Question that makes no sense!

OP

I am still confused :s

Reply 5

AJ-24

djpailo

I am still confused :s

Well, someone's weight in newtons, is their mass, times the gravity on earth?

Sooo, you know that they're 60 kg (which would be 'm' in the equation)

And you know gravity is 9.81 metres per second per second (which is g)

So, by w=mg

w = 9.81 x 60 = 589 Newtons

It's basically just their mass (which you were given previously) times g.

Reply 6

OP

Okay, but then why is that worth three marks??

Reply 7

djpailo

This was the last part of the question. It was a Unit 4 June 2005 Edexcel Paper. It wants the scale reading. My answer for the above question was 2.03N

So how do you do it?

Made 10x worse by the fact that it is worth 3 marks. I know it is w=mg but how do you apply it because I am confused.

F=mg=weight

therefore definately not 2N? unless the girl is a fairy lol

answer is:

60 x 9.81 = 588.6 N
= 589 N (3 sig.figs)

a weighing scale calibrated to newtons means that the weighing scale actually does this conversion itself, therefore the girl will see her weight in Newtons at 589

Reply 8

OP

MC REN

erm if she is standing on the scales when in the orbit then the force pulling her down isn't just mg, it is the centripetal force

I checked the markscheme and it says something similar to that, like you have to take away the centripetal force. I really don't understand this resultant business and whats going on. I've tried looking at notes but to be honest it just makes it all the more confusing.

Reply 9

AJ-24

djpailo

I checked the markscheme and it says something similar to that, like you have to take away the centripetal force. I really don't understand this resultant business and whats going on. I've tried looking at notes but to be honest it just makes it all the more confusing.

Its such a weird question ... lol

I clearly didnt read the other bit - if she's in orbit ... then the centrepetal force will be:

F = (mv^2)/r i think

So that'll be the force .. in newtons ... acting downwards.

Whats the markscheme say?

Reply 10

MC REN

14

djpailo

I checked the markscheme and it says something similar to that, like you have to take away the centripetal force. I really don't understand this resultant business and whats going on. I've tried looking at notes but to be honest it just makes it all the more confusing.

this is a similar example:
http://www.physicsforums.com/archive/index.php/t-184609.html

might help

Reply 11

djpailo

I checked the markscheme and it says something similar to that, like you have to take away the centripetal force. I really don't understand this resultant business and whats going on. I've tried looking at notes but to be honest it just makes it all the more confusing.

oh i get this now, its like this

F=mg
F=(mv^2)/r

therefore: F = (mg) - ((mv^2)/r)

therefore: F = (60 x 9.81) - ((60 x [angular speed at b)i)]^2 / [the radius of the planet in/excluding the girl's height])

Reply 12

silver_j

Don't you work out the weight and minus (or add? :s) the centripetal force, to get the resultant force?

Reply 13

OP

MC REN

this is a similar example:
http://www.physicsforums.com/archive/index.php/t-184609.html

might help

Thats got me more worried. I don't have a clue whats going on there.

I managed to do that centripetal force. But why do you minus them?

I'm trying to think about all the force acting on the scale. The weight is acting downwards, but what direction does the centripetal force act???

Reply 14

AJ-24

djpailo

Thats got me more worried. I don't have a clue whats going on there.

I managed to do that centripetal force. But why do you minus them?

I'm trying to think about all the force acting on the scale. The weight is acting downwards, but what direction does the centripetal force act???

her weight is acting in the same direction as the centripetal force (ie downwards) i think.

Reply 15

F=mg => the gravitational force, causing your weight to be shown

F=mv^2 /r => the centripetal force caused due to the velocity of the rotation of the human on a rotational sphere which is the planet she is on

both of these are completely different forces, therefore in order to find the real force that is measured on a scale, you need the resultant force of these two.

Reply 16

OP

liquidblot

F=mg => the gravitational force, causing your weight to be shown

F=mv^2 /r => the centripetal force caused due to the velocity of the rotation of the human on a rotational sphere which is the planet she is on

both of these are completely different forces, therefore in order to find the real force that is measured on a scale, you need the resultant force of these two.

Yup okay I think I get that now. Why do you minus them as opposed to adding them to finding the resultant though?

Reply 17

silver_j

My understanding (or guess heh) is that the mass exerting the centripetal force also exerts an equal and opposite force in the other direction - Newton's third law. Therefore if you go on the weighing scale, you do the weight - that force

Reply 18