Gravity Week of October 22nd

Journal Entry 10/22/12 What is gravity?

Gravity A force of attraction between objects that is due to their masses.

Affects on Motion The force of gravity can change the motion of an object by changing its speed, direction, or both.

The Earth The Earth’s gravitational force pulls everything toward the center of Earth. Must apply a force to overcome the Earth’s gravity.

Sir Isaac Newton Newton observed a falling apple and concluded that an unbalanced force on the apple made it fall. He applied this idea to the moon and develop the law of universal gravitation.

Law of Universal Gravitation All objects in the universe attract each other through gravitational force. Two factors affect the size of the force.

Gravitational force increases as mass increases. Factor 1 Gravitational force increases as mass increases. Why do astronauts bounce when they walk on the moon?

Factor 2 Gravitational force decreases as distance increases. Why do the planets stay in orbit?

Center of Mass The point at which all the mass of an object can be considered to be concentrated.

Finding Center of Mass Regular shaped objects: center of object Irregular shaped objects: the point around which the object spins is its center of mass. http://video.pbs.org/video/1607949294/ http://www.youtube.com/watch?v=DY3LYQv22qY

Weight vs. Mass Weight: a measure of the gravitational force exerted on an object; its value can change with the location of the object in the universe. Mass: a measure of the mount of matter in an object.

Differences between weight and mass Weight changes when gravitational force changes. Mass does not change. What would your weight be on Jupiter? What would be your mass on Jupiter?

Journal Entry 10/24/12 If a golf ball and a ping-pong ball are dropped from the roof of HMS, which will fall to the ground first? Why?

Galileo vs. Aristotle Aristotle said that an object with more mass will fall first, but he never tested his theory. Galileo tested the theory by dropping two cannon balls from the Leaning Tower of Pisa and proved Aristotle wrong.

Gravity & Acceleration Objects fall to the ground at the same rate because acceleration due to gravity is the same for all objects. All objects accelerate toward Earth at a rate of 9.8 m/s2

Velocity of Falling Objects To calculate the change in velocity of falling objects use the following equation: Δv = g x t g= is acceleration due to gravity on Earth (9.8m/s2) t= time the object takes to fall in seconds

Practice #1 A foul ball is hit straight up in the air and falls from top of its motion for 1.4s before being caught by the catcher. What is the velocity of the ball as it hits the catcher’s glove?

Practice #2 A brick falls from the top of a building and strikes the ground with a velocity of 19.6 m/s downward. How long does the brick fall?

Practice #3 A penny at rest is dropped from the top of a tall stairwell. What is the penny’s velocity after it has fallen for 2s?

Air Resistance The force that opposes the motion of objects through air. Amount of air resistance depends on size, shape and speed of the object.

Terminal Velocity The constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity. (Net force of 0N) Acceleration stops when terminal velocity is reached.

Free Fall The motion of a body when only the force of gravity is acting on the body. Free Fall only occurs in space and in a vacuum.

Orbiting An object is orbiting when it is traveling around another object in space. Gravity provides the centripetal force that keeps objects in orbit.

Projectile Motion The curved path that an object follows when thrown, launched, or otherwise projected near the surface of Earth. Projectile motion has two components: horizontal motion and vertical motion. Gravity affects only the vertical motion of projectile motion.