Laws Of Motion

Question 1

What is Newton‘s First Law of Motion?

Answer 1

Every object continues to be in its state of rest or uniform motion until an external unbalanced force is applied on it.

Question 2

Define Momentum. Give its formula, unit and dimension.

Answer 2

When a force is applied on a body, the motion can be described with its mass and velocity. The product of mass and velocity is called the momentum of a body.
Momentum; p = mv
Unit = kgm/s
Dimension: MLT^-1

Question 3

What is Newton’s second law of motion?

Answer 3

The second law of motion states that the rate of change of momentum of a body is proportional to the external unbalanced force applied in the direction of force.
F = ma

Question 4

Define impulse. Mention it’s unit.

Answer 4

Impulse of a force is the product of force and time during which the force acts on the object.
i = F • t
Unit: Ns (Newton • second)

Question 5

What is Newton’s third law of motion?

Answer 5

For every action, there is an equal and opposite reaction.
Extra: Action and reaction forces are always equal in magnitude, opposite in direction and act on different bodies. Therefore they do not cancel each other.

Question 6

State the law of conservation of linear momentum.

Answer 6

It states that in the absence of an external force, the total linear momentum of an isolated system remains constant.
Learn proof using 2nd law and 3rd law

Question 7

Explain the recoil of a gun.

Answer 7

Consider a shot fired from a gun. The shot acquires a large forward momentum due to explosion of gunpowder. Consequently, the gun gets an equal backward momentum and is pushed backward with a velocity V. This backward motion of a gun is called recoil of a gun and the velocity V of the gun after firing is called recoil velocity of a gun.

Question 8

What is muzzle velocity?

Answer 8

The velocity of the bullet after firing is called the muzzle velocity of the bullet.
Let v be the muzzle velocity of the bullet, M be the mass of the gun and m be the mass of the bullet.
Acc to Law of Conservation of momentum,
Total momentum before firing = total momentum after firing
=MV + mv =0
MV=-mv
V=-mv/M

Question 9

What is equilibrium?

Answer 9

When all the forces that act upon an object are balanced, then the object is said to be in a state of equilibrium.
The net external force acting on an equilibrium is zero.

For 2 Forces: F1 and F2
F1 + F2 =0
F1 = -F2

For 3 Forces: F1, F2 and F3
F1 + F2 + F3 =0
F1 + F2 = -F3

Question 10

What are coplanar concurrent forces?

Answer 10

When forces P,Q and R are acting at a point on the same plane, they are called coplanar concurrent forces.

Question 11

What are the conditions to be satisfied for a point to be an equilibrium?

Answer 11

For a point to be an equilibrium, one of the following conditions must be satisfied.

1. The vector sum of P and Q must be equal and opposite to R.
   P + Q = -R
2. If P, Q and R can be represented by the sides of a triangle taken in order, then
   P/LM = Q/LN = R/MN = a constant.
3. Each force must be proportional to the sine of the angle between the other two.
   P/sinalpha = Q/sinbeta = R/singamma = a constant.
   This is known as Lami’s Theorem.

Question 12

What is known as Lami’s theorem?

Answer 12

Each force must be proportional to the sine of angle between the other two, for a point to be considered an equilibrium.

P/sinalpha = Q/sinbeta = R/singamma = a constant.

This is known as Lami’s Theorem.