# physics

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a body of mass 1kg on the equator rotates about the axis of the earth with a period of 1 day. (mass of earth = 5.98x10^24kg n may b assumed to b concentrated at its centre n radius of earth = 6380km)
first question asks to calculate gravitational force of attraction between earth n mass....simple enuff
second qs asks to work out centripetal force on mass...easy
then it asks us to work out the difference between these two forces n use the answer as the resultant force to calucalte g at the equator using F =ma; what i've been taught is centripetal force is the resultant force so then y the need to find resultant force as the qs says. and then wat i also don't understand is why the resultant force is the difference between two forces that are acting in the same direction.
if someone cud clarify id b gr8ful

• physics -

The resultant force that they want is the modified force M g' that a scale would indicate for the object at the equator. It is less than the gravitational weight alone (M g) because

Gravity force - Indicated (scale) weight = Centripetal force
Therefore
Indicated weight = Gravity force - Centripetal force

g' = g - (V^2/R)

What you are really calculating is the resultant of the gravity force and the fictitious centriFUGAL force in the opposite direction.