# Physics Kinematics Questions help?

1.If a rocket starting from rest is given a costant acceleration and moves 1000 meters during the first 20.0 seconds, how far does it move during the third second (ie 2s to 3s)?

2. An autmobile going 30 km/hr can be stopped by the brakes in 25m. If this auto were going 60 km/hr in what distance could it be stopped, assuming that the brakes produce the same negative acceleration.

3. A stone is dropped over a cliff 78.0m high. It accelerates at 9.81m/s^2. The sound when it strikes the rocks below is heard 4.230 seconds later. Find the velocity of sound.

4. A stone is dropped from a tower 68.6m high above the surface of the ground into a well, 0.75 of a second elapses between the time the stone passes the surface of the ground and the time it strikes the bottom of the well. a) find the velocity of the stone as it passes the surface of the ground. b) find the depth of the well.

5. A train started from rest and moved with constant acceleration. At one time it was travelling 30 m/s and 160m father on it was travelling 50m/s. Calculate a) the acceleration, b) the time required to travel the 160m mentioned, c) the time required to attain the speed of 30m/s, d) the distance moved from rest to the time the train had a speed of 30m/s.

6. how long will it take an object to fall to the ground from the top of the Empire State building which is approximately 310m high? what will be it's veloctiy when it hits the ground?

### 1 Answer

- 5 years agoFavorite Answer
I am a bit tired so I will give you a very brief to the point answer.

1) s = ut + 1/2a * t^2

so, 1000 = 0 + 1/2 *a* 20^2

1000 = 200*a

so, a = 1000/200 = 5m/s^2

Now the question asks what was the 's' from 2 to 3 sec.

so, the displacement would be = displacement till 3rd sec - displacement till 2nd sec

= (u*3 + 1/2a*3^2) - (u*2 + 1/2a*2^2)

= (0 + 4.5*5) - (0+ 2 * 5)

= 22.5 - 10

= 12.5 m

I think you can do the rest

you just have to use the three equations of uniform acceleration :

v = u+at

v^2 = u^2 + 2as

s = ut + 1/2*a*t^2