Pressure

Grade 8 Atlantic                                                                                          Teacher : Ibu Nuri Martini
Concepts of PRESSURE 

PRESSURE ON SOLID 
Why would a lady in high heels standing on your foot hurt more than an elephant standing on your foot? 
The elephant has a larger weight than the lady, but the contact area between its feet and the foot is far larger. A Force spread over a large area produced a lower pressure.


The relationship between FORCE and AREAS on which the force acting on it is called PRESSURE


PRESSURE ON LIQUIDS

Liquid can produce Pressure. How? Take one plastic bag and fill it with water until it is full. Then make a hole in the side of the plastic. What will happen? Water will spout out from the plastic bag, why? because of the water pressure applied on the hole. 

Properties of Pressure on Liquid 

• Pressure on Liquid depends on a GRAVITYSea-divers feel the water pressure on their bodies from every direction. The deeper they   dive, the bigger the pressure they receive. Consequently, when we dive deeper, our ear will feel hurt, because every water molecule is attracted downward by the Earty's gravitational force. Attracted downward because of gravitation acceleration (Percepatan Gravitasi)  

• Pressure on Liquid depends on the DEPTHThe pressure becomes greater when it goes deeper. As a result, the water spouts reach a longer distance.
  Students do some experiment for prove this properties.

• Pressure on Liquid depends on the DENSITY of the LiquidIf different kinds of liquid are poured into a communicating vessel, do you think that the pressure will still be the same? 
  Remember : 
  Pressure is caused by gravity or the weight of the liquid. 
  Weight of an object is the product of the mass of that object and the gravity in which the object is located. 
  W = m * g 
  Mass of a substance is defines by its density

From properties above, it can be concluded that :
"Pressure on a liquid depend on its DEPTH, GRAVITY and DENSITY but doesnt depend on shape of its container". This statement is known as the main law of Hydrostatics

ARCHIMEDES' LAW 
Have you ever experienced the following phenomenon?

• After swimming when you wanted to go out from the swimming pool, you felt that your body was heavier than before
• It is felt that lifting a weight in the water is lighter than lifting in the air. 

Why is it so?
The volume of the Load immersed in the water is equal to the volume of the displaced liquid. The weight of the displaced liquid is the buoyancy force (GAYA APUNG) 
Archimedes Law says
"Any object, wholly or party immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object"
Students do experiment for proving this statement.

 
Why ship or boat can float on the water? 
An Object may float, suspend, or sink when it is in the water. How can it happen? Students do experiment of Archimedes' Law. Here the video when they do experiment