Spring balances

1. Feb 28, 2012

rishch

Q. A spring balance read 10 kg when a bucket of water is suspended from it. An iron piece having some mass is suspended by another string and is then immersed in the bucket with half of its volume. The reading of the spring balance will -

(A)decrease
(B)increase
(C)remain the same
(D)another spring balance needed

My answer to this question would be (C) as the iron piece is suspended by another string so it does not apply any force on the spring balance. However, according to the answer book the correct answer is (B) and the reason they give is this - The iron piece experiences buoyant force exerted by water. The iron piece will exert an equal and opposite downward force on water. So reading of balance will increase.

Is it possible for an object to exert a reaction force while it is stationary ? Wouldn't it exert a force on the string that its attached to instead of on the water ? Is that possible ? Could an object exert a reaction force on an object apart from the one which exerted the action force ?

2. Feb 28, 2012

tiny-tim

hi rishch!
A reaction force usually is between stationary objects.
it exerts (and receives) a force from both

remember, the water doesn't know the string is there

it only knows that the iron is shoving its way in!
no

reaction forces come in pairs, always between the same two bodies

any other pair of forces … even if they are equal and opposite … aren't a reaction pair

3. Feb 28, 2012

Thanks !