SCIENCE STUDY GUIDE – Chapter 3 – Gravity, Friction and Pressure
Gravity – The force that objects exert on each other because of their masses.
more mass = more gravitational force
more distance = less gravitational force
Acceleration due to Earth’s gravity = 9.8m/s at the Earth’s surface = g
F=mg Force= mass x acceleration (due to Earth’s gravity = g)
Two objects dropped from the same height in a vacuum, have the same acceleration (even if they
have different masses)
Weight – the force of gravity on an object
Orbit – elliptical path of one body (ex. - the Moon) follows around another body (ex – the Earth) due to
the influence of gravity.
Microgravity – A space ship in orbit is in a free fall. Gravity is acting on the ship and astronauts. The
ship and astronauts are falling around Earth at the same rate. The astronaut’s weight does not
press against the floor – this environment is called microgravity.
Friction – the force that resists the motion between two surfaces in contact. Ex – your feet and the
ground, rubbing your hands together…It produces heat
Factors that determine friction
Surface – a tile floor will create less friction than a carpeted floor
Weight – more weight, more friction
Fluid – a substance that can flow easily Example- water, air (gases and liquids)
Air resistance – friction due to air It depends on surface area and speed of an object.
1. larger surface area- contact with more molecules = increase in air resistance
2. speed of an object – speed increases = air resistance increases
~ if you drop 2 objects with the same mass but different sizes, the smaller ball will hit first
because there is less air resistance
DRAG- when an object moves through a fluid (liquid or gas, such as water or air) the object pushes the
molecules out of the way. The molecules of the fluid push back with an equal and opposite force– this is
drag (this is a type of friction).
Pressure- the measure of how much force is acting on a certain area.
Pressure = Force/Area
Increase in force = increase in pressure
Decrease in area = increase in pressure
Pascal – the unit for pressure
Pressure in a fluid – depends on density and depth
Pressure in water- the deeper you go under water, the more water there is above you. The weight of
the water above you puts pressure on you. The deeper you go under water, the pressure
increases.
Buoyant force – upward force on objects in a fluid
Objects floating/sinking in a fluid
~ depends on the density of both the object and the fluid.
1. Object less dense than fluid – object will float. The buoyant force is balancing the
weight.
2. Object more dense than fluid – object will sink. The sinking object has a weight greater
than the buoyant force.
Bernoulli’s principle – An increase in the speed of the motion of a fluid decreases the pressure
within the fluid – the faster a fluid moves, the less pressure it puts on surfaces it flows over
Pascal’s principle – when an outside pressure is applied at any point to a fluid in a container, that
pressure is transmitted with equal strength.
~examples: piston, hydraulic jack