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Maths A level projectile question help!

I'm stuck on part 1. of the question. I looked at the mark scheme and it says the initial velocity is P. I am a bit confused as to why it is not Pcosα. Please could someone help me understand this question.

(edited 4 years ago)

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Reply 1

Pangol

Original post by Anonymous_4657

If the initial velocity were v in the direction that the projectile is moving, then yes, the initial horizontal velocity would be v cos(α). But the initial velocity is pi + qj, which means that the horizontal component of the velocity (the bit in the i direction) is p and the vertical component of the velocity (the bit in the j direction( is q.

Reply 2

ApexCoder

Since its a vector, you don't need to apply sin or cos when resolving the forces

Reply 3

username4506446

Original post by ApexCoder

Since its a vector, you don't need to apply sin or cos when resolving the forces

Thanks. How come? it it just a rule that I have got to learn?

Reply 4

ApexCoder

Original post by Anonymous_4657

Thanks. How come? it it just a rule that I have got to learn?

Answer this question for me, why do you use sin or cos in the first place

Reply 5

username4506446

Original post by ApexCoder

Answer this question for me, why do you use sin or cos in the first place

Don’t you draw a triangle and resolve forces? Maybe I’m wrong

Reply 6

ApexCoder

Original post by Anonymous_4657

Don’t you draw a triangle and resolve forces? Maybe I’m wrong

Basically, you use either sin or cos, to find the vertical or horizontal magnitude of a force

Reply 7

simon0

You are resolving vertically and you are already given a vertical magnitude of the velocity (or the vertical component of the velocity) by the j-component of the velocity vector.

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Sometimes for a vector (a force, a velocity, impulse etc.), the direction of it given to you may not be acting along the direction you are resolving in.

This is why when finding a vector's effect (in the direction you are resolving in), you have to use sin or cos.