# Acceleration and speed .

1. Oct 20, 2009

### nina09

Hi , I'm new to to the forum . I have a very hard time with physic or maybe is undestanding the professor . I currently trying to solve this acceleration problems but I end up not understanding that I need to do next or how to solve them. If any you can help me I will really apreaciate it.

1)The tires of a car begin to lose their grip on the road at an acceleration of 5 m/s2. At this accelaration , how long does the car need to reach speed of 25m/s starting from10 m/s?

A=5 m/s2
V1= 10 m/s
V2=25 m/s

T= 10m/s
------ = 2s
5m/s2

2) A car start from rest and reaches a speed of 40m/s in 10s. If its aceleration remain the same how fast will it be moving 5s later?

V1=0
V2=40m/s
T=10s

A= 40m/s -0 40 ms
------------ =--------
10s 10s
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 20, 2009

### tiny-tim

Welcome to PF!

Hi nina09! Welcome to PF!
No.

Acceleration is rate of change of velocity …

so how much is the velocity changing? and how long does it take to change?
Yes … keep going!

3. Oct 20, 2009

### Delphi51

Hi Nina and welcome to PF!
It is very natural to have trouble with any new skill at first; you just have to work hard at it and it may turn out that you have a lot of talent.
is a good start. Look in your list of formulas for one that relates the velocities, acceleration and time. The one I'm thinking of is
a = Δv/Δt. The Δ on the t doesn't make any difference since we start at time zero, but the Δ on the v does because you start at v=10. So you get Δt = Δv/a = (25-10)/5

In the second problem, it is difficult to follow your calc because this forum removes spaces. I think you wrote
a = Δv/Δt = 40/10 = 4 m/s^2.
This is correct and knowing the acceleration you should be able to use the same formula again over 15 seconds to find the velocity.

4. Oct 20, 2009

### nina09

I wrote each step under the othe so is easier to see the results

This is my solution to the second one , Did I did it correctly?
1) 40m/s -0
-------------=
15s

2)40m/s
-------- =
15s

3) 2.6m/s^2
------------
15 s

I'm still having dificulty's with the first one since I can't find the formula.

5. Oct 20, 2009

### Delphi51

For the second one, it should be
a = Δv/Δt = 40/10 = 4 m/s^2
The car goes from 0 to 40 m/s in 10 seconds, not 15.
In a second step you use the "same acceleration" of 4 with a time of 15 to get the speed after 15 seconds.

In the first one, the formula is a = Δv/Δt. Your only mistake was to say Δv = 10 when it was actually 25-10.

6. Oct 20, 2009

### nina09

This my second try at the first one, is it wrong?

25m/s-10ms
5m/s^2= -------------=
T

15m/s x 5m/s^2
------------ =
T

=0.6m/s^2

7. Oct 20, 2009

### Delphi51

First one should be Δt = Δv/a = (25-10)/5 = 3 m/s^2.

8. Oct 20, 2009

### nina09

This is what I understood from the explanation for the second one:

40m/s- 0
---------=
10s

40m/s
------- = 4m/s^2
10s

2 step

4m/s^2
--------
15s

=1m/s^2

9. Oct 20, 2009

### nina09

First one

25m/s-10m/s
-------------=
5m/s^2

25m/s-10m/s
-------------= 3m/s^2
5m/s^2

10. Oct 20, 2009

### Delphi51

First one looks good!
Second one, second step should be
a = Δv/Δt
a*Δt = Δv after multiplying both sides by Δt
Δv = a*Δt = . . .

11. Oct 20, 2009

### nina09

Does this * means multiplication?

12. Oct 20, 2009

### Delphi51

Yes, * means multiply. If you don't use some *s, things can get unreadable.

13. Oct 20, 2009

### nina09

This the second one, I know I'm doing something wrong but I just can't pinpoint where exactly I'm making the mistake.

40m/s- 0
---------=
10s

40m/s
------- = 4m/s^2
10s

Second step

The acceleration is 4m/s^2 and the time is 15

A*t
-----=
V

4m/s^2 *15s = 60m/s
------------
V

60m/s
------- =4
15s

???

14. Oct 20, 2009

### Delphi51

I said a = Δv/Δt
Use a=4, Δt = 15 and this is
4 = Δv/15
Multiply both sides by 15 to get the Δv by itself:
15*4 = Δv
60 = Δv

Will try writing this in your notation. Have to use dots to make it line up.
......Δv
4 = ----
......15

............Δv * 15
15*4 = ----
............15

............Δv
15*4 = ----
............1
60 = Δv

15. Oct 20, 2009

### nina09

I think I got it , thank you very much.

40m/s- 0
---------=
--10s

40m/s
------- = 4m/s^2
--10s
------V
4 = ----
------15

-------Vx15
15*4 = ----
-------15

----------V
15*4 = ----
--------- 1

60 = V

16. Oct 20, 2009

### Delphi51

I love that trick with the white dashes! Thank you!
Another good trick is, if you need a symbol like Δ or θ, you just go to
https://www.physicsforums.com/blog.php?b=347 [Broken]
and copy it.

Last edited by a moderator: May 4, 2017
17. Oct 21, 2009

Thank you .