# Model airplane in a circle

1. Dec 19, 2007

### aham925925

1. The problem statement, all variables and given/known data
A model airplane is controlled to fly in a horizontal circle of radius 24 metres with a constant speed of 20ms-1. An observer beneath the plane notices that the plane is flying anticlockwise. Find the magnitude of the change in velocity of the plane after:
a- a full revolution
b- a quarter of a revolution
c- a half revolution

2. Relevant equations
I have no idea on any equations. We haven't started the study of circular motion yet.

3. The attempt at a solution
I really don't know where to start here. I guessed that the change in velocity after a full revolution would be 0 and that was correct (displacement =0) but im not sure on the other ones (B and C). They seem like random answers. Any ideas??

2. Dec 19, 2007

### Shooting Star

After half a rev, in which direction do you thing the plane is moving?

3. Dec 19, 2007

### aham925925

Well, the plane would be flying downwards..

4. Dec 19, 2007

### Shooting Star

No, no, the plane is flying in a horizontal circle.

5. Dec 19, 2007

### aham925925

ahh yes...see i'm not really sure what that means....which is probably part of the reason why I don't know where to start...

6. Dec 19, 2007

### Shooting Star

Can you visualize a model car going round in circles on the floor? Well, the floor is a horizontal plane. Just visualize the whole thing taking place up in the air.

7. Dec 19, 2007

### aham925925

hmm i see what you're getting at but i still don't know the direction that the airplane will be moving...

I thought all circles were the same...

Last edited: Dec 19, 2007
8. Dec 19, 2007

### Shooting Star

Tell me in which direction the model car would be moving on the floor after it has completed half a revolution, that is, when it is at point diametrically opposite to its starting point?

9. Dec 19, 2007

### aham925925

Sorry but would it be moving in the same direction?

10. Dec 19, 2007

### Shooting Star

No. Take some object at hand, pretend it's a car, and identify the front and the back -- that is to say, it'll move to the front. Then just roll or slide it along the floor in a circle like a toy car. Now see how it's moving when you have completed half the circle.

11. Dec 19, 2007

### aham925925

It would always be moving forward...

I'm sorry...like i said...I haven't done circular motion yet and we have just started holidays here in Australia so i can't ask my teacher...

12. Dec 19, 2007

### Shooting Star

Isn't it going in exactly the opposite direction in which it started? I mean, if it was going, say, north when it started, isn't it going exactly south now?

13. Dec 19, 2007

### aham925925

ohh yes of course.... ha ha thank you for that

how would you find the change in velocity now that we finally have that figured out??

14. Dec 19, 2007

### Shooting Star

Tell me first, if the initial velocity was 20 m/s towards north, what is the velocity after half the revolution?

Give speed and direction.

15. Dec 19, 2007

### aham925925

Well....it's not -20ms-1 because the answer at the back of the book says that it's 28ms-1 (at 135 degrees anticlockwise to original motion direction)

In order to get the answer, would you have to go about resolving vectors or anything?

16. Dec 19, 2007

### Shooting Star

(It's not supposed to be -20 m/s, but -40 m/s.)

1. Have you checked the answer to the correct question?

2. Was there any other information in the problem which you have not written down? Like how much high the plane was?

17. Dec 19, 2007

### rl.bhat

the answer at the back of the book says that it's 28ms-1 (at 135 degrees anticlockwise to original motion direction
This answer is for ( b) i.e for quarter revolution when the initial velocity and final velocity are perpendicular. To find the change in the velocity, vf - vi, Change the direction of the initial velocity vi and find the resultant with vf

18. Dec 19, 2007

### aham925925

Yes sorry I was looking at the wrong question

So if it is travelling at 20ms-1, what would it be travelling at a quarter revolution (then you cold work out change in velocity)

So the change in velocity at half a revolution would be final velocity(-20) minus the initial velocity(20) equals -40 ms-1? Thats what the book says..

Still having trouble with the quarter revolution...

19. Dec 19, 2007

### Shooting Star

Have you done cartesian vectors i, j, k?

20. Dec 19, 2007

### aham925925

cartesian vectors?

nope, don't think i've heard of those before.