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Laws of Motion

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Physics Part I

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Class 10 Class 12
A nucleus is at rest in the laboratory frame of reference. Show that if it disintegrates into two smaller nuclei the products must move in opposite directions.

Let mm1, and m2 be the respective masses of the parent nucleus and the two daughter nuclei.

The parent nucleus is at rest.

Initial momentum of the system (parent nucleus) = 0

Let v1 and v2 be the respective velocities of the daughter nuclei having masses m1 and m2.

Total linear momentum of the system after disintegration = m1v1 + m2v

According to the law of conservation of momentum, 

Total initial momentum = Total final momentum 

                                 0 = m1v1 + m2v

                                v1 = -m2v2 / m

Here, the negative sign indicates that the fragments of the parent nucleus move in directions opposite to each other.

When the branches of an apple tree are shaken, the apples fall down. Why?

The apple fall from an apple tree when it shaken because of inertia of rest. Apple is in a state of rest and when the tree is suddenly shaken, apples still tends to remain in it's same state of rest whereas branches move. 

So, the apples fall down. 

Is Aristotle’s law of motion now correct?

No. Aristotl'e law of motion is false. 

What is Aristotle’s law of motion?

Aristotle’s law of motion states that an external force is required to keep the body in motion.

Define inertia.

The property by virtue of which the body cannot change its state of rest or uniform motion in a straight line, unless an external force is acting on the body is called as Inertia. 

State Galileo’s law of motion.

Galileo’s law of motion states that, a body continues to move in the same direction with constant speed, if no force is acting on the body.