# A level Physics Help needed...Watch

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#1
Have I just completely lost all ability to do maths or what? Right:
This is from OCR Physics A 2011 January Q3 B)ii ,in case you need to check it.

The question is
"The angle between the slope and the horizontal is 7 degrees. The weight of the car is 9.0 x 10^3. The car travels is the slope at a constant speed of 18ms. A resistive force of 300 N acts on the car down the slope.

Calculate the component of the weight of the car acting down the slope"

BTW, the slope is going diagonally upward to the right if that helps.
I drew a vector triangle, so I has 7 degrees as angle bottom left, a 90 degree angle bottom right, and therefore 83 degree angle top right. The vertical component of weight being 9 x 10^3 as opposite side to angle of 7 degrees, and labelled the bit I want (the hypotenuse to the 7 degree angle) as a bold W. Then, basic trig tells me that Sin(7) = opp/hyp = 9x10^3 / W

I rearrange that to W = 9 x 10^3 / sin(7) and get the wrong answer, even after checking ect, please help! Absolutely no idea what I've done wrong, which is why I know I've missed something massive. Thanks in advance!

P.S I keep getting 1.37 x 10^4, the answer is 1.1 x 10^3
Also, if you're able to help, please explain why so I know for future, cheers!
0
5 years ago
#2

Not sure if this helps you, I am bad at explaining using words.

the top angle is still 7
it's the bottom left is 83

To find the opp, you can use sin, ie sin (opp/hyp)
sin7 x 9x10^3
= 1096.82
round it up 1.1x10^3

(ps if im wrong, please correct me)
1
5 years ago
#3
Its mgsin7, not mg/sin 7
Sin of the angle = opposite/hypotenuse
The component of weight down the slope is the opposite.
So the answer is mg x sin7
0
#4
So sin(7) does not = opp/hyp ?? Brilliant thanks! But why doesn't just using different information work the same way? So confusing, I completely understand how and why that works, but I don't know how I'm supposed to know which triangle to pick to do in the real thing.
1
5 years ago
#5
Hmm do you mean like how to resolve it into a triangle when you see another slope angle thingie?
0
5 years ago
#6
If this is what u learn in physics. Im doubting doing it

Posted from TSR Mobile
0
5 years ago
#7
(Original post by AsandaLFC)
If this is what u learn in physics. Im doubting doing it

Posted from TSR Mobile
Don't worry, I'm sure your teacher will help you! Plus this is only in AS , A2 is kind of different
0
5 years ago
#8
(Original post by Makashima)
Don't worry, I'm sure your teacher will help you! Plus this is only in AS , A2 is kind of different
OPs post brought some acute stress. I dont understand a thing

Posted from TSR Mobile
0
#9
Nah this is pretty much the hardest thing you do in Unit 1 of 2 AS level, it's really easy other than a trick question here and there IMO
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