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Laws of Motion

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Physics Part I

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Class 10 Class 12
A truck starts from rest and accelerates uniformly at 2.0 m s–2. At = 10 s, a stone is dropped by a person standing on the top of the truck (6 m high from the ground). What are the (a) velocity, and (b) acceleration of the stone at = 11 s? (Neglect air resistance.)


Initial velocity of the truck, u = 0

Acceleration, a = 2 m/s2

Time, t = 10 s


According to the equation of motion, we have

                   v = u + at

                     = 0 + 2x10

                     = 20 m/s

The final velocity of the truck and hence, of the stone is 20 m/s.

At t = 11 s, the horizontal component (vx) of velocity, in the absence of air resistance, remains unchanged.

i.e.,                vx = 20 m/s 

The vertical component (vy) of velocity of the stone is given by the first equation of motion as,

                     vy = u + aδ


δt = 11 – 10 = 1 s, and 

y = g = 10 m/s

∴           vy = 0 + 10 × 1

                  = 10 m/s

The resultant velocity (v) of the stone is given as:

straight v space equals space square root of straight v subscript straight x squared space plus space straight v subscript straight y squared end root space

space space space equals space space square root of 20 squared plus space 10 squared end root space

space space space equals space 22.36 space straight m divided by straight s

Let, straight theta be the angle made by the resultant velocity with the horizontal component of velocity vx,


tan space straight theta space equals space begin inline style straight v subscript straight y over straight v subscript straight x end style

straight theta space equals space tan to the power of negative 1 end exponent space open parentheses 10 over 20 close parentheses

space space space equals space 26.57 to the power of straight o 


When the stone is dropped from the truck, the horizontal force acting on it becomes zero. However, the stone continues to move under the influence of gravity. Hence, the acceleration of the stone is 10 m/s2 and it acts vertically downward. 


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. 

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.