# Help! Kinematics

1. Sep 5, 2006

### Chutzpah

Okay, I am new to physics and for some reason I am not connecting the dots. I would appreciate if a combination of anyone could explain these solutions. I know the 4 kinematics equations but am really unsure if I need to know other things...like the rate at which objects will always fall or rise? Is it always 9.8m/s? Thank you, this is due tomorrow at 8 am. I am new and think this is great. There are 8 simple questions. I really appreciate any help that might be offered.

8. A tennis ball is dropped from 1.36 meters above the ground and rebounds to a height of 0.969 meters.
a. With what velocity does it hit the ground? Hint: Velocity is a vector.
WebAssign will check your answer for the correct number of significant figures. m/s
b. With what velocity does it leave the ground?
WebAssign will check your answer for the correct number of significant figures. m/s
c. If the ball is in contact with the ground for 0.0136 seconds, find the ball's acceleration while touching the ground.
WebAssign will check your answer for the correct number of significant figures. m/s/s

7. During a baseball game, a batter hits a high pop-up. If the ball remains in the air for 6.54 seconds, how high does it rise? Hint: Calculate the height using the second half of the trajectory.

6. A pitcher throws a fastball at a speed of 34.2 meters per second. The acceleration occurs as the pitcher holds the ball in his hand and moves it through an almost straight-line distance of 3.66 meters. Calculate the acceleration, assuming it is uniform. answer is in m/s squared

5. An engineer designs a runway to accommodate airplanes that must reach ground speed of 77.0 meters per second before they can take off. These planes are capable of accelerating uniformly at 2.89 meters per second per second.
a. How long will it take the planes to reach takeoff speed?
WebAssign will check your answer for the correct number of significant figures. s
b. What must be the minimum length of the runway?
WebAssign will check your answer for the correct number of significant figures. m

4. An astronaut drops a feather from 2.21 meters above the surface of the moon. If the acceleration due to gravity is 1.63 meters per second per second down, how long does it take the feather to hit the surface?

3. How far does a plane fly in 24.1 seconds if its velocity is changing uniformly from +160 meters per second to +68 meters per second?

2. Rocket-powered sleds are used to test the responses of humans to acceleration. Starting from rest, a sled can reach a speed of 409 meters per second in 1.99 seconds and can stop again in another 2.32 seconds.
a. Calculate the acceleration of the sled when starting.
WebAssign will check your answer for the correct number of significant figures. wrong check mark m/s/s
b. Calculate the acceleration of the sled when braking.
WebAssign will check your answer for the correct number of significant figures. wrong check mark m/s/s

So far I have attempted a few of these without much success. Thank you again.

2. Sep 5, 2006

### Andrew Mason

All objects at the same place on the earth fall at the same acceleration. That acceleration may vary slightly depending on where you are on the surface of the earth, but 9.8 m/sec^2 is a pretty good approximation of the acceleration. As you move away from the centre of the earth, gravity of course falls off as 1/r^2.

Why not show us what you have done so we can see where your problems are.

AM

3. Sep 5, 2006

### Chutzpah

Well, for number 2 I divided 409 meters per second in 1.99 and 409 by 2.32. In theory, I would think this would give me a negative answer for part 5 since we are braking. According to my internet assignment, these answers are not right. I got a-205.5276382 b--176.2931034 Must I do some dimensional analysis since the answer is in meters per second squared?

In number 3 I used the equation d=vi+vf/2*t, again i am unsure of if and when i need dimensional analysis because for this problem i got 2747m/s2 which is wrong according to the internet checker. Like I said, this is due 8 am tomorrow so any answers would be appreciated. Basically I did d=(160+68)/2*24.1s...right?

I am unsure of when i can use dimensional analysis as well. More to come, but would appreciate any feedback.

At number 4 I used the same equation as above to get 2.712 for time. This is also wrong I guess. My knowns are velocity initial is 1.63 and final is 0(IS this correct?) acceleration is 1.63m/s and distance or displacement is 2.21m. Now I only need to find T. I plug in the equation and get 2.712 rounded which according to mr computer is not right. There has to be something universal i am justn ot seeing or doing wrong.

For number 5 I sued the equation vfsquared=vinitialsquared+2ad. Plugged this in to get 5929+0=5.78d which gave me 1025.779 which would gave me the runway length which according to mr computer once again is wrong. To find time I divided 77 by 2.89 to get 26.644 which is the number of seconds it takes to get to top speed. Why are these wrong?

Last edited: Sep 5, 2006
4. Sep 6, 2006

### Andrew Mason

What about significant figures? The answers should be 206 and -176 m/sec^2.

t is in the numerator:

$$d = v_{avg}\Delta t = \frac{v_i + v_f}{2}\Delta t$$

Make sure you get the sig. figs. right.

AM

5. Sep 8, 2006

### Raiza Camlian

Hello everyone!
I am hoping that you could help me with this one:
A shaft 15 cm in diameter is to b turned on a lathe which has a linear speed of 180m/min, wat is its angular speed? Would I be correct If I use w=v/r to compute for its angular speed? Plsssssss help. I need this badly....