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## Answers

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*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

account the revolution of earth around Sun here.

Our speed = radius of earth * angular velocity

= 6400 km * 2 π / 24 hours = 1675.5 kmph

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.

average speed = 60 / (3/4) = 80 kmph

5. Speed of Birds flying in air = distance traveled / time

= 15 meters /sec = 15 * 18/5 = 54 kmph

6. In an industry usually there are moving parts inside machines - they

generally move with uniform speeds:

Their rotational speed = 100 rpm - 1000 rpm.

7. Hard disk inside the computer. It rotates at uniform speed.

speed = 4,000 rpm - 10,000 rpm

8. the motor to pump water from sump to the tank. the rotor in that moves

at uniform speed of around 300 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 p rad/sec

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 to 75/min

Average distance from heart to a hand/leg & back = 2.5 m

Speed of blood flow = around 2.5 m * 72 / min

= 180 m / min = 180 * 60/1000 = 10.8 kmph

12. The water from tank that comes in our taps flows at uniform speed over a short duration

of time. Speed depends on the level of water in the tank.

*Bodies moving with non uniform speed*

*:*1. In a cricket match the cricket ball is bowled and it travels the pitch at

an average speed in air and after bouncing from ground =

The ball takes 0.7 to 1 sec to reach batsman - observation.

speed = 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

3. weather satellites move with a uniform and high speed in space

4. An object thrown in to air, like stone, moves at non-uniform speed.

5. 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.

Linear Speed at perihelon = 2 π R / T = 105,000 kmph

Linear Speed at apihelon = 2 π R / T = 109,000 kmph

6. 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.

So acceleration of an elevator is = 20/100 = 0.2 m/s²

Speed = 0.2 * time m/sec