firing

https://img-nm.mnimgs.com/img/study_content/curr/1/9/8/121/606/Chapter%209_html_28a56525.gif

The negative sign indicates that the rifle recoils backwards with a velocity of 0.4375 m/s.

Also Read : Page-No-127:-Chapter-9-Force-&-Laws-Of-Motion-class-9-ncert-solutions-Science

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