# Determining the rate at which speed changes

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1. Jan 10, 2016

### x2017

1. The problem statement, all variables and given/known data
A golfer sinks a hole-in-one on a mini-putt golf course. The ball leaves the putter face with a speed of 4.04 m/s, travels a distance of L1 = 4.38 m, and bumps into a bank with a speed of 3.34 m/s, rebounds at 90° and travels a length of L2 = 3.26 m into the hole.

Assuming a constant acceleration of the golf ball as it traveled over the distance L1, what was the rate at which the speed of the golf ball changed over the distance L1?

2. Relevant equations

3. The attempt at a solution
(3.34-4.04)/4.38=-0.1598 (This is wrong)

2. Jan 10, 2016

### WrongMan

Why did you divide by L1?
Have you tried thinking about relevant equations that might help you solve the problem?
for instance what are the basic motion equations?
What do you understand by "rate at which speed changes"?

3. Jan 10, 2016

### SteamKing

Staff Emeritus
And you would know that it's wrong if you had looked at the units of this calculation.

What is m/s divided by meters? Hint: it's not m/s2.

You should review the SUVAT equations. There is one which applies directly to this problem.

http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/MEI/MechSuvatEquns.html

4. Jan 10, 2016

### haruspex

Dividing a speed by a distance is not going to yield an acceleration.
Do you know what I mean by SUVAT equations? Can you find one that involves the three things you know (initial speed, final speed, distance) and the unknown you want to find (acceleration)?

5. Jan 10, 2016

### x2017

I divided by L1 because I was trying to make a rate. I'm not sure about equations, my professor only gave us equations that involve time and we don't have that variable in this equation that's why I'm so confused! "Rate at which speed changes" meant acceleration to me, which it says is constant. The equations I have to solve for acceleration all involve time...

6. Jan 10, 2016

### x2017

Thank you for linking me to the SUVAT equations. I tried v2 = u2 + 2 a s & got -0.58, which the computer told me was wrong also...

7. Jan 10, 2016

### x2017

I have just been introduced to the SUVAT equations today through this post!
I tried using v2 = u2 + 2 a s and got -0.58, which my online assignment told me was wrong as well. I'm not sure where I am going wrong...

8. Jan 10, 2016

### WrongMan

are you rounding the number properly?

9. Jan 10, 2016

### WrongMan

Oh and fyi you could have figured the result with the formulas your teacher gave you.
For example, using the displacement formula (since its given the acceleration is constant), you substitute "a" for "(vf-vi)/t" gettign rid of the squared t, then you find t, and then solve the velocity equation for a

10. Jan 10, 2016

### x2017

No, that was a silly mistake! -0.59 was the answer. Thank you for your help!

11. Jan 10, 2016

### x2017

OHHHHHH, awesome thank you! This stuff is review for my third year biomechanics university class & I remember almost nothing from my grade 12 physics. I knew acceleration being constant meant something, I just couldn't remember how it affected the problem. Looks like I should definitely spend some time online reading over some physics intro... Thanks again!