Page 134 - Physics

Physics for Secondary Schools

Procedure However, when some detergent is added
1. Fill the trough with water. to the water, the same objects sink to the
2. Gently place the needle, razor blade bottom of the trough, as shown using a
and paper clip on the water surface. razor blade in Figure 6.10. This means that
3. Observe what happens. the introduction of the detergent reduced
4. Sprinkle some chalk powder on the the surface tension of the liquid.
water surface.
5. Note down your observations.
6. Add a little detergent such as soap to
the water in the trough and observe
what happens.

Questions
(a) What did you observe when the
needle, paper clip, razor blade and
chalk powder were gently placed on
the water surface?
(b) What did you observe when some
detergent was sprinkled on the
water?

Figure 6.10: Razor blade sinking in soapy
When the needle, paper clip, and razor water
blade were placed on the surface of the
water, they all fl oated on top, as shown If an object is placed on the surface of
using a razor blade in Figure 6.9. The a liquid, its weight pushes the liquid
surface tension of the water was large. downward causing a deformation, which
increases the surface area of the liquid.
The surface tension resists the increase in
the area by pushing upward the object, as
shown in Figure 6.11.

Light needle Surface of liquid

Weight of object
Surface tension pushing upward

Figure 6.11: Surface tension supporting an
Figure 6.9: Razor blade fl oating in fresh water object

128
Student’s Book Form One

Physics Form 1 Final.indd 128 16/10/2024 20:57