A-level Physics (Advancing Physics)/Circular Motion/Worked Solutions
1. A tennis ball of mass 10g is attached to the end of a 0.75m string and is swung in a circle around someone's head at a frequency of 1.5Hz. What is the tension in the string?
2. A planet orbits a star in a circle. Its year is 100 days, and the distance from the star to the planet is 70 Gm from the star. What is the mass of the star?
100 days = 100 x 365.24 x 24 x 60 x 60 = 3155673600s
3. A 2000kg car turns a corner, which is the arc of a circle, at 20kmh-1. The centripetal force due to friction is 1.5 times the weight of the car. What is the radius of the corner?
20kmh-1 = 20000 / 3600 = 5.56ms-1
This is a bit unrealistic, I know...
4. Using the formulae for centripetal acceleration and gravitational field strength, and the definition of angular velocity, derive an equation linking the orbital period of a planet to the radius of its orbit.
So, orbital period squared is proportional to radius of orbit cubed. Incidentally, this is Kepler's Third Law in the special case of a circular orbit (a circle is a type of ellipse).