Overview
This lecture explains the concepts of gravity, weight, and gravitational potential energy, providing key formulas and example calculations relevant to physics studies.
Gravity and Gravitational Fields
- Gravity is a force of attraction between two objects, depending on their masses and distance apart.
- Small masses or large distances make gravity negligible, while large, nearby objects (like Earth or the Moon) have a strong gravitational effect.
- The gravitational field is the area around an object where its gravity can be felt.
- Gravitational field strength (g) measures the strength of gravity on an object; on Earth, g ≈ 9.8 N/kg, and on the Moon, g ≈ 1.6 N/kg.
Weight and Mass
- When an object is in a gravitational field, it experiences a force called weight.
- Weight is calculated as: Weight = Mass × Gravitational Field Strength (W = mg).
- Example: A 60 kg person on Earth has a weight of 60 × 9.8 = 588 N.
- Mass is an intrinsic property of an object; weight is the force due to gravity acting on that mass.
- In physics, mass and weight are distinct; mass is measured in kilograms (kg), and weight in newtons (N).
Gravitational Potential Energy
- Lifting an object against gravity requires energy, which is stored as gravitational potential energy (GPE).
- GPE formula: Ep = mgh (mass × gravitational field strength × height).
- GPE is measured in joules (J).
- Example: Raising a 0.1 kg apple 3 m on Earth gives Ep = 0.1 × 9.8 × 3 = 2.94 J.
Key Terms & Definitions
- Gravity — the attraction between two objects due to their mass.
- Gravitational Field — the region around a mass where its gravity affects other objects.
- Gravitational Field Strength (g) — force per kilogram due to gravity (N/kg).
- Weight (W) — the force on a mass in a gravitational field (W = mg), measured in newtons.
- Mass (m) — the amount of matter in an object, measured in kilograms.
- Gravitational Potential Energy (Ep) — energy stored due to an object’s position in a gravitational field (Ep = mgh).
Action Items / Next Steps
- Practice distinguishing between mass and weight in example problems.
- Memorize the formulas for weight (W = mg) and gravitational potential energy (Ep = mgh).
- Convert units correctly (e.g., grams to kilograms) in calculations.