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MS_SUMTER2010 · Feb 26, 2010, 05:00 PM

A jogger on a riverside path sees a rowing team coming toward him. If the jogger is moving at 10 km/h, and the boat is moving at 20 km/h, how quickly does the jogger approach the boat?

A. 3 m/s
b. 8 m/s--- this is suppose to be the answer.
C. 40 m/s
d. 100 m/s

Unknown008 · Feb 27, 2010, 08:46 AM

It's a problem about relative speed.

Let's say the boat is moving towards the right, and the jogger towards the left.

The boat moves to the right at a speed of 20 km/h.
The jogger moves to the left at a speed of 10 km/h.

Does the jogger meet the boat faster than if he stayed still, or not? You'll guess that yes, since he is moving towards the boat, he'll be meeting it faster.

That means, the speed of the jogger relative to the boat is 20+10 = 30 km/h. Which is: when someone from the boat sees the jogger, he appears to come at 30 km/h and to the jogger, the boat appears to come at 30 km/h.

So that is the speed the jogger approaches the boat.

Now convert 30 km/h into m/s.

MS_SUMTER2010 · Feb 27, 2010, 08:53 AM

OK I will try it and see if it gets that

MS_SUMTER2010 · Feb 27, 2010, 10:03 AM

A colorfully feathered lead cannonball is shot horizontally out of a circus cannon 25 m/s from the high-wire platform on one side of a circus ring. If the high-wire platform is 52 m above the 80-m diameter ring, will the performers need to adjust the cannon (will the ball land inside the ring, or past it)?

Unknown008 · Feb 27, 2010, 11:28 AM

How about your past question?

For this one, you need to find the time it takes to fall 52 m.

$s = ut + \frac12 at^2$

$52 = 4.905t^2$

Solve for t.

Then, use that time to know what distance the ball covers, using

$s = ut$

$s = (25)t$

If s is greater than 80 m, then the canon has to be adjusted.

MS_SUMTER2010 · Feb 27, 2010, 11:33 AM

It was I got 82 m for that so it has to be adjust 2 m.

Unknown008 · Feb 27, 2010, 11:45 AM

I got 81.4 m. Anyway the canon needs to be adjusted.