sorry for the delay......but finally done

Force and Laws of Motion

(By Abhishek sir)

Q: Define force?

Ans: A force is a physical quantity which causes or tends to cause a motion in an object at rest or changes or tends to change the direction of motion of a moving object.it is a vector quantity.its SI unit is Newton and CGS unit is DYNE.

Q: What are the effects of force?

Ans: Force can change –

       i.            The state of rest or motion of an object.

    ii.            Size of the object.

 iii.            Shape of the object.

  iv.            Speed of the object.

     v.            Direction of motion of an object.

Force is of two types:

1.    Balanced force

2.    Unbalanced force

Balanced force: - When two forces of equal magnitude but acting in opposite directions on an object simultaneously, then the object is continues in its state of rest or uniform motion on a straight line, such forces acting on an object is called balanced force.

i.                   When we push a wall, the wall does not move at all it remains at rest.

ii.                In a tug of war two teams pull the rope in any direction, in spite of the fact that both the teams are applied equal forces, then the force applied by both the team is called balanced force.

Unbalanced force – When two forces of unequal magnitude acts in opposite direction on an object simultaneously, then the object move in direction of larger force these forces acting on an object is called unbalanced force.

E.g.

       i.            When a boy drags a box in a floor, then an unbalanced force is acting on the box.

    ii.            Bicycle gets slow down, if the rides stop pedaling it because the force of friction will act in the direction opposite to the direction of motion of the bicycle.

NOTE: force of friction opposes the relative motion of an object.

 iii.            The net force acting on an object is not a when unbalanced force is acting on it.

Q: Who proposed the laws of motion?

Ans: Isaac Newton.

Q: Explain the Newton’s 1^st law of motion?

Ans:  According to this law of a body is at rest or in the state of uniform motion in a straight line unless some external force acts on it and changes.

Newton’s first law of motion is also famous as ‘Law of Inertia’ proposed by Galileo Galleli which states that when no unbalanced force acts on a body at rest, then it continues at the state of rest or when no unbalanced force acts on a body moving with constant velocity it continues in a state of motion because of their state.

Q: Define inertia.

Ans: The tendency of a body of rest or uniform motion is called inertia of a bod.

Inertia is of three types –

i.                   Inertia of rest.

ii.                Inertia of motion.

iii.             Inertia of direction.

Examples of inertia of rest

1.    When a bus suddenly starts moving forwards the passengers falls backward.

Reason: This is because the lower part of the body of passenger being in contact with the floor of a bus come in motion along with the bus on the other hands, the upper part of the body remain at rest due to inertia of rest. Hence, the passengers fall backward.

2.    The carpet is beaten with a stick to remove the dust particles.

Reason: When the carpet is beaten with a stick, the fibers of the carpet come in motion and hence move forward. On the other hand, the dust particles remain at rest due to inertia of rest. Therefore, they fall down.

3.    When a tree is vigorously shaken, some of the leaves fall from the tree.

Reason: When a tree is falling vigorously shake, the branches of the tree come in motion but the leaves tend to continue in their state of rest due to inertia of rest.  As a result of this, leaves get separated from the branches of the tree and hence fall down.

4.    A pile of a one rupee coins remains intact when the lowest coin in the pile is stuck quickly.

Reason: this is because, the lowest coin comes in motion when the stuck. But the remaining coins in the pile remain in form of pile. Similarly, a pile of carom coin remains intact when the lowest coin is stuck by a striker.

·       The tendency of a body to oppose any change in its state of uniform motion is known as inertia of motion.

Examples of inertia of Motion

1.    The passengers fall forward when a fast moving bus stops suddenly.

Reason: When a moving bus suddenly stops, the lower part of the bodies of the passengers comes to rest as soon as the bus stops. But, the upper part of their body continues to move forward due to inertia of motion.

2.    A luggage is usually tied with a rope on the roof of a bus.

Reason: When a moving bus suddenly stops, the luggage on its roof tends to continues in the state of motion due to inertia of motion. Hence the luggage falls down from the roof of the bus.  Similarly, when a bus suddenly starts, the luggage on the roof of the buss tends to continues in the state of rest of the bus. Thus, to avoid the falling of the luggage, it is tied with a falling of the luggage; it is tied with a rope of the bus. 

·       The tendency of a body to oppose any change in its direction of motion is known as inertia of direction.

Examples of inertia of direction

1.    A stone tied to a string is whirling in a horizontal circle. If the string breaks, the stone flies away tangentially.

Reason: the stone moving in a circular path has the direction of motion along the tangent at any point of the circle. The pull of hand keeps it in a circular path. As soon as the string breaks , the force acting on the string ceases and the stone continues to move along the tangent of the point of the circle due to inertia of direction.

Inertia and Mass

Consider two stationary objects say a small table and a big table. Now if we apply the same amount of force on those two tables, then it is difficult to move the bigger table than the smaller table. It means, the heavy table opposes more to the change in the state of rest than the light or small table. Thus , greater is the mass, the greater is the inertia. Hence, inertia of a body is equal to the mass of the body.

·       Inertia of a body depends on its mass and equal to the mass of the Body.

Momentum

The quantity of motion posses by a moving body is known as the momentum of a body. Momentum of the body is equal to the product of a mass (m) of a body and equal to the velocity (v) of the body. It is denoted by

P= m x v

 P  m

P  v

·       It is a vector quantity.

·       The direction of a momentum of a body is same as the direction of the velocity of the body.

·       It depends on both mass and velocity of the body

Therefore, if either of the both changes the momentum of a body are also changes.

S.I. unit of momentum = Kg. m/s

C.G.S. unit- gm cm/s