Characteristics of uniform motion :
1. Same distance is covered (traveled) in equal intervals of time.
2. Distance Time graph shows a straight line. s = v t
3. Speed is constant. If the body is moving in a straight line, velocity is constant and
acceleration is zero. There is no net force acting on the body. Displacement always
increases and is in one direction.
4. If body is moving in a circle, the speed v is constant and acceleration = v²/R . Acceleration
points towards the center of circle. There is a centripetal force acting on the body.
Displacement increases and decreases in magnitude and changes in direction.
Centripetal force is constant.
1. Body moves unequal distances in same time intervals.
2. Distance Time graph is not a straight line. Speed is not a constant.
3. If the body is moving along a straight line, velocity is in the same direction and
acceleration is also in the same direction. Velocity & acceleration are not constant.
4. If the body is moving in a circle, then speed, velocity, acceleration are not constant.
5. Angular velocity is not constant. Centripetal force is not constant.
Bodies moving in uniform speed
1. Earth rotates around itself with a uniform speed (almost)
So all of us are
moving with a uniform speed (tangential) in circles, with
respect to a
stationary observer outside earth. We have not taken into
revolution of earth around Sun here.
Our speed = radius of earth * angular velocity
= 6400 km * 2 π / 24 hours =
2. Trains and air planes move with uniform
speed in between halts. Near
stations or airports, they accelerate or decelerate.
An express train
covers usually around 60 km in 3/4 hour non-stop
between two big stations.
= 60 / (3/4) = 80 kmph
5. Speed of Birds flying in air = distance traveled / time = 15
meters /sec = 54 kmph
7. Hard disk inside the computer. It rotates at uniform speed.
speed = 4,000 rpm - 10,000 rpm
9. Light wave moves at uniform speed of c = 3 * 10^8 m/sec
10. A fan (ceiling) rotates at a uniform speed, same with the blade in a
kitchen - mixie or mixer.
Tip of Fan wing : radius = 0.80 meter,
smallest speed = 5 rotations per sec
Angular speed = 5 * 2 p radians/sec = 10
Uniform Linear speed of the tip (far end
of a wing)
= 0.80 * 10 p m/sec =
25.13m/s = 90.5 kmph
11. Blood inside our body moves (circulates) at uniform speed
Pulse rate = 72 /min
distance from heart to a hand/leg = 2.5 m
Speed of blood flow = around 2.5m
* 72 / min
= 180 m / min = 180 * 60/1000 = 10.8 kmph
Bodies moving with non
uniform speed :
1. In a
cricket match the cricket ball is bowled and it travels the
an average speed in air
and after bouncing from ground =
The ball takes 0.7 to 1 sec to reach batsman - observation.
= distance / time = 20 meters / 0.7 sec = 20/0.7 * 18/5 = 103 kmph
for a pacer
and 20 m / 1 sec = 72 kmph for a spinner.
2. moon moves around the Earth with almost uniform speed
4. An object thrown in to air, like stone, moves
at non-uniform speed.
4. Earth moves in an elliptical
orbit non-uniform speed - its orbital speed
changes depending on its
distance from the Sun. During summer Earth is
at 147 million km
and (perihelon) during winter, it is at 152 million km.
at perihelon = 2 π R / T = 105,000 kmph
at apihelon = 2 π R / T = 109,000 kmph
5. Lift used to go up or down in a building - it
moves at non-uniform speed -
accelerates and decelerates
also. Lift reaches 4th floor (10meters) in
10 seconds with
uniform acceleration starting from rest.
acceleration of an elevator is = 20/100 = 0.2 m/s²
Speed = 0.2 * time m/sec