# Acceleration Word Problem

1. Jul 11, 2006

### JassC

A hockey puck sliding on a frozen lake comes to rest after traveling 175m.
If it's initial velocity is 1.0m/s, what is its acceleration if that acceleration is assumed constatnt?

I first divided 175/1.9 to get 92.11s. I then put 1.9/92.11 to get 0.02063. But it's wrong.

Any ideas?

Thanks.

2. Jul 11, 2006

### d_leet

Where did 1.9 come from for the first thing. For the second this object is accelerating so what equations do you know for accelerated motion that take account of the information you have?
-The distance it travels
-It's initial velocity
-It's final velocity

3. Jul 11, 2006

### Staff: Mentor

Not sure what you're doing here. What's the 1.9 signify?

Start by writing the kinematic relationship between velocity, acceleration, and distance.

4. Jul 11, 2006

### JassC

Sorry....the initial velocity is suppose to be 1.9m/s....my bad.

5. Jul 11, 2006

### Staff: Mentor

In that case, realize that the speed is not constant. To find the time as you attempted, you must use the average speed. (What's the average speed?)

6. Jul 11, 2006

### JassC

1.9/2 = 0.95 (Average speed)

175/.95 = 184.2105s (Time took for puck to stop)

I'm stuck here now.

7. Jul 11, 2006

### Staff: Mentor

Now apply the definition of acceleration.

8. Jul 11, 2006

### JassC

0.95/184.2105 = 0.005157

1.9 + (0.005157)(184.2105) = 2.85 m/s^2

Hopefully I've gotten it right.

9. Jul 11, 2006

### Staff: Mentor

Again, I'm not sure what you are doing with these equations. What's the definition of acceleration? That's all you need.

10. Jul 11, 2006

### JassC

Umm the definition? The rate of change? or do you mean speed over time......

11. Jul 11, 2006

### Kurdt

Staff Emeritus
the velocity was 1.9m/s and it changed to 0m/s in a time you calculated. the correct definition is the rate of change of velocity with time.

Does this help?

12. Jul 11, 2006

### Staff: Mentor

$$a = \Delta v / \Delta t$$

13. Jul 11, 2006

### JassC

Is the answer 0.005157? I'm so confused right now.

14. Jul 11, 2006

### Staff: Mentor

Just apply the definition. What's the change in velocity? (You already calculated the time.)

15. Jul 11, 2006

### JassC

I did use the definition.....speed over time and which is 0.95/184.2105 = 0.005157

I have no clue how to find the change in velocity....

Sorry, I'm new to phyiscs. Just started learning....

16. Jul 11, 2006

### Staff: Mentor

The definition is change in speed over time, not just speed over time. To find the change in anything, take the final value and subtract the initial value. What's the final speed? What's the initial speed?

Don't confuse the average speed, which you had to use to find the time, with the initial or final speed, which is what you need to find the acceleration.

17. Jul 11, 2006

### JassC

I'm so lost....I don't know how to find the final value........

My best guess is 0.95 final speed(since thats what I got from 175/184.2105) - 1.9 initial speed.....which gives you -0.95.

18. Jul 11, 2006

### Staff: Mentor

Sure you do, if you read the problem carefully. I'll quote: "A hockey puck sliding on a frozen lake comes to rest..."

No guessing!

19. Jul 11, 2006

### JassC

well then its 0......

Last edited: Jul 11, 2006
20. Jul 11, 2006

### Staff: Mentor

Of course. The final velocity is zero. So what's the change in velocity?