Unit 5: Circular Motion and Gravitation
Why turning is a kind of acceleration, and the one force law that keeps both an apple and the Moon falling.
7 topics · 27 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Uniform Circular Motion
- How it worksHow does velocity keep changing direction even while its size stays exactly the same?
- Worked exampleFind the speed of a satellite completing one 40000 km orbit every 5000 seconds.
- Common mistakeWhy doesn't constant speed mean constant velocity for something moving in a circle?
- Going furtherWhat happens to the centripetal acceleration of an object in circular motion when its speed doubles but the radius stays fixed?
- Ties togetherHow does uniform circular motion reappear as the sideways half of a satellite's orbital motion around a planet?
Your turn1 of 2
Find the speed of a satellite completing one 42392 km orbit every 5367 seconds.Centripetal Force
- How it worksHow does a net force pointed toward the center keep an object curving around a circle instead of flying straight?
- Worked exampleFind the centripetal force needed to keep a 2 kg ball moving in a 0.5 m circle at 4 m/s.
- Common mistakeWhy isn't there really a centrifugal force flinging you outward on a spinning ride?
- Common mistakeWhy doesn't a ball on a string fly straight outward away from the center the instant the string snaps?
- Going furtherWhat happens to an object in circular motion the instant the centripetal force disappears?
Your turn1 of 2
Find the centripetal force needed to keep a 13.96 kg ball moving in a 6.56 m circle at 18.68 m/s.Newton's Law of Universal Gravitation
- How it worksHow does gravitational force depend on both masses and fall off with the square of the distance between them?
- Worked exampleFind the gravitational force between two 1000 kg objects separated by 10 meters.
- Common mistakeWhy doesn't doubling the distance between two objects only cut the gravitational force in half?
- Going furtherWhat happens to the gravitational force between two planets when their distance apart is tripled?
Your turn1 of 2
Find the gravitational force between two objects of mass 3497 kg and 3263 kg separated by 93 meters. Use G = 6.674 times 10−11 N m2/kg2.Orbital Motion
- How it worksHow does a satellite's sideways speed combine with its constant falling to trace a closed curve around a planet?
- Common mistakeWhy isn't there zero gravity in low Earth orbit even though astronauts float?
- Going furtherWhat happens to a satellite's orbital speed when it is moved into a higher orbit?
Your turn1 of 2
Find the orbital speed of a satellite at a distance of 31253162 meters from Earth's center, using Earth's standard gravitational parameter GM of 3.986 times 1014 m3/s2.Weightlessness and Apparent Weight
- How it worksHow does free fall make a scale read zero even while gravity keeps pulling at full strength?
- Common mistakeWhy isn't the Moon a place with no gravity at all?
- Common mistakeWhy doesn't climbing a few hundred kilometers above Earth put astronauts far enough away to escape most of its gravity?
- Going furtherWhat happens to your apparent weight on a scale during the first second an elevator drops?
Your turn1 of 2
Astronauts float freely inside the International Space Station, drifting through the cabin as if gravity had no effect on them at all, yet Earth's gravity at that altitude is only slightly weaker than it is on the ground below. Why do astronauts float inside the International Space Station even though gravity there is almost as strong as on Earth's surface?Kepler's Laws of Planetary Motion
- How it worksHow does a planet sweeping out equal areas in equal times fall right out of gravity acting alone?
- Common mistakeWhy isn't a planet's orbit around the Sun a perfect circle?
- Going furtherWhat happens to a planet's orbital period when its average distance from the Sun increases?
Your turn1 of 2
A comet following an elongated elliptical orbit noticeably speeds up as it swings closest to the Sun and noticeably slows down as it swings out to the farthest point of its orbit, rather than moving at one constant speed the whole way around. Why does a planet speed up when it swings closest to the Sun and slow down when it's farthest away?Artificial Gravity from Rotation
- How it worksHow does the centripetal force from spinning substitute for the pull of gravity pressing you against a floor?
- Common mistakeWhy isn't the floor of a spinning space station actually pulling astronauts downward with gravity?
- Going furtherWhat happens to the artificial gravity astronauts feel when a space station's rotation speeds up?
Your turn1 of 2
A spinning space station has a radius of 350 meters, and a point on its rim moves at 39 m/s. What artificial gravitational acceleration does that rotation create at the rim?