Transcript 5-8 - mrhsluniewskiscience

Chapter 5
Circular Motion; Gravitation
Objectives
1. Use Newton's second law of motion, the
universal law of gravitation, and the concept of
centripetal acceleration to solve problems
involving the orbital motion of satellites.
2. Explain the "apparent" weightlessness of an
astronaut in orbit.
5-8 Satellites and “Weightlessness”
Satellites are routinely put into orbit around the
Earth. The tangential speed must be high
enough so that the satellite does not return to
Earth, but not so high that it escapes Earth’s
gravity altogether.
5-8 Satellites and “Weightlessness”
The satellite is kept in orbit by its speed – it is
continually falling, but the Earth curves from
underneath it.
5-8 Satellites and “Weightlessness”
Objects in orbit are said to experience
weightlessness. They do have a gravitational
force acting on them, though!
The satellite and all its contents are in free fall, so
there is no normal force. This is what leads to the
experience of weightlessness.
5-8 Satellites and “Weightlessness”
More properly, this effect is called apparent
weightlessness, because the gravitational force
still exists. It can be experienced on Earth as
well, but only briefly:
• The velocity of a satellite keeps it in orbit
• Even when moving, the satellite is actually
accelerating toward the Earth (this is what
keeps it in its circular path)
• Its acceleration results in a curved path which
is the same as the curve of the Earth
• Gravity is providing the centripetal force
Perception of
Weightlessness
• There is still gravity acting in a satellite
(about 8.9 m/s2), so why do we feel
weightless?
• In an free falling elevator, if the FA is equal
to the FG, there is no FN
• No force is felt feel weightless – called
apparent weightlessness
• Weightlessness that you feel in a
satellite is like the weightlessness in an
elevator
• The satellite and everything on it are all
accelerating toward the earth at the
same rate
Practice Problem # 1
• Calculate the force of gravity on a
spacecraft 12,800 km (2 Earth radii) above
the Earth’s surface if its mass is 1400 kg.
Practice Problem #2
• At the surface of a certain planet, the
gravitational acceleration g has a
magnitude of 12.0 m/s2. A 2.10 kg brass
ball is transported to this planet. What is:
– The mass of the brass ball on the Earth and
on the planet?
– The weight of the brass all on the Earth and
on the planet?
Practice Problem #3
• A satellite moves in a circular orbit around
Earth at a speed of 5,000 m/s.
(a) Determine the satellite's altitude above
the surface of Earth.
(b) Determine the period of the satellite's
orbit.
• RE = 6378 km and ME = 5.98 x 1024 kg
Practice Problem #4
• A 600-kg satellite is in a circular orbit
about Earth at a height above Earth equal
to Earth’s mean radius. Find (a) the
satellite’s orbital speed, (b) the period of
its revolution, and (c) the gravitational
force acting on it.
Homework
• Problems in Chapter 5
• #43, 45, 48, 50, 60