Surface Tension of needle on water

In summary, the conversation discusses the calculation of the maximum mass of a needle that can be supported by the surface tension of still water. The conditions and formula for this calculation are provided, and there is a clarification on the necessary adjustments for the maximum weight.
  • #1
Aleisha
25
0
I attempted the question but it was wrong... I don't understand where i went wrong my working seems logical, can someone please help.

When a needle is gently placed on the surface of still water (γ = 0.0730 N.m-1) it can be supported by surface tension if the mass of the needle is small enough.
Calculate the maximum mass of a needle that is 4.85 cm long that can be supported by the water. Take the acceleration due to gravity to be 9.80 m/s/s.
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  • #2
Aleisha said:
I attempted the question but it was wrong... I don't understand where i went wrong my working seems logical, can someone please help.

When a needle is gently placed on the surface of still water (γ = 0.0730 N.m-1) it can be supported by surface tension if the mass of the needle is small enough.
Calculate the maximum mass of a needle that is 4.85 cm long that can be supported by the water. Take the acceleration due to gravity to be 9.80 m/s/s.View attachment 224645 View attachment 224646
You only included half the force.

P.S., Please use the Homework Template in the future.
 
  • #3
So all I have to do is double my answer to get the full force?
 
  • #4
Aleisha said:
So all I have to do is double my answer to get the full force?
Yes. You also have to understand that, at the maximum weight, ##\theta=0##.
 
  • #5
I understand that now it makes sense, thank you!
 

What is surface tension?

Surface tension is the force that causes the surface of a liquid to behave like an elastic membrane, allowing it to resist external forces and form droplets or maintain a distinct surface.

Why does a needle float on water?

A needle floats on water due to the surface tension of the water. The surface tension creates a thin layer of water molecules on the surface, which the needle can rest on without breaking through.

How does the thickness of a needle affect its ability to float on water?

The thickness of a needle does not significantly affect its ability to float on water. As long as the surface tension of the water is strong enough to support the needle's weight, it will float regardless of its thickness.

How does surface tension change with temperature?

Surface tension decreases as temperature increases. This is because higher temperatures cause water molecules to have more energy and move more freely, making it harder for them to maintain a strong cohesive force at the surface.

How does adding soap affect the surface tension of water?

Adding soap to water decreases the surface tension. Soap molecules are amphipathic, meaning they have both hydrophobic and hydrophilic properties. The hydrophobic tails disrupt the cohesive forces between water molecules, reducing the surface tension. This is why soap creates bubbles when agitated on the surface of water.

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