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Laws of Motion

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Physics Part I

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Class 10 Class 12
A constant force acting on a body of mass 3.0 kg changes its speed from 2.0 m s–1 to 3.5 m s–1 in 25 s. The direction of the motion of the body remains unchanged. What is the magnitude and direction of the force?


Mass of the body, m = 3 kg

Initial speed of the body, u = 2 m/s

Final speed of the body, v = 3.5 m/s

Time, t = 25 s

Using the first equation of motion, 

                     v = u + at

Acceleration(a) produced in the body, a =fraction numerator straight v minus straight u over denominator straight t end fraction

straight a space equals space fraction numerator 3.5 space minus space 2 over denominator 25 end fraction space equals space 0.06 space straight m divided by straight s squared

Now, according to Newton's second law of motion, we have

F = ma

   = 3 x 0.06

   = 0.018 N

Since the application of force does not change the direction of the body, the net force acting on the body is in the direction of its 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. 

Is Aristotle’s law of motion now correct?

No. Aristotl'e law of motion is false. 

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. 

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. 

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.