Unit 2 Lesson 6: Gravity

​Science Root of the Day:

DO NOW: Write what you think the example words mean in your lab manual.

Extra Credit: Find three additional words that use this root and write them and their definition in your lab manual (6 Points Max)

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Gravity

Multiple Choice

What is acceleration due to gravity on Earth?

• 100 m/s²

• 10 m/s²

• 1.0 m/s²

Multiple Choice

The equation for POTENTIAL ENERGY is

• PE = mass x gravity x height

•      PE= height / work

• PE = gravity x weight

• PE = work / time

Multiple Choice

The equation for KINETIC ENERGY is

• KE = mass x gravity

• KE = mass²

• KE = 1/2 height x speed

• KE = 1/2 mass x v²

Multiple Choice

Where would you have more mass, on Earth or on the Moon?

• Earth
• Moon
• It is the same, mass doesn´t change

Lesson Objectives

• Define and describe Newton's law of universal gravitation

• Describe the rule by which gravity decreases with distance

• Compare weight vs weightlessness

• Explain How Satellites stay in motion

What is Gravity?

• Gravity: A force that can act at a distance to pull objects toward each other

  • First Described by Sir Isaac Newton in ~1665

    • Newton's apple-while watching apples fall, he had the insight that the same Force that pulls the apple to the ground also pulls the moon towards the Earth​

• Gravity is the force that pulls objects to the Earth and makes them fall

If objects with Mass are attracted to one another, why do they not fly off into space toward each other? 

Universal Gravity

• Newton realized that gravity acts all around us, not just on the Earth

  • Including Space

• The Law of Universal Gravitation: The force of Gravity acts between all objects that have mass

  • Anything that has mass feels a pull towards the objects with mass around them

  • Keeps the Planets in Orbit

The Falling Moon

• ​Why doesn't the Moon crash into the Earth?

  • The moon is moving in a pattern that follows the curvature of the Earth, so it travels in a curve around the earth as a Satellite

• Tangential velocity: a velocity parallel to the Earth's Surface

Newton's Law of Universal Gravitation

• Everything pulls on everything else through the force of gravity

• The force of Gravity is simple: it only involves Mass and distance

• Law of Universal Gravitation: every mass attracts every other mass with a force that is directly proportional to the product of the interacting masses, and inversely proportional to the square of the distance separating them

  • When on Earth, the distance we measure to is the Earth's Center

• Gravity gets weaker the farther apart two masses are

Gravity & Distance

• ​Gravity gets weaker with distance

  • inverse square law: Gravity decreases as the inverse square of the distance

• Because all matter has some form of mass, No matter how far away two objects get, the force of gravity between them will never be equal to zero.

The Universal Constant of Gravitation: G

• ​G is the universal constant of Gravitation

• G=6.67 x 10^-11

• Gravity is a very weak force compared to other forces,

  • we only feel it so strongly because of the massive size of the Earth​

Ocean Tides

• ​The tides are connected to the moon through the force of Gravity

• As the moon revolves around the Earth, the force of Gravity changes depending on which side of the Earth it is on​

Multiple Choice

Gravity always causes objects to

• Repel Each Other

• Attract Each other

• Circle Each other

• Bounce off of each other

Multiple Choice

The gravitational attraction between two objects would increase if both masses are –

• increased, and the distance between them is increased.

• increased, and the distance between them is decreased.

• decreased, and the distance between them is decreased.

• decreased, and the distance between them is increased.

Multiple Choice

Unlike friction, gravity

• Is a Force

• acts over a distance

• acts between two objects not touching

• Can affect the movement of an object

Multiple Choice

True or False: The force of gravity on your body by the Earth is less at the top of a mountain.

• True

• False

• It depends on the weather

Weight and Weightlessness

• ​Say you are standing on a scale in an elevator

  • What forces act on you when the elevator stands still​?

    • Gravity and the Normal force

• When the elevator accelerates, the scale would change its reading based on the direction

  • If the acceleration increases, the pressure increases, and the scale reading goes up

  • If the acceleration decreases, the pressure decreases, and the scale reading goes down

Weight and Weightlessness

• If the elevator breaks and goes into free fall, there is no pressure on the scale, and you are in a weightless state

• Weightlessness is ​not the absence of gravity, but the absence of the support force

Projectile Motion

• Gravity causes an object to fall in a curving motion when thrown

• Projectile: any thrown object that continues to move due to inertia​

  • Two parts: a vertical component and a horizontal component

    • Only the vertical component is affected by gravity

    • The horizontal force has no affect on gravity

• If you throw one object and drop another object of the same mass at the same time, they will hit the ground at the same time because the force of gravity is the same for both

Centripetal Force

• Centripetal Force: any force that causes an object to follow a circular path

  • "Center Seeking"

  • The gravitational force of the Earth on the moon keeps it following a circular path

Satellite

• satellite: any projectile moving fast enough to fall continuously around the Earth

• To be a satellite the horizontal velocity must be great enough for its falling distance to match the Earth's Curvature

• Satellites have a tangential velocity, parallel to the Earth's Surface​

Multiple Choice

True or False: An ordinary projectile, if moving fast enough, can become an Earth satellite.

• True

• False

Multiple Choice

What is weightlessness?

• A complete end of Gravity

• An overwhelming force of gravity

• The absence of the support force

• An overwhelming support force