A-level Maths Mechanics

Hi, the markscheme for this question resolves the vertical forces and solves for R to find the magnitude, but how do you know it is in equilibrium and not accelerating?

(edited 1 year ago)

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Reply 1

mqb2766

Original post by Har6547

Hi, the markscheme for this question resolves the vertical forces and solves for R to find the magnitude, but how do you know it is in equilibrium and not accelerating?

Youre finding the resulting force. It wont be in equilibrium with only P and Q acting on the particle, but youre not interested in the motion.

Reply 2

Har6547

Original post by mqb2766

Youre finding the resulting force. It wont be in equilibrium with only P and Q acting on the particle, but youre not interested in the motion.

Hi, yes but in the markscheme they assume the vertical forces are equal. For this to be true, wouldn't the particle have to be in vertical equilibrium?

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Reply 3

mqb2766

Original post by Har6547

Hi, yes but in the markscheme they assume the vertical forces are equal. For this to be true, wouldn't the particle have to be in vertical equilibrium?

R is the resultant force, so no.
Once R is calculated, you can remove P & Q and just consider the effect of R.

Reply 4

Har6547

Original post by mqb2766

R is the resultant force, so no.
Once R is calculated, you can remove P & Q and just consider the effect of R.

Oh I think I understand now. Because R is the resultant, the sum of the constituent forces is equal to R, and therefore the sum of the vertical components of the constituent forces is equal to the vertical component of the resultant force?

Reply 5

mqb2766

Original post by Har6547

Yes, just think of the force triangle/parallelogram you learn in gcse physics (for example). Here theyve given you slightly diferent info and as you don't know the magnitude of the horizontal force, simply resolve in the vertical direction to "ignore" it.