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Laws of Motion

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Physics Part I

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Class 10 Class 12
The rear side of a truck is open and a box of 40 kg mass is placed 5 m away from the open end as shown in Fig. 5.22. The coefficient of friction between the box and the surface below it is 0.15. On a straight road, the truck starts from rest and accelerates with 2 m s–2. At what distance from the starting point does the box fall off the truck? (Ignore the size of the box).

We have, 

Mass of the box, m = 40 kg

Coefficient of friction, μ = 0.15

Initial velocity, u = 0

Acceleration, a = 2 m/s2

Distance of the box from the end of the truck, s' = 5 m

According to Newton's second law of motion,

Force on the box caused by the accelerated motion, F = ma 

               = 40 × 2

                = 80 N 

According to Newton’s third law of motion, a reaction force of 80 N is acting on the box in the backward direction.

The backward motion of the box is opposed by the force of friction f, acting between the box and the floor of the truck.

This force is given by, 

                 f = μmg  

                   = 0.15 × 40 × 10

                   = 60 N 


Net force acting on the block, 

              Fnet = 80 – 60 = 20 N, acting backward 

The backward acceleration produced in the box is, 

Acceleration, aback = straight F subscript net over straight m space equals space 20 over 40 space equals space 0.5 space straight m divided by straight s squared space
Using the second equation of motion,

Time t is, 

                   s' = ut + (1/2) abackt

                   5 = 0 + (1/2) × 0.5 × t

∴                  t = √20 s 

Hence, the box will fall from the truck after square root of 20 s from the start. 

The distance s, travelled bytruck in √20 s is given by, 

                   s = ut + (1/2)at

                     = 0 + (1/2) × 2 × (square root of 20)

                     = 20 m 

Therefore, at a distance of 20 m, the box will fall off the truck. 

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. 

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.