Revision: Term - 1 >> Force and Motion Science SSLC (English Medium) Class 7 Tamil Nadu Board of Secondary Education

The length of path travelled by a body in certain interval of time is called distance.

• Distance is the total length of the path travelled by a moving object, regardless of the direction. It is a scalar quantity, which means it only has magnitude (size), not direction.
• Example: If Ranjit walks from point A to B to C to D, the distance is the total of AB + BC + CD.

• Displacement is the shortest distance in a straight line between the starting point and the final point, along with a direction. It is a vector quantity, meaning it includes both magnitude (distance) and direction.
• Example: In Ranjit’s case, the displacement is the straight line AD from his house to the school.

Displacement of an object between two points is the shortest distance between these two points.
“It is the unique path that can take the body from its initial to the final position.”

The displacement of a moving body is defined as the change in its position along a particular direction

Speed is the rate of change of distance.
Formula: Speed = distance /time
Unit is meter/second (m/s)

The distance covered by a body in a unit time is called its speed. It is also defined as the rate of change position of a body in any direction.
Speed = distance/time

Velocity is the rate of change of displacement. It is the displacement in unit time.

Velocity is the rate of change in displacement.
Formula: Velocity (v) = displacement / time
SI unit of velocity is meter / second (m/s).

“Rate of change of displacement with time is called velocity.”
OR
“The time rate of change of displacement of an object is called the velocity.”
V = distance/time

Negative acceleration or deceleration or retardation:
If the velocity of an object decreases with respect to time, then the object is said to be in negative acceleration or deceleration, or retardation.

Positive acceleration:
If the velocity of an object increases with respect to time, then the object is said to be in positive acceleration or just acceleration.

The Centre of Gravity (c.g.) of a body is the point around which the resultant torque due to the force of gravity on the body is zero.

or

The centre of gravity (C.G.) of a body is the point about which the algebraic sum of moments of the weights of all the particles constituting the body is zero. The entire weight of the body can be considered to act at this point, howsoever the body is placed.

The centre of gravity is an imaginary location where the body’s whole weight is assumed to be concentrated.

When a number of forces acting on a body produce no change in its state of rest or of linear or rotational motion, the body is said to be in a state of equilibrium.

The momentum of a body is the product of the mass of the body and its velocity, i.e. p = mv.

• The weight of a body acts through a single point called the centre of gravity (C.G.), where the sum of moments of all particles' weights is zero.
• The position of the C.G. depends on the shape and mass distribution of the body and changes if the body is deformed.
• The C.G. may lie outside the material of the body (e.g., a ring or hollow sphere).
• A body balances when supported exactly at its centre of gravity, as seen in a metre rule or square lamina.
• The C.G. of an irregular lamina can be found by suspending it from multiple points and tracing the intersection of plumb line paths.

Conditions for Equilibrium of Bodies:

1. The resultant of all the forces is zero, so they do not change either the state of rest or the state of linear motion of the body, and
2. The algebraic sum of moments of all the forces about the fixed point is zero, so they do not change the rotational state of the body.