13 Universal Gravitation Instructions on using this PowerPoint: Please review this PowerPoint while simultaneously completing the EXERCISES: Weight, Weightlessness, and Gravity Notes. I would recommend skimming the pages in your textbook that correspond with your notes; they are labeled on your Notes (p.186, p ) It is in your best interest to think about the answer first BEFORE revealing the answer on the PowerPoint – challenge yourselves!

13 Universal Gravitation

Pressure against Earth is the sensation we interpret as weight Weight and Weightlessness

13 Universal Gravitation The force of gravity, like any force, causes acceleration. Objects under the influence of gravity are pulled toward each other and accelerate. We are almost always in contact with Earth, so we think of gravity as something that presses us against Earth rather than as something that accelerates us. #1: because we are always in contact with Earth…we are used to it! 13.3 Weight and Weightlessness

13 Universal Gravitation  Stand on a bathroom scale that is supported on a stationary floor. The gravitational force between you and Earth pulls you against the supporting floor and scale.  By Newton’s third law, the floor and scale in turn push upward on you. #2 – This pressure against Earth’s surface is the sensation we interpret as weight Weight and Weightlessness

13 Universal Gravitation The sensation of weight is equal to the force that you exert against the supporting floor. -If you stand on the scale, your weight is “normal” -When the elevator accelerates upward, the bathroom scale and floor push harder against your feet. The scale would show an increase in your weight. -#3 B -#5 C 13.3 Weight and Weightlessness

13 Universal Gravitation When the elevator accelerates downward, the support force of the floor is less. The scale would show a decrease in your weight. If the elevator fell freely, the scale reading would register zero. According to the scale, you would be weightless. You would feel weightless, for your insides would no longer be supported by your legs. **Remember – weightlessness is really a LACK of SUPPORT FORCE # 4 D # 6 A 13.3 Weight and Weightlessness

13 Universal Gravitation #7 - Rather than define your weight as the force of gravity that acts on you, it is more practical to define weight as the force you exert against a supporting floor; OR support force! According to this definition, you are as heavy as you feel. #8 – False # 9: The condition of weightlessness is not the absence of gravity, but the absence of a support force Weight and Weightlessness

13 Universal Gravitation Look at the picture: Both people are without a support force and therefore experience weightlessness. **Astronauts still have GRAVITY acting on them…they just lack a SUPPORT FORCE** 13.3 Weight and Weightlessness

13 Universal Gravitation What sensation do we interpret as weight? 13.3 Weight and Weightlessness

13 Universal Gravitation From within a rotating frame of reference, there seems to be an outwardly directed centrifugal force, which can simulate gravity. *It does NOT create gravity…it just FEELS like it* # 10 – The centrifugal force experienced in a rotating reference frame is not a real force. #11 – Real forces include: gravitational, nuclear, & electromagnetic. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation Consider a colony of ladybugs living inside a bicycle tire. If the wheel falls through the air, the ladybugs will be in a weightless condition and seem to float freely while the wheel is in free fall. ***This is similar to astronauts in space If the wheel is spinning, the ladybugs will feel themselves pressed to the outer part of the tire’s inner surface. At the right spinning speed, the ladybugs will experience simulated gravity. ***This is similar to a hypothetical rotating space station – if it is large enough, and spun at the right speed, it SIMULATES the FEELING of Earth gravity. #12 – true 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation Gravity is simulated by centrifugal force. #13 – To the ladybugs, the direction “up” is toward the center of the wheel, or the center of rotation. The “down” direction to the ladybugs is what we call “radially outward,” away from the center of the wheel. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation Need for Simulated Gravity Today we live on the outer surface of our spherical planet, held here by gravity. What happens if we can no longer inhabit Earth? In the future, people may live in huge, slowly rotating space stations where simulated gravity allows them to function normally. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation Support Force Occupants in today’s space vehicles feel weightless because they lack a support force. Future space travelers need not be subject to weightlessness. Their space habitats will probably spin, effectively supplying a support force and simulating gravity. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation The man inside this rotating space habitat experiences simulated gravity. As seen from the outside, the only force exerted on the man is by the floor. The floor presses against the man (action) and the man presses back on the floor (reaction). The only force exerted on the man is by the floor. It is directed toward the center and is a centripetal force. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation As seen from inside the rotating system: In addition to the man-floor interaction, the man FEELS a centrifugal force pressing him on the wall. It seems as real as gravity. Yet, unlike gravity, it has no reaction counterpart. Centrifugal force is not part of an interaction, but results from rotation. It is therefore called a fictitious force. Centrifugal force is actually the acceleration you feel from rotation 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation Challenges of Simulated Gravity The comfortable 1 g we experience at Earth’s surface is due to gravity. Inside a rotating spaceship the acceleration experienced is the centripetal acceleration due to rotation. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation Small-diameter structures would have to rotate at high speeds to provide a simulated gravitational acceleration of 1 g. Sensitive and delicate organs in our inner ears sense rotation. Although there appears to be no difficulty at 1 RPM, many people have difficulty adjusting to rotational rates greater than 2 or 3 RPM. *** Remember the ‘Vomit Comet’ video?** To simulate normal Earth gravity at 1 RPM requires a large structure—one almost 2 km in diameter. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs

13 Universal Gravitation This NASA depiction of a rotational space colony may be a glimpse into the future. #14 – Rotation will allow people inside the space station to experience a support force that simulates normal Earth gravity. 9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs