Question

 

 

1) Discuss Newton's three laws of motion. Show that the second Law implies the First Law.

 

2) Usinh Newton's third law of motion ( Action - Reaction law ) explain how a rocketship is propelled.

 

3) In terms of Newtons's Universal Law of Gravitation, explain why Galileo found that al objects fall on Earth at the same rate, regardless of their mass

 

 

Answer 1

1.

First law :

According to Newton's first law, an object maintains its state of rest or motion in absence of an external force. As object at rest continues to be at rest unless acted upon by a force. the same can be said about an object in motion.

Second law :

According to Newton's second law, force on an object is directly proportional to time rate of change of its momentum.

F = dP/dt

F = d(mv)/dt

F = m (dv/dt)

F = ma

hence acceleration of an object is directly proportional to the force applied on it.

Third law :

According to Newton's third law, Every action has equal and opposite reaction. If an object applies force on another object, it experiences same amount of force back on it in opposite direction.

According to newton's second law ,

F = ma

when there is no force on the object

F = 0

m a = 0

Since mass can not be zero. hence

a = 0

this explains that an object at rest remains at rest and an object in motion keeps moving at constant velocity. which is nothing but the newton's first law

hence Newton's second law implies first law

2)

In a rocket, hot gases are ejected from the back of the rocket by some force. hence force is applied on the hot gases in backward direction. According to newton's third law, the hot gases applies same amount of force on the rocket is forward direction. hence making the rocket to move.

3)

According to Newton';s gravitational law

F = G Mm/r2

here m = mass of the object

we also know that

F = ma

So comparing the two equations , we get

a = G M/r2

For earth, Mass and radius are constant , hence the acceleration of the object falling towards earth is constant. that is why the objects fall on earth at same rate.