Motion

Kepler’s laws, Gravitational attraction, and projectile motion

An object is orbiting when it is traveling in a

circular or nearly circular path around another object

When a space shuttle orbits Earth, it is moving forward, but it is also in free fall toward earth.

These two motions combine to cause orbiting

as the shuttle follows the curve of Earth’s surface

What causes orbiting?

An object in circular motion is constantly

changing direction. An unbalanced force is needed to change the motion of any object.

The unbalanced force that causes objects to move in a circular path is called centripetal force.

Gravity provides the centripetal force that keeps objects in orbit

What causes orbiting?

1) The orbit of every planet is an ellipse with the

Sun at one of two locations

2) A line joining a planet and the Sun sweeps out equal areas during equal intervals of time

http://upload.wikimedia.org/wikipedia/en/6/69/Kepler-second-law.gif

Kepler’s Laws

Gravity is a force of attraction between objects

that is due to their masses

The law of universal gravitation describes the relationship between gravitational force, mass, and distance

The law states that all objects in the universe attract each other through gravitational force. The size of the force depends on the masses of the objects and the distance between them.

Gravitational attraction

Gravitational attraction is small between

objects with small masses and large between objects with larger masses

If the distance of two objects is increased, the gravitational force between them is reduced

F = Gm1m2/r2

Gravitational attraction

Swinging tray demo

At the Earth's surface, all objects experience a downward force due to gravity. The greater the mass, the greater the force.

F = mg

An object traveling in a circle behaves as if it is experiencing an outward force. This force is the centrifugal force. It depends on the mass of the object, the speed of rotation, and the distance from the center.

F = mv2/r

Gravitational attraction

Step 1: The water in the cup is experiencing a

force downward due to gravity. The cup does not fall because it is being supported by the tray.

Step 2: When the tray is swung in a circle, there is still a force downward due to gravity, but when the tray is upside down the cup is no longer being supported by the tray. The water does not fall, however, because it is experiencing an upward force due to its circular motion. This force is great enough to cancel out the force due to gravity.

Gravitational attraction

The curved path an object follows when thrown or

propelled near the surface of the Earth

Has both horizontal and vertical motion

The horizontal and vertical motion are independent– they have no effect on each other, but when they combine they form a curved path

What are some examples of projectile motion you can think of?

Projectile motion

Horizontal motion is parallel to the ground

Vertical motion is perpendicular to the ground

If you have a ball that is dropped and another ball that is thrown from the same height, is the acceleration the same for both?

Horizontal and vertical motion