Gravity

What is Gravity? An attractive force between any two objects that depends on the masses of the objects and the distance between them Force increases as the mass of either object increases or as the objects move closer Earth has enough mass and is close enough that you feel its gravity Name 3 objects (only 2 can be on Earth) whose gravity you cannot feel

Four Basic Forces Gravity 2. Electromagnetic 3. Strong nuclear 4. Weak nuclear You already know about this one! Electricity and magnetism are caused by the electromagnetic force, as are chemical interactions between atoms and molecules Nuclear forces only act on particles in the nuclei of atoms

Law of Universal Gravitation Formulated by Isaac Newton, 1687 Based on his observations of stars and planets

Law of Universal Gravitation What happens to the gravitational force as the distance between two objects increases from 1 m to 2 m? m1m2 d2 F = G gravitational force = (constant) x (mass 1) x (mass 2) (distance)2 No matter how far apart, gravitational force never disappears…called a long range force.

Finding Other Planets 1840s most distant planet known was Uranus But all planets affect motion of all other planets due to gravity, so…

Motion of Uranus disagreed with predicted motion from law of universal gravitation Two astronomers independently calculated this and led to the discovery of Neptune in 1846

Earth’s Gravitational Acceleration If air resistance is the same, two objects of different masses will fall at the same acceleration = free fall Close to Earth’s surface free fall acceleration is 9.8 m/s2 and is given the symbol g Gravitational force (N) = mass (kg) x gravitational acceleration (m/s2) F = mg

Example problem What is the gravitational force on a skydiver with a mass of 60 kg? F = ? m = 60 kg g = 9.8 m/s2 F = mg F = 60 kg x 9.8 m/s2 F = 588 N

Example problem What is the gravitational force on a 150 kg piano falling from a skyscraper? F = ? m = 150 kg g = 9.8 m/s2 F = mg F = 150 kg x 9.8 m/s2 F = 1470 N

Weightlessness and Free Fall Weightlessness is when an object does not experience gravity Video of astronauts in orbit A typical shuttle mission orbits the Earth at an altitude of about 400 km Gravity inside shuttle is 90% that on Earth Why do objects feel weightless?

If you stand on a scale, net force is zero You exert force on scale, scale exerts equal force upward to you Suppose you stand on that scale in an elevator in free fall Scale no longer exerts a force on you, only force is gravity, scale would read zero ONLY FORCE ON AN OBJECT IN FREE FALL IS GRAVITY

Space shuttle in orbit is also in free fall Falling around the Earth Objects in shuttle seem to float because they all fall with the same acceleration How does something orbit the Earth? Why doesn’t the moon fall down?

Projectile Motion Anything that’s thrown or shot through the air is called a projectile Earth’s gravity causes projectiles to follow curved path Video showing many projectile objects (4.17 min)

Projectile Motion con’t Horizontal and Vertical Motions Thrown ball force comes from your hand pushing ball in horizontal motion When it leaves your hand, gravity pulls it down, vertical motion Therefore it appears to travel in a curved path

Projectile Motion con’t Horizontal and Vertical DISTANCE A thrown ball and one that is dropped from the same height will hit the ground at the same time Both balls travel the same vertical distance in same amount of time Thrown ball travels farther horizontally

Centripetal Force When a ball travels through a curved tube it may speed up or slow down during straight sections = acceleration BUT even if its speed doesn’t change in the CURVED parts it is STILL ACCELERATING because it is a change in the direction of the velocity Change is towards the center of the curved path = CENTRIPETAL ACCELERATION

Centripetal Force Car rounding a curve has centripetal force, this comes from traction (friction of tires on road) If friction is too small, car will move in a straight line (off the road) Anything that travels in a circle is doing so from centripetal force, accelerating it toward the center Demo: bucket of water, penny on a coat hanger Same reason moon orbits Earth