Physics I Honors 1 Specific Forces Fundamental Forces Universal Gravitation
Physics I Honors 2 Objectives Calculate the gravitational force given two masses and the distance between them. · Calculate the change in gravitational force when the mass or distance is changed. · Add gravitational force vectors · Explain how a person's weight is related to the Law of Universal Gravitation. · Define 'gravitational field strength' and relate it to the acceleration due to gravity
Physics I Honors 3 SPECIFIC FORCES Fundamental Forces Gravitational Force Centripetal Force Restoring force
Physics I Honors 4 What Are The Fundamental Forces? Gravitational Force Electromagnetic force Strong Nuclear Force Weak Nuclear Force
Physics I Honors 5 Gravitational Force An attractive force that exists between all masses. It is the basis of planetary motion. It is the weakest force. It acts over long distances Is universal We experience it as –Weight –Planetary orbits –Satellite motion
Physics I Honors 6 Comparing the Gravitational and Electromagnetic Forces The gravitational force, which only attracts, is much weaker than the electric force. electric force = 10 35 gravitational force Just as the space around a planet and every other mass is filled with a gravitational field, the space around every electric charge is filled with an electric field.
Physics I Honors 8 THE LAW OF UNIVERSAL GRAVITATION Newton proposed that an attraction between bodies is universal. Gravitational force is extremely weak between ordinary objects. Objects with enormous mass have significant gravitational force. Creates orbits Creates tides Is known as weight for objects on the surface.
Physics I Honors 9 According to Newton's law of gravity, her weight (not mass) decreases as she increases her distance from the Earth's center (not surface).
Physics I Honors 10 Equal and Opposite Forces m 1 -F +F m 2 r
Physics I Honors 11 Newton’s Correlations! He hypothesized that the net force on a planet must vary inversely with the square of its distance from the sun. F 1/d 2
Physics I Honors 12 WEIGHT ON A PLANET Weight is dependent upon the acceleration due to gravity where the weight is measured.
Physics I Honors 13 WEIGHT GRAPH Universal Gravitation, and therefore weight, follow the inverse square law.
Physics I Honors 14 Variation of g with Altitude 1 typical space shuttle altitude 2 altitude of communication satellites 3 distance to the moon Altitude (km)G (m/s 2 ) 09.83 59.81 109.80 509.68 1009.53 400 1 8.70 35,700 2 0.225 380,000 3 0.0027
Physics I Honors 15 The math… FF = (G m 1 m 2 )/ d 2 Newton’s Inverse Square Law. FG = 6.67 10-11 N·m 2 /kg 2 –It is the Universal Gravitation Constant. –Discovered later by Cavendish.
Physics I Honors 20 How Can I be Weightless on Earth? The sensation of weight (your apparent weight) equals the force with which you press against the supporting floor. If the floor accelerates up or down, your apparent weight varies.
Physics I Honors 22 If a star collapses to half its radius and there is no change in its mass, gravitation at its surface would increase by a factor of four.
Physics I Honors 23 Which falls toward the other, A or B? Do the accelerations of each relate to their relative masses?
Physics I Honors 24 Gravitational Field Strength Is the gravitational force per unit of mass acting at a point g = F g / m Units for gravitational field strength are N/kg
Physics I Honors 25 Field lines represent the gravitational field about the Earth. Where the field lines are closer together, the field is stronger. Farther away, where the field lines are farther apart, the field is weaker.
Physics I Honors 26 Calculations about Satellites We can calculate the velocity of a satellite using the equation _________ v = (Gm e ) / r
Physics I Honors 28 What is Escape Velocity? It is the slowest speed with which we can launch a projectile so that it will never fall back to the earth.
Physics I Honors 29 What kinds of orbits? A launch speed of –8 km/s will give a circular orbit –8 to 11.2 km/s will give an elliptical orbit –11.2 km/s orbit is parabolic, it escapes –Above 11.2 km/s, the orbit is hyperbolic
Physics I Honors 30 What about the Period of the Satellite? We use the equation ___________ T = 2 (r 3 ) / (Gm e )
Physics I Honors 31 Geosynchronous Orbits The satellite remains stationary above one place on the earth. Characteristics –Must have the same period as the earth’s period of rotation Which is 24 hours.
Physics I Honors 32 The Astronomical Unit This is a unit of measure equal to the average distance between the earth and the sun. It is designated as 1 A.U. 1 = 93 million miles. 1 A.U. = 1.5 X 10^11 m. The A.U. is used to measure astronomical distances.