Chapter 9 Forces and Buoyancy

Force An influence that causes an object to undergo a change in motion Generally a push or a pull

Balanced forces These forces cancel each other out – Equal in strength – Opposite in direction Results in no movement

Unbalanced forces These forces do not cancel each other out – Unequal in strength – Result in a change of motion for the object Speed Direction

Difference between Mass and Weight Mass The amount of matter an object has – Cells and atoms Stays the same – Kilograms Weight Measure of push or pull on an object – Gravity Measured in Newtons – 9.8 N/Kg

Buoyancy The upward force on objects either floating or submerged in water. If the object floats it has neutral buoyancy

Archimedes’ principle The amount of buoyant force exerted on a submerged object equals the weight of the water that has been displaced.

Density and buoyancy If the density of the object is higher than the fluid then the object will sink! What if the object does not have one specific density? – Average density needs to be calculated Total mass divided by total volume

Average Density Total mass that makes up a substance divided by the total volume Mass total /volume total

Benefits of average density Causes large ships to float Fish have a swim bladder (air bladder) – Mixture of air and water – Air decreases, fish can go lower – Adapted for submarines

Benefits of Average Density

Which will Float? Wooden boat vs. water logged stick. Metal block vs. metal boat Sealed empty bottle vs. full water bottle

Pressure Force acting on a certain area of surface – Larger the force, greater the pressure – Smaller the area, greater the pressure

Calculating Pressure Unit of pressure is Pascal (Pa) – Can determine pressure if you know force and area

Sample problems An aquarium is filled with water that weights 10,000 N. If the base of the aquarium has an area of 1.6 m 2, what pressure does the water exert on its base? P = F/A P = 10,000N/1.6m 2 P = 6250 Pa

Sample Problems If the atmospheric pressure is 101,200 Pa and you are holding your hand, the atmosphere is exerting a force on your hand. If the area of your palm is 0.006m 2, calculate the force on your hand. F = P x A F = 101,200 Pa x m 2 F = 607N

Atmospheric Pressure Pressure exerted by the layers of air surrounding the Earth – Reaches roughly 160 km above the Earth

Pascal’s Law Pressure applied to an enclosed fluid is transmitted with equal force throughout the container – Basics for hydraulic and pneumatic devices