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Motion Near the Earth's Surface
Gravitational Forces
- Forces cause accelerations
- All forces are applied without contact. The chair you sit on, is not really touching you. What keeps you from falling onto the floor, is the forces between the electrons in the atoms in your body, and the electrons in the atoms in the chair you sit on. Like charges repel each other, and so the electrons in your body repel the electrons in the chair, and this hold you up.
- In spite of forces applying without contact, Newton's laws apply in all situations.
- Objects in free fall experience a force that accelerates them at -9.81 m/s2. This force is the force due to gravity (Fg).
- Gravitational field constant (g) - The ratio of the gravitational force to the mass.
- Gravitational field constant is an acceleration.
- For near the Earth's surface it is -9.81 m/s2 or -9.81 N/kg.
- Gravitational field constant is a vector.
- The direction of this vector is towards the center of the object producing the gravity.
Weight
- Weight - The gravitational force acting on a mass.
- The SI units of weight are units of force, in other words, the Newton (N). Note: the kilogram is a unit of mass and not a unit of force!
- Weight is a vector quantity.
- If gravity is present, then an object has weight.
- Weight is measured by using a spring scale.
- In this last equation, both k and m are constants. Therefore, if the gravitational field constant changes (as when you go to the moon g = -1.6 N/kg), then the amount of stretch of the spring changes. This means that you would weigh differently.
- Joey is in an elevator. His mass is 55 kg. Joey is a bit strange. He likes to bring his bathroom scale with him when he rides in the elevator. As the door closes, if Joey is standing on his bathroom scale, what weight does it read?
- Now, suppose that after the elevator doors close, the elevator accelerates upwards at 1.5 m/s2. What is the new reading on the scale?
- As the elevator comes to a stop it must slow down. If the rate of slowing is 1.5 m/s2 [down], what does the scale read?
- As the door opens, Joey sees Ricky outside the elevator door with a chain saw and a very large grin on his face. Joey quickly closes the door and pushes the main floor button. The elevator now accelerates downwards at 1.5 m/s2. What value would the scale now read?
- Ricky cuts a hole through the closed elevator door with the chain saw, and also cuts the elevator cable. While the elevator falls, and Joey enjoys the ride down, what would he notice the scale read?
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