# Super easy driving question - instantaneous acceleration

1. Sep 25, 2011

### physicsgurl12

super easy driving question -- instantaneous acceleration

1. The problem statement, all variables and given/known data
a Person in a car is driving down a straight road. The instantaneous acceleration is decreasing with time but is directed in the same direction as the cars motion the speed of the car is constant or increasing with time. explain

3. The attempt at a solution
i know its not decreasing with time.

Last edited: Sep 25, 2011
2. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Hi again physicsgurl12!

For this you need to understand a little bit about acceleration and speed.

The definition of acceleration is: the change in speed per unit of time.
(Similarly the definition of speed is: the change in position per unit of time.)

So if acceleration is positive, this means that the change in speed is also positive, meaning that speed increases.

Can you tell whether the acceleration is positive or negative?

3. Sep 25, 2011

### CheckMate

Re: super easy driving questioninstantaneous acceleration

If the direction of acceleration is directed in the same direction as the car's velocity than it must be positive.

If the car's acceleration is positive (greater than zero) than the velocity must be increasing per unit time, regardless if the acceleration is decreasing in terms of magnitude.

Example : Velocity starts at 10 m/s forward at t=1
Acceleration is 10 m/s^2 (same direction as velocity) at t=1s forward but decreases to 2 m/s^2 at t=2s. Speed at t=2s (new speed) will be 14 m/s instead of being 20 m/s if the acceleration was constant.

The car is still going forward and it's speed is increasing because you still have an acceleration in the same direction.

4. Sep 25, 2011

### physicsgurl12

Re: super easy driving questioninstantaneous acceleration

negative. because its decreasing?

5. Sep 25, 2011

### CheckMate

Re: super easy driving questioninstantaneous acceleration

No decreasing means going from a big value to a lesser value. Going from 10 to 3 is decreasing.

Negative means less than zero. A negative acceleration would be an acceleration against the car.

If you drive a car into a building the car will stop and go backwards because the building is doing a negative acceleration relative to the car's initial trajectory.

6. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Consider the speed of a skateboard.
Let's assume that the speed of the skateboard is decreasing.
Does that mean the speed is negative, or in other words that your skateboard is moving backward?

7. Sep 25, 2011

### physicsgurl12

Re: super easy driving questioninstantaneous acceleration

moving backwards? because its distance and if the speed decreases it doesnt mean it has to go behind the orgin.

8. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Uhh, right.
So when you're moving forward with a decreasing speed, you're still going forward, meaning you have a positive speed.
That is, until the speed becomes zero, after which you would be going backward.

Now back to the problem.
Does the problem statement say in which direction the acceleration is (even though it is decreasing)?

9. Sep 25, 2011

### physicsgurl12

Re: super easy driving questioninstantaneous acceleration

nope it just its decraseing and opposite of the car.

10. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Hmm, I read: "The instantaneous acceleration (is decreasing with time but) is directed in the same direction as the cars motion".

11. Sep 25, 2011

### physicsgurl12

Re: super easy driving questioninstantaneous acceleration

oh. just kidding. thats what i get for trying to do two at once

12. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Sooooooo?

13. Sep 25, 2011

### physicsgurl12

Re: super easy driving questioninstantaneous acceleration

so then its moving the same direction as the car

14. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Uhh?

What is moving in the same direction as the car?

15. Sep 25, 2011

### physicsgurl12

Re: super easy driving questioninstantaneous acceleration

the acceleration. isnt that what we were talking about?

16. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Ahhh, well, "acceleration" isn't "moving"...

So the acceleration is in the same direction as the car.
What does that tell you about the speed of the car?

17. Sep 25, 2011

### physicsgurl12

Re: super easy driving questioninstantaneous acceleration

that its accelerating

18. Sep 25, 2011

### I like Serena

Re: super easy driving questioninstantaneous acceleration

Good! :shy:

19. Sep 25, 2011

### physicsgurl12

Re: super easy driving question -- instantaneous acceleration

um i feel like its constant because its decreasing acceleration even though its going the same direction as the car

20. Sep 25, 2011

### I like Serena

Re: super easy driving question -- instantaneous acceleration

Uhh... exactly what is constant?