GRAVITY

The force of gravity Gravity is the attraction between all objects in the universe. Sir Isaac Newton, an English scientist, first described this force, so it is known as Newton’s Law of Gravity, or Universal Gravitation.

Two things count: The amount of gravity felt between any two objects depends on: 1) The objects’ masses AND 2) the objects’ distance from each other.

Galileo Galilei Galileo Galilei died the year Isaac Newton was born. Galileo proposed that all objects fall with the same acceleration (the same rate of speed). This became an important part of Newton’s theory of gravity.

Gravity The acceleration of gravity (g) for objects in free fall at the earth's surface is 9.8 m/s2. Galileo found that all things fall at the same rate, and accelerate (speed up) as they fall. He did NOT, however, drop things from the Tower in Pisa, as is commonly thought.

Free Fall The speed an object falls at is always 9.8 m/s faster than it was the second before.

Downward… Because the Earth is so big compared to things on the Earth, we feel attracted to the center of the Earth. We call the pull of gravity our weight, and in the US we measure it in pounds (lbs). The metric system measures the pull in Newtons (N).

Gravity Gravity – force that acts between any two masses Non-contact, attractive force Gravity is directly proportional to the masses of the objects Gravity is inversely proportional to the square of the distance between the masses

What does this mean? If distance is constant, doubling mass would quadruple the force of gravity. If distance is constant, tripling the mass would increase the force of gravity NINE times. If mass is constant, doubling the distance would lessen the force of gravity by a factor of 4. If mass is constant, tripling the distance would lessen the force of gravity by a factor of 9.

Earth’s Gravity Earth’s gravity pulls you downward toward the center of Earth’s mass and your gravity pulls the earth upward toward the center of your mass! Do you see that if you have more mass than someone else, the force of gravity is larger for you?

Falling Objects An object falls toward Earth due to gravity – object is in free fall if it moves toward Earth only due to gravity. Gravity (force) causes the object to accelerate toward the Earth’s surface at 9.8 m/s/s

Falling Objects If two objects of different masses are dropped from the same height at the same time, which one hits the ground first? http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

Acceleration due to Gravity (g) 9.8 m/s/s – what does this mean? Every second the object falls, it adds 9.8 m/s to its velocity! Is there a limit to how fast an object can get as it falls through the air? As the object falls through the air, is there any force acting on the object besides gravity?

YES Air resistance is a force that opposes gravity. However, as gravity is greater than air resistance, there still a net force down, and the object will continue to fall, just at the same speed (terminal velocity).

Falling Objects If gravity and air resistance become equal, the object is still falling but is no longer accelerating. Terminal Velocity is the constant velocity of falling object when Fair = Fg (Fnet = 0)

Falling Objects Some objects encounter more air resistance and thus take less time to reach terminal velocity than others. This depends on object’s mass to surface area ratio. The smaller the ratio, the quicker terminal velocity is reached http://www.physicsclassroom.com/mmedia/newtlaws/efar.cfm What about when there’s no air resistance, as in a vacuum? Ex: On the Moon, there is gravity, but no air. All objects accelerate toward the surface of the moon at the same rate – 1.62 m/s/s http://www.youtube.com/watch?v=5C5_dOEyAfk - Hammer vs. Feather on the moon

Mass vs. Weight Mass – measure of how much matter an object contains Weight – force of gravity acting on an object Weight = Mass x Acceleration due to gravity w = m x g Ex: If you have a mass of 67 kg (about 150 lbs), you would multiply your mass by 9.8 m/s/s if you wanted to find your weight on Earth in Newtons w = (67 kg) x (9.8 m/s/s) = 642 kgm/s/s = 642 N If you traveled to the moon, your mass would stay the same but the acceleration due to gravity on the moon is only 1.62 m/s/s, so your weight would not remain the same! w = (67 kg) x (1.62 m/s/s) = 108.5 N Let’s look at your weight on different astronomical bodies http://www.exploratorium.edu/ronh/weight/index.html

Weightlessness Weightlessness is a sensation that one experiences due to the lack of a contact force (normal force) Astronauts experience this in space because they are falling at the same rate that their spacecraft is falling and thus do not have a contact force to provide them with a sensation of weight – you get the same sensation during the big drops on roller coasters (momentary loss of contact force as you and rollercoaster car fall together)

The Space Station & Weightlessness The Space Station is orbiting at 400 km above Earth’s surface. Doubling distance from Earth’s surface = 12,756 km Still have weight but have no sensation of weight. What are the health affects of prolonged weightlessness? Headaches Vomiting Nausea Muscle atrophy (decreased muscle mass)

Things that Orbit Earth How does an object remain in orbit? These objects are projectiles that fall around the earth instead of into it because they are moving fast enough. They move in a circular motion because the earth’s gravity provides a center-directed force that constantly changes their direction This center-directed force is called a centripetal force.