Gravity, Friction and Pressure

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Gravity, Friction and Pressure

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

Source : http://www.centennialsd.org/cms/lib4/PA01001061/Centricity/Domain/356/science_study_guide_Motion_and_Forces_ch.3.doc

Web site link to visit: http://www.centennialsd.org/

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