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When to use Force or Average Force in Work Done?

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tnas11:
Im re-sitting Edexcel Physics Unit 1 tomorrow!! Code: 6PH01

When do we use Force x Displacement and Average Force x Displacement in Work Done?

Work Done = Force x Displacement   and   Work Done = F. Average x Displacement

Average Force = Initial Force + Final Force / 2

Its question 21) (a) (ii) (the chest expander question), in "New Template Exemplar" pass paper >> http://www.scribd.com/doc/24557363/GCE-Physics-6PH01

after searching, i understood that we use Average Force when the Force is not constant.. is this right?
so in all springs we use average force?
and what other situations do we use average force?


Exam is tomorrow, good luck to all!! :D

elemis:
Your link is broken.

elemis:
30 J ?

tnas11:
no its 75J
this is how they solved it, i know how to solve it but i dont get why they used F. Average?

Work Done= F. Avg x Displacement
= 60*5/2 x 0.5
= 75 Joules

My question is when to use Average Force and when to use Force?
i think that average is used when the force is not constant as i found on the internet, but i want someone to tell me this is correct

The Golden Girl =D:
to be honest my teacher is ALWAYS telling us that Work done in a Wire *as in Strain Energy* is measured as follows ;

Work done = 0.5*force*extension.


When to use this ,is whenever they ask your for Work Done in a material that has elastic potential energy in it *in this case Spring*.

Hence,

Work done = 0.5 *force * extension * 5    blue ; cuz there are FIVE springs ;)
               = 0.5 *(60) * ( 0.5)  * 5
               = 0.5 * (30) * (5)
               = 0.5 * 150
               = 75 J

good luck to u too ;)

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