Forces component question

http://www.mediafire.com/view/?40faentnevqki77#

I need help on the question in the link above. It's annoying me so much! The answer's D, but how? Any ideas on how to arrive at D??

Thanks a lot.

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Reply 1

L'Evil Fish

W = mass

You use sin theta when angle isn't included between the forces you want.

So if they asked for the weight acting perpendicular to plane then it'd be Wg cos theta

Its just basic trig but you'd know when its included it is cos theta and when it's excluded its sin theta

(edited 11 years ago)

Reply 2

krisshP

Original post by L'Evil Fish

W = mass

You use sin theta when angle isn't included between the forces you want.

So if they asked for the weight acting perpendicular to plane then it'd be W cos theta

Its just basic trig but you'd know when its included it is cos theta and when it's excluded its sin theta

But weight=mass X gravitational field strength (9.81)

Reply 3

L'Evil Fish

Original post by krisshP

But weight=mass X gravitational field strength (9.81)

That too

The mass isn't all acting perpendicular to a plane.

So it's

Wg Cos Theta

Reply 4

krisshP

and can you please explain really slowly as I am not so good when it comes to these inclined surface situations :frown:

Reply 5

L'Evil Fish

I can't upload photos as the app won't let me :frown:

?????

I do not understand at all really :frown:

Reply 8

L'Evil Fish

Okay draw me a right angled triangle.

Now draw a box on the hypotenuse.

Draw a line vertically down from the centre of the box down through the plane and triangle.

Now draw two dotted lines, one parallel to the plane (hypotenuse) from the centre of the box.

And one perpendicular to the one you just drew.

Tell me when that's done.

Reply 9

FaisalTreShah

Original post by krisshP

http://www.mediafire.com/view/?40faentnevqki77#

I need help on the question in the link above. It's annoying me so much! The answer's D, but how? Any ideas on how to arrive at D??

Thanks a lot.

You just need to find the Parallel and Perpendicular (of the slope) components of W, and the angle between W-Perpendicular and W is the same as Theta.

With this in mind, make a triangle, with W as the hypotenuse, W-Parallel as the opposite, and W-Perpendicular as the adjacent side, and it's just simple trigonometry from there.

The picture I attached should hopefully help you. :biggrin:

DSC_0001.jpg496.9KB

Reply 10

krisshP

Original post by L'Evil Fish

Is the second dotted line suppose to through the centre of the box on hypotenuse??

thanks

Reply 11

L'Evil Fish

Original post by krisshP

Is the second dotted line suppose to through the centre of the box on hypotenuse??

thanks

Yeah towards the plane...

The post above is a good way too

Reply 12

krisshP

Original post by L'Evil Fish

Yeah towards the plane...

The post above is a good way too

i'm done!

Reply 13

L'Evil Fish

Original post by krisshP

i'm done!

Okay, now the angle in between your vertical line and your perpendicular line is the same as the angle of your original right angled triangle (inclined from horizontal) label them both with theta

Reply 14

krisshP

Original post by L'Evil Fish

Okay, now the angle in between your vertical line and your perpendicular line is the same as the angle of your original right angled triangle (inclined from horizontal) label them both with theta

done.

Reply 15

krisshP

Oh I get

thanks so much L'Evil Fish and FaisalTreShah. Also if you use your hands and twist both a bit, it makes even more sense :smile:

! You two are so smart to recognise the angles and stuff so quickly and fast! Can't believe the question was in the multiple question section A side of the paper. It's got some difficulty though.

THANKS AGAIN !

Reply 16

L'Evil Fish

Original post by krisshP

done.

Now if you join up the dotted perpendicular line with the vertical line to create a right angle triangle. How would you find the length of the dotted perpendicular line?

Cos theta = adjacent / hypotenuse

Hypotenuse = Vertical Line (which has a magnitude of mass x gravity) label this line w.

So the dotted line is found with:

Wg Cos Theta

Reply 17

krisshP