# By how much does the speed change each second?

1. Jan 30, 2010

### kremit

1. The problem statement, all variables and given/known data
A vehicle moves in a straight line with an
acceleration of 3 km/h2.
By how much does the speed change each
second?

2. Relevant equations
Not sure

3. The attempt at a solution
sqrt 3 to get a base answer then divide by 3600
I have also tried doing the problem directly. 3 divided by 36002 . Gives me a large number im unsure is correct.

But when i don't get it right i start guessing.

Last edited by a moderator: Mar 7, 2013
2. Jan 30, 2010

### tiny-tim

Welcome to PF!

Hi kremit! Welcome to PF!

(try using the X2 tag just above the Reply box )

What equations do you know relating speed and acceleration?

3. Jan 30, 2010

### kremit

Re: homework help

i have a change in speed over a change in time to equal velocity. I also have an average and instantaneous speed too. I also fixed the ^2 to the correct super script. I think that is what that is called. lol

It's strange though. There is no other figures, that could be why im stuck.

Last edited: Jan 30, 2010
4. Jan 31, 2010

### tiny-tim

Hi kremit!

(just got up :zzz: …)
No, change in distance over change in time equals velocity (∆x/∆t = v).

ok, that doesn't help here, so …

do you have a similar equation that includes acceleration?

Yeah! Super!

5. Feb 4, 2010

### kremit

Re: homework help

Why must this be so hard for me. I don't understand that.

I looked in my book,on the web, and not making sense of what a "resultant displacement" is.

The problem to this is simple.

Guy passes a 241km marker, passes 172km marker, and hits a 148km marker. he then backtracks to the 172km marker. What is the resultant displacement

6. Feb 5, 2010

### tiny-tim

Hi kremit!

(are you ok on the previous question?)

Displacement means total change in position.

For example, if you go half-way round a circle of radius r, then you have gone a distance (or arc-length) of πr, but your displacement is only 2r.

And if you go all the way round a circle of radius r, then you have gone a distance (or arc-length) of 2πr, but your displacement is zero!

(and "resultant" always simply means "overall", you take everything into account, and then calculate the displacement )

So what is the overall displacement in this case?​

7. Feb 6, 2010

### kremit

Re: homework help

A change in total position. Wonder why they couldn't say that?

Guy passes a 241km marker, passes 172km marker, and hits a 148km marker. he then backtracks to the 172km marker

He went the entire distance of

93km from 241km -> 148km
69km from 241km -> 172km
24km from 172km -> 148km

93km - 24km ?
93km + 24km?

I doubt it's that simple, but he traveled the whole distance and came back.

I like how our lab or lecture has nothing to do with our homework. Then must rely on others or tutors to teach me equations. :uhh:

8. Feb 6, 2010

### tiny-tim

Hi kremit!

Where did he start from? The 241 km marker?

If so, just follow the definition … total displacement is |241 - 172|.