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Laws of Motion

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Physics Part I

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Class 10 Class 12
The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg. 

Initial speed of the three-wheeler, u = 36 km/h = 10 m/s

Final speed of the three-wheeler, v = 0 m/s

Time, t = 4 s

Mass of the three-wheeler, m = 400 kg

Mass of the driver, m' = 65 kg

Total mass of the system, M = 400 + 65 = 465 kg

According to the first law of motion,

Acceleration (a) of the three-wheeler is, 

                       v = u + at 


 a = fraction numerator left parenthesis straight v space minus space straight u right parenthesis over denominator straight t end fraction space equals space fraction numerator left parenthesis 0 minus 10 right parenthesis over denominator 4 end fraction space equals space minus space 2.5 space straight m divided by straight s squared

The negative sign indicates that the velocity of the three-wheeler is decreasing with time.

Now, using Newton’s second law of motion, the net force acting on the three-wheeler is, 

       F = Ma

         = 465 × (–2.5)
         = –1162.5 N 

The negative sign indicates that the force is acting against the direction of motion of the three wheeler. 

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. 

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. 

Is Aristotle’s law of motion now correct?

No. Aristotl'e law of motion is false.