Gravity and Motion

Gravity is what gives the universe its _________ A universal force that acts on _________ the objects in the universe Every particle in the universe is drawn ________ every other particle in the universe Gravity is what also controls the motion of astronomical bodies, holding planets in orbit Gravity and motion are tightly connected in the universe

Solving the Problem of Astronomical Motion Ancient astronomers didn’t make the connection between gravity and motion that we understand today. How come we don’t fly off the face of the Earth? What keeps the planets moving? ___________________ was the first to make the connection in the 1600s with a series of carefully constructed experiments using projectiles and balls rolling down planks These experiments demonstrated the power of the _________________________

Inertia _________________: the tendency of a body at rest to remain at rest and a body in motion to keep moving. Both Aristotle and Kepler recognized inertia, but it was Galileo who demonstrated its importance through experimentation _________________ was the first to use the term inertia Newton recognized the special importance of inertia

Inertia Galileo was the first to demonstrate it by real experimentation –Rolled balls down an incline – speed _____________ until it reached the bottom –Rolled balls up an incline – speed ________________ as it approached the top –Hypothesized that a ball rolled along a flat surface with no other forces acting on it would neither increase or decrease –In the absence of outside forces, inertia keeps an object already in motion moving at a fixed speed

Newton’s Laws of Motion Newton’s first law of motion (The Law of Inertia): a body continues in a state of _________ or uniform motion in a straight line unless made to change that state by forces acting on it. ______________: something that pushes or pulls Net force: the total of ___________ forces acting on a body _______________ forces lead to _______ acceleration

Newton’s Laws of Motion If a body in not moving in a ________________ line at constant speed, some net force must be acting on it Newton actually wasn’t the first to state this law (Christiaan Huygens was), but Newton showed how to ___________________ it to the universe Because the planets and other heavenly bodies follow a curved path, some net force must be acting upon them.

Orbital Motion and Gravity Kepler recognized that something must be holding the planets in orbit around the sun and proposed that it was something similar to ____________________ (this was 100 years before Newton) ______________________ suggested it was _____________ that held the planets in motion (suggested several years before Newton published the laws of gravity in 1687) Newton was the one to publish his findings and demonstrate the properties that gravity must have to fulfill its role as the controller of planetary motion Newton also came up with the mathematical ____________ that describe gravity and its effect on motion Newton correctly deduced that Earth’s gravity, if weakened by distance, could explain the motion of the moon.

Orbital Motion and Gravity If a projectile moves fast enough, the Earth’s surface will curve away below the projectile such that it will __________________ hit the ground. This is the nature of orbital motion; the __________________ between inertia and the force of gravity maintain the orbit The question becomes: how fast does the object need to move to maintain its orbit?

Newton’s Second Law of Motion Motion of an object is a change in its position which can be characterized by the direction the object is moving and its speed ________________: nonuniform motion; a change in speed either increasing or decreasing We can produce an acceleration by changing the _______________ a body is moving.

Newton’s Second Law of Motion A body moving in a circular orbit is ________________ accelerating even though its speed is not changing (the direction is changing) For a body to accelerate a force must act upon it Acceleration is proportional to the force that is applied It also depends on the amount of _____________ being accelerated

Newton’s Second Law of Motion ________________ :the amount of matter an object contains It is also technically is a measure of ________________ The more mass the more inertia What is the difference between mass and weight? Weight measures the force of gravity acting upon a given mass

Newton’s Second Law of Motion The law states: the amount of acceleration (a) that a force (f) can produce depends directly on the mass (m) of the object being accelerated __________________________ This is what allows us to launch satellites and send them to a pinpoint destination

The Law of Gravity Every mass exerts a force of attraction on every other mass. The strength of the force is directly proportional to the product of the masses divided by the square of their separation F = GMm r 2 G = 6.67 X meters 3 /(kilogram-sec. 2 ) G= Universal Gravitational Constant

The Law of Gravity What does this law tell us? If M or m increases and the other factors remain the same, the force increases If r increases the force gets weaker by the square of the distance (if we double the distance the forces gets weaker by a factor of 4) The gravitational force _______________ really disappears What does that mean?

Newton’s Third Law (The Law of Action and Reaction) When two bodies interact, they create equal and opposite forces on each other. The forces acting on the Sun and Earth are exactly equal The Earth orbits the sun because its mass is much smaller (300,000 times) than the sun because acceleration is inversely proportional to the mass of an object The acceleration a body feels is inversely proportional to its mass –The more massive the body is the greater the force that is needed to accelerate that mass –The Sun accelerated 300,000 times less than the Earth because it is 300,000 times more massive Therefore the Earth does the moving and not the Sun

Surface Gravity This is a measure of the gravitational ________________ at a planet’s or star’s surface A body’s weight depends on its mass and the acceleration of gravity Determines what an object _____________ on its surface Influences a planet’s _______________ and whether it has an _______________________ –Asteroids are not spherical because the surface gravity is too weak to press them into round shapes The strength of surface gravity is different from body to body.

Escape Velocity The force need to overcome a planet’s gravitational force and ______________________________ This will also influence whether a planet has an atmosphere It also influences the nature of ____________________. V esc = √2GM/R –V = Escape Velocity –G = universal gravitational constant 6.67 X m 3 /kilogram – sec 2 –M = Mass of the Body to be escaped from –R = Radius of the body to be escaped from