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# AS Mechanics - Projectile Motion Watch

1. Hey guys, so I'm a little stuck on a question from a past paper I'm doing as revision. I've attached a copy of the question, so hopefully you can get it alright. x

Basically I've done part one with no trouble. And came out with the correct 5u/g.
For the second part, I got the displacement as '30u^2/g' which is correct according to the mark scheme - so I'll skip over this part as well.

It's the next section I'm having trouble with. "Find V, in terms of u." Sounds simple right? And I thought it would be, my working is as follows;

Vertical Motion

V = u + at
V = 2.5u + g(5u/g)
V = 2.5u + 5ug/g (here the 'g' can cancel out)
V = 2.5u + 5u
V = 7.5u or 7 1/2u

That is what I got, but the answer in the mark scheme is actually 6.5u, not 7.5u. So can anyone explain to me why? What it is I have done wrong?

The working out the mark scheme has is;

Speed^2 = (6u)^2 + (2.5u)^2
Speed = 6.5u

But I can't work out what the formula they have used is?

Thanks for reading, any help would be much appreciated.
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2. (Original post by Pixie-Bee)
Hey guys, so I'm a little stuck on a question from a past paper I'm doing as revision. I've attached a copy of the question, so hopefully you can get it alright. x

Basically I've done part one with no trouble. And came out with the correct 5u/g.
For the second part, I got the displacement as '30u^2/g' which is correct according to the mark scheme - so I'll skip over this part as well.

It's the next section I'm having trouble with. "Find V, in terms of u." Sounds simple right? And I thought it would be, my working is as follows;

Vertical Motion

V = u + at
V = 2.5u + g(5u/g)
V = 2.5u + 5ug/g (here the 'g' can cancel out)
V = 2.5u + 5u
V = 7.5u or 7 1/2u

That is what I got, but the answer in the mark scheme is actually 6.5u, not 7.5u. So can anyone explain to me why? What it is I have done wrong?

The working out the mark scheme has is;

Speed^2 = (6u)^2 + (2.5u)^2
Speed = 6.5u

But I can't work out what the formula they have used is?

Thanks for reading, any help would be much appreciated.

3. I never thought of Trig Identities. xD But I don't understand how that shows the Final Velocity? o.O
4. (Original post by Pixie-Bee)
I never thought of Trig Identities. xD But I don't understand how that shows the Final Velocity? o.O
They give you 2 equations for V in the question - just square them and add them together

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