Presentation on theme: "The Universal Law of Gravitation"— Presentation transcript:
1 The Universal Law of Gravitation Mr.RockensiesRegents Physics
2 Gravity r A remote force of mutual attraction between any two masses Magnitude of the force depends on the distance between the masses and their sizem1m2rGravityDistance between the centers
3 Newton’s Law of Universal Gravitation Fg = Gm1m2/r2Works everywhere for all massesFg = The force due to gravitym1 and m2 = The massesr = the distance between the center of the two massesG = The Universal gravitation constant = 6.67x10-11N·m2/kg2G can be found on the front of the reference tableNewton’s Law of Universal Gravitation
4 The forces due to gravity are small for ordinary objects The forces due to gravity are small for ordinary objects. In order to see a large noticeable force, there needs to be large scale masses – planets, moons, stars, etc.G was measured in a Cavendish Experiment a century after NewtonNewton’s Universal Law of Gravitation
6 Weight Revisited 100 kg box Fg = (GmEmbox)/rE2 mE = 5.98 x 1024 kg rE rE = 6.37x106 mboth on reference tablerEEarthFg = (6.67 x 10-11N•m2/kg2)(5.98 x 1024 kg)(100 kg)(6.37x106 m)2Fg = 983 N – same as Fg = mg = 100(9.81) = 981 NWeight Revisited
7 Weight off of Earth 2rE Gravity is an inverse-square law Fg α 1 r2 100 kgEarthFg α 1r2Weight off of Earth
8 What do we do when a question asks… A question asks you what will happen to the Force of Gravity when the radius between two objects is doubled. How do you find out what will happen?If we multiply r by… We multiply Fg by…/22 = ¼/32 = 1/9/102 = 1/100½ /(½)2 = 1/¼ = 4So in the example from the previous slide, a 100 kg box 2rE from Earth’s center weighs 981/22 = 245NWhat do we do when a question asks…
9 The Explanations of Gravity Newton’s (what we will use)Space around a mass is altered to be a gravitational field. The field exerts a force on a second mass.EinsteinSpace is warped by mass. Traveling in a straight line is impossible. Objects orbit by the following curves in space.ModernMasses exchange particles (called Bosons) which bind them together.MFgm1m2mThe Explanations of Gravity
10 How does our weight change when we ride in an elevator? Apparent Weight on an ElevatorHow does our weight change when we ride in an elevator?
11 Apparent WeightScales will read normal force, which is the “apparent weight”Free-Body DiagramElevatorFN = FscalemscaleFg
12 4 cases:Standing still; v = 0, a = 0,FNET = 0 FN = Fg2) Moving at a constant speed (up or down) a = 0FNET = 0 FN = FgAccelerating up, FNET is up therefore FN > Fgscale reads above true weight – you feel heavierAccelerating down, FNET is down therefore Fg>FNscale reads below true weight – you feel lighterIf the elevator is in free fall, FN = 0!