Unit 5 of Physics: Circular Motion and Gravity.
This unit covers centripetal force, orbital motion and universal gravitation — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers centripetal force, orbital motion and universal gravitation — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Centripetal Force
Centripetal force is the net force directed toward the center of a circular path that keeps an object moving in a circle. It is not a new type of force — it is provided by real forces such as tension, gravity, friction, or normal force. Without it, the object would move in a straight line (Newton's First Law).
Key Points
- Formula: Fc = mv²/r, where m is mass, v is speed, r is radius
- Always points toward the center of the circle (inward)
- Greater speed or smaller radius requires greater centripetal force
- Centripetal acceleration: ac = v²/r (directed inward)
A 1,200 kg car travels at 20 m/s around a flat circular curve of radius 80 m. What centripetal force is needed, and what provides it?
Use Fc = mv²/r: Fc = (1200)(20²)/80 = (1200)(400)/80 = 6,000 N. On a flat road, friction between the tires and road is the only horizontal force, so friction provides the 6,000 N centripetal force. If friction cannot supply this force (e.g., icy road), the car slides outward.
2 Orbital Motion
An orbiting object is in continuous free fall toward the central body while moving fast enough sideways that it keeps missing it. Gravity provides the centripetal force for circular orbits. For a circular orbit, setting gravitational force equal to centripetal force allows you to find orbital speed and period.
Key Points
- Orbital speed: v = √(GM/r), depends on central mass and radius, NOT the satellite's mass
- Orbital period: T = 2πr/v (combine with above to get T in terms of r and M)
- Higher orbit → slower orbital speed, longer period
- Kepler's Third Law: T² ∝ r³ (for orbits around the same central body)
The International Space Station orbits Earth at a radius of 6.77 × 10⁶ m. Earth's mass is 5.97 × 10²⁴ kg and G = 6.67 × 10⁻¹¹ N·m²/kg². Find the ISS orbital speed.
Use v = √(GM/r): v = √((6.67×10⁻¹¹)(5.97×10²⁴) / 6.77×10⁶). The numerator is approximately 3.98×10¹⁴, divided by 6.77×10⁶ gives 5.88×10⁷, and the square root is approximately 7,668 m/s (~7.7 km/s). Notice the ISS mass was never needed — orbital speed is independent of the orbiting object's mass.
3 Universal Gravitation
Newton's Law of Universal Gravitation states that every mass attracts every other mass with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. This force acts along the line connecting the two masses and follows Newton's Third Law — both objects feel equal and opposite forces.
Key Points
- Formula: Fg = Gm₁m₂/r², where r is the distance between centers of mass
- G = 6.67 × 10⁻¹¹ N·m²/kg² (given on most exams)
- Doubling distance reduces force by a factor of 4 (inverse-square law)
- On Earth's surface, Fg = mg; combining with the universal law gives g = GM_Earth/R_Earth²
Two objects have a gravitational attraction of F. If the distance between them is tripled and one object's mass is doubled, what is the new gravitational force in terms of F?
Original force: F = Gm₁m₂/r². New force: F_new = G(2m₁)(m₂)/(3r)² = 2Gm₁m₂/9r². Comparing to the original, F_new = (2/9)F. Tripling the distance alone would reduce F by 1/9; doubling a mass multiplies by 2, giving a net factor of 2/9.
Questions, answered.
What is Circular Motion and Gravity?
Circular Motion and Gravity is Unit 5 of Physics, covering centripetal force, orbital motion and universal gravitation.
How to study for Physics Unit 5?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 28+ review questions across 5 different game modes.