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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 body of mass 5 kg is acted upon by two perpendicular forces 8 N and 6 N. Give the magnitude and direction of the acceleration of the body.

Mass of the body, m = 5 kg

The given situation can be represented as follows:

The resultant of two forces is given by, 

R = square root of left parenthesis 8 right parenthesis squared plus left parenthesis negative 6 right parenthesis squared end root space equals space square root of 64 space plus space 36 end root space equals space 10 space straight N
straight theta is the angle made by R with the force of 8 N.


straight theta space equals space tan to the power of negative 1 end exponent space open parentheses negative 6 over 8 close parentheses space equals space minus 36.87 to the power of straight o
The negative sign indicates that θ is in the clockwise direction with respect to the force of magnitude 8 N.

According to Newton’s second law of motion

Acceleration (a) of the body is given as, 

F = ma 


a =straight F over straight m space equals space 10 over 5 space equals space 2 space m divided by s squared


Is Aristotle’s law of motion now correct?

No. Aristotl'e law of motion is false. 

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. 

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.