# When will the sled attain this speed?

1. Feb 21, 2013

### Sneakatone

a bobsled slides down an icy track making of 29 degrees with the horizontal. Assume that the motion is frictionless.
a) how far must the bobsled slide in order to attain a speed of 91 km/h if initially at rest?
I converted km/h to 25.27 m/s and I used d = (v^2)/(2gsin(theta)) where d= 67.1 m, which is correct.

b)when will the speed be attained?
I tried d/v -> 67.1/25.27 and had 2.65 s and 25/67.1
But the answer is wrong. So I am stuck on part b.

2. Feb 21, 2013

### PeterO

d/v presumably means you want to use distance / time to calculate here. The speed you should be using then is the average speed. You have used the final speed.

3. Feb 21, 2013

### Sneakatone

how would I find average speed based on the data?

4. Feb 21, 2013

### PeterO

For motion under constant acceleration, it is fortunate that the average velocity is simply the average of the initial and final velocities.
Although the path a bobsled takes if far from straight, it is not too wrong to consider that path as straight, and so the direction of velocity is always positive (or negative - depending which way you defined positive) so the average speed is the average of the initial and final speeds.

5. Feb 21, 2013

### Sneakatone

so would initial speed equal zero so the average is just 25.27 m/s /2=12.63 m/s ?

6. Feb 21, 2013

### PeterO

You could try that and see if you get the right answer. Be bold and try it!

7. Feb 21, 2013

### Sneakatone

yes! I did 67/12.63 and had around 5 seconds. I didnt try at first because I did not want to run out of chances on the online HW. Thanks a lot!