# Projectiles problem, help needed

1. Feb 11, 2015

### Aaron Curran

Here is the problem (Q3 (A));

Here's what I have so far;

Any help would be appreciated, thank you!

2. Feb 11, 2015

### RUber

It appears you made an algebra error when you went from $21 \cos \alpha = \frac {30}{ t}$ to $\cos \alpha = \frac {630}{ t}$.
Also, I would first find your time of impact based on your horizontal component.
With that time of impact, you can make an expression that is in terms of alpha only.

3. Feb 11, 2015

### Aaron Curran

Yes I realised this after I posted, I'll try that, thanks!

4. Feb 11, 2015

### RUber

I made a substition of x = cos^2(alpha) and manipulated it into a quadratic equation to get the two solutions.
part b comes right out of part a if you have already solved for time of impact.

5. Feb 11, 2015

### TSny

Aaron,

Something else to note: If you have the expression $\tan \alpha = \frac{A}{B}$, then you cannot conclude that $\sin \alpha = A$ and $\cos \alpha = B$.

6. Feb 11, 2015

### Aaron Curran

Could you elaborate on this a bit? Not quite sure what you mean..

7. Feb 11, 2015

### RUber

Do you have an expression that can be written as some trig functions of alpha = 0?
Use sin = sqrt(1-cos^2) to convert everything in terms of cos.