Physical Science / Unit 7: Motion and Forces
7.5 Friction and Gravity
Notes
- Friction opposes motion between surfaces in contact. Types: static (before sliding), sliding, rolling, and fluid friction (air resistance). Rough surfaces and heavier loads increase friction.
- Gravity is the attractive force between all masses. Near Earth's surface, free-fall acceleration is $g = 9.8 \text{ m/s}^2$.
- Weight is the force of gravity: $W = mg$. Mass is the same everywhere; weight depends on gravity.
- Example: A 5 kg object weighs $5 \times 9.8 = 49$ N.
- Without air resistance, all objects fall at the same rate. Terminal velocity is reached when air resistance equals weight.
Practice
What is the weight of a 60 kg person on Earth?
$W = mg = 60 \times 9.8 = 588$ N.
Which would reduce sliding friction between a box and the floor?
Wheels and lubricants reduce friction.
An astronaut on the Moon has a mass of 70 kg. Compared to Earth, her mass on the Moon is:
Mass does not depend on location; only weight does.
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