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Laws of Motion

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Physics Part I

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Class 10 Class 12
A stream of water flowing horizontally with a speed of 15 m s–1 gushes out of a tube of cross-sectional area 10–2m2, and hits a vertical wall nearby. What is the force exerted on the wall by the impact of water, assuming it does not rebound?


Speed of the water stream, v = 15 m/s

Cross-sectional area of the tube, A = 10–2 m2

Volume of water coming out from the pipe per second,


                     = 15 × 10–2 m3/s 

Density of water, ρ = 103 kg/m

Mass of water flowing out through the pipe per second = ρ ×V 

           = 150 kg/s

The water strikes the wall and does not rebound.

Therefore, according to Newton's second law of motion, 

Force exerted by the water on the wall,

F = Rate of change of momentum = fraction numerator increment straight p over denominator increment straight t end fraction

                                                    = mv over straight t

                                                    = 150 × 15

                                                    = 2250 N

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. 

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.

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. 

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.