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Laws of Motion

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Physics Part I

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 ms–1. How long does the body take to stop?

Retarding Force, F = - 50 N

Mass of the body, m = 20 kg 

Initial velocity of the body, u = 15 m/s

Final velocity of the body, v = 0 

Acceleration produced in the body can be calculated as: 

   F = ma

-50 = 20 x a

  a = fraction numerator negative 50 over denominator space 20 end fraction space equals space minus 2.5 space m divided by s squared

Using the first equation of motion,

                          v = u + at 

Time (
t) taken by the body to come to rest can be calculated as,


                 t = negative straight u over straight a space equals space fraction numerator negative 15 over denominator negative 2.5 end fraction space equals space 6 space s


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. 

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.

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. 

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. 

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