# Homework Help: Bunch of questions

1. Sep 22, 2004

### Kdawg

I have 5 problems on my homework that I can get right. Probably because of somthing stupid but I have to get these done soon. I dont need the answers, just some hints or tips. I have included my work or a description on all of them.

1.) A race car traveling at +44 m/s is uniformly accelerated to a velocity of +24 m/s over an 12-s interval. What is its displacement during this time?
Vi = 44m/s
Vf = 24m/s
t = 12
d = ?

a = Vf-Vi
----
t
a = -1.6
d = 1/2at^2
d = -115.2
with correct sig figs
120

2.) A car accelerates at a constant rate from 16 m/s to 23 m/s while it travels 130 m. How long does this motion take?
Vi = 16m/s
Vf = 23m/s
d = 130m
t = ?

Dont know what formula/s to use for this one.

3.) An airplane accelerates from a velocity of 22 m/s at the constant rate of 2.8 m/s2 over +504 m. What is its final velocity?
Vi = 22m/s
a = 2.8m/s^2
d = 504m
Vf = ?

t = sqr((2*504)/2.8)
t = 19
Vf = Vi + at
Vf = 75.2
With correct sig figs 75

4.) A person wearing a shoulder harness can survive a car crash if the acceleration is smaller than -300 m/s2. Assuming constant acceleration, how far must the front end of the car collapse if its crashes while going 93 km/h?
Vi = 93km/h
Vf = 0km/h
d = ?

What am I missing in this one?

5.) A woman falls 1.6 m to the floor. How fast is she going when she hits the floor?
Vi = 0m/s
a = 9.8m/s^2
d = 1.6m
t = .57s
Vf = ?
Vf = Vi +at
Vf = 5.6

2. Sep 22, 2004

### christinono

for the first one, find the average velocity and use the formula: d=vt

3. Sep 22, 2004

### christinono

for the 2nd question, use the formula: Vfinal squared= V1initial squared + 2ad
(sorry, my subscript and superscript buttons don't work)

4. Sep 22, 2004

### christinono

sorry, screwed up the formula. it is: Vfinal squared = Vinitial squared + 2ad

5. Sep 22, 2004

### Kdawg

How do I get a without time? THanks for the fast responce btw.

6. Sep 22, 2004

### christinono

what question are U talking about?

7. Sep 22, 2004

### Kdawg

The second one, you said to use Vfinal squared = Vinitial squared + 2ad but I dont know how to get a without time. I am going of the formulas I have so there might be one.

8. Sep 22, 2004

### christinono

NOw I see what you mean...
since d=vt, (average velocity, of course), you can just isolate t and solve it.

9. Sep 22, 2004

### Sirus

You can use that equation because it does not require you to know time and works. I don't think you can use the other formulas.

10. Sep 22, 2004

### Sirus

Nevermind.

11. Sep 22, 2004

### christinono

Sirus can help with the rest of the questions :(!!)
Sorry, I'm tired and wanna go to bed... :zzz:

12. Sep 22, 2004

### Kdawg

np, thanks for the help.

13. Sep 22, 2004

### Kdawg

I got 1 and 2, now im looking for 3-5.

14. Sep 22, 2004

### Sirus

#3: $$v_{f}^{2}=v_{i}^{2}+2ad$$
#4: You are missing acceleration, which should be set at -300 meters per sec. squared. Also convert all your values from km/h to m/s.
#5: Use the same formula as #3.

Most of these question use that formula, so make sure you know it for future problems.

15. Sep 22, 2004

### Kdawg

for number 3 im getting 5.6, is that right?

16. Sep 22, 2004

### Kdawg

I got one more I have to throw in
A bus traveling at +29 km/h accelerates at a constant +3.7 m/s2 for 6.3 s. What is its final velocity in km/h?
I was working on it here, but I cant get a correct answer. Either that or webassign is being gay.

17. Sep 23, 2004

### Sirus

Try number three again.

For this latest problem: Do you have a formula that relates velocity, accleration, and time? Rather than trying to convert meters per sec. squared to km/h, convert km/h to m/s.

18. Sep 23, 2004

### christinono

remember that acceleration is equal to the change in velocity (in the same units) over the time interval