A van is travelling north - Vector question.

[shamswahid]

Hi, I have just started A-level physics and bought the Letts AS/A2 study guide from which I am working. However I have gotten stuck right at the beginning! Please can someone figure this out with an explanation? Also any recommendations on good but concise textbooks? I was thinking of buying Physics for Dummies.

1. A van is traveling north at a speed of 28ms. After turning a corner it is heading 40 degrees east of north at 25ms. Work out the change in velocity of the van



Thanks!

 

[tiny-tim]

Velocity is a vector …

1. A van is traveling north at a speed of 28ms. After turning a corner it is heading 40 degrees east of north at 25ms. Work out the change in velocity of the van

Hi shamswahid! Welcome to PF!

The question gives you two velocities (speed-plus-direction), and asks you to find the difference.

Velocity is a vector.

What that means (among other things) is that you can combine them using vector diagrams (usually triangles).

So draw the triangle (roughly), and calculate the unknown side and angle!

 

[tiny-tim]

Hi tiny-tim,

Ok but it has been such a long time since I did maths and physics that I cannot see how to do what you suggested.

Would you be able to provide a working out please if that's ok?

Thanks,
Shams

Hi Shams!

No, you have to do the work!

Draw the triangle: one side will be 28, straight up, and one side will be 25 at an angle of 40º to it.

You can find the length of the third side either by using x and y coordinates, or by using the trig formula:

$$A^2\,=\,B^2\,+\,C^2 -2BC\,cosa\,.$$​

(Using x and y coordinates is probably easier.)

Have a go!

™​

 

[koat]

Well i tried using pythagoras 25²=28²+x^2.
but i cannot work out x.
what did i do wrong?
please help.

 

[tiny-tim]

uhh? what does Pythagoras have to do with it?

either use the trig formula, or use x and y coordinates