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# Projectiles question watch

1. Hi I have a problem with a projectiles question, A ball is a projected from the point 0. The horizontal and vertical components of the initial velocity are 5ms^-1 and 4ms^-1 respectively.

When the particle is at the point X, its velocity is at right angles to its initial velocity. Find the angle which the straight line joining 0 to X makes with the horizontal.

But if the horizontal velocity never changes how does it turn from 5ms^-1 to 4ms^-1?

Help appreciated.
2. (Original post by swifty)
Hi I have a problem with a projectiles question, A ball is a projected from the point 0. The horizontal and vertical components of the initial velocity are 5ms^-1 and 4ms^-1 respectively.

When the particle is at the point X, its velocity is at right angles to its initial velocity. Find the angle which the straight line joining 0 to X makes with the horizontal.

But if the horizontal velocity never changes how does it turn from 5ms^-1 to 4ms^-1?

Help appreciated.
the initial direction is arctan(4/5). presuming air resistance is being neglected then the horizontal comp. will remain 5m/s.
quick sketch and work from there.
3. But if the initial gradient of the path of the projectile is 4/5, then the perpendicular to this is 5/4 because ( [5/4] x [4/5] ) = -1

So the horizontal component changes from 4 m/s to 5 m/s, right?? But it can't because we are using a model which excludes the effects of air resistance, so the horizontal velocity must remain constant.

I get what you're saying Mrm but why doesn't the above apply in this case?
4. Mrm is quite right. the horizontal velocity is fixed permanently at 5 m/s. the vertical changes. it starts at 4m/s .

although the ratio changes from 4/5 to -5/4 that does not mean that the new components are -5 and 4.

when you have a ratio it can be expressed using different numbers : 3/8 is the same as 6/16 for instance.

so you can have a ratio of -5/4 with the underneath number being 5...

?/5 = -5/4 so the ? must be -25/4

i hope this makes it clearer for you
5. Any ideas?
6. Yeah that makes it a lot clearer thanks!

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