# Describing motion

By mentioning a reference point, we are able to explain the location of an object. Give us an example to help us comprehend this. Let's say a school in a community is located 2 km from the train station. The location of the school in relation to the train station has been specified. The train station serves as the reference point in this illustration. Other reference locations might have been used based on convenience. Therefore, we must define a reference point known as the origin in order to describe the position of an item.

# MOTION ALONG A STRAIGHT LINE

The simplest type of motion is the motion along a straight line.It is also called rectilinear motion.

### Displacement-When a body covers a path,displacement is the shortest distance between the initial point and the final point of the body.When a body reaches its original path total displacement is zero is initial point and final point is same.

Note-Distance may be equal to displacement(When body is travelling in straight line) but it cannot be less than displacement

Displacement may be equal to distance(When body is travelling in straight line) but it cannot be more than distance.

# Uniform motion

As the object covers equal distances in equal intervals of time, it is said to be in uniform motion. The time interval in this motion should be small.Eg a boy running in a straight path with constant speed.

# Non-Uniform motion

As the object covers unequal distances in equal intervals of time, it is said to be in non-uniform motion.for example, when a car is moving on a crowded street or a person is jogging in a park. These are some instances of non-uniform motion.

# Speed

Speed, which is a scalar quantity, is the the rate at which an object travels a distance.Its si unit is m/s (metre/sec).

• Speed=distance/time

## Average speed

The average speed of an object is obtained by dividing the total distance travelled by the total time taken.

That is, average speed = Total distance travelled/Total time taken

# Speed with direction(Velocity)

• If we include the direction of an object's motion in addition to its speed, we can get a more complete picture of its rate of motion. Velocity is the term for the quantity that identifies both of these features.

• The speed of an object travelling in a specific direction is known as its velocity.

• An object's velocity may be constant or variable. It can be changed by altering the object's motion's speed, direction, or both.

• When an object is moving along a straight line at a variable speed, we can express the magnitude of its rate of motion in terms of average velocity. It is calculated in the same way as we calculate average speed.

### Average velocity = (initial velocity + final velocity)/2

• initial velocity is denoted by u

• final velocity is denoted by v

• time is denoted by t

• speed is denoted by S

• acceleration is denoted by a

# Rate of Change of Velocity(Acceleration)

• The velocity is constant throughout time when an object is travelling uniformly in a straight path.

• In this situation, the object's change in velocity over all possible time intervals is zero.

• The velocity, on the other hand, fluctuates with time in non-uniform motion.

• At various times and locations along the path, it has various values. As a result, the object's change in velocity during any time period is not zero.

• This kind of motion is known as accelerated motion.

• The acceleration is taken to be positive if it is in the direction of velocity and negative when it is opposite to the direction of velocity. The SI unit of acceleration is m s–2 .

• If an object travels in a straight line and its velocity increases or decreases by equal amounts in equal intervals of time, then the acceleration of the object is said to be uniform.

• The motion of a freely falling body is an example of uniformly accelerated motion

• . On the other hand, an object can travel with non-uniform acceleration if its velocity changes at a non-uniform rate.

• For example, if a car travelling along a straight road increases its speed by unequal amounts in equal intervals of time, then the car is said to be moving with non-uniform acceleration.

### Acceleration is the rate at which velocity changes with time

• acceleration = change in velocity/time taken

• acceleration = (final velocity-initial velocity)/time taken

• a=(v-u)/t

• v=u+at

• S=ut+0.5att

• v^2=u^2+2aS

# Uniform Circular Motion

• When the velocity of an object changes, we say that the object is accelerating. The change in the velocity could be due to change in its magnitude or the direction of the motion or both.

• If the athlete moves with a velocity of constant magnitude along the circular path, the only change in his velocity is due to the change in the direction of motion. The motion of the athlete moving along a circular path is, therefore, an example of an accelerated motion.

• We know that the circumference of a circle of radius r is given by 2 r π . If the athlete takes t seconds to go once around the circular path of radius r, the speed v is given by v=2πr/t.

• When an object moves in a circular path with uniform speed, its motion is called uniform circular motion.