Gravity and Motion

1. Gravity and Motion Pgs. 36 - 42

2. Falling Objects • If you drop a marble and a baseball from the same height at the same time, which hits the ground first? • Give up? Well in the late 1500’s a scientist by the name of Galileo proved that they hit at the same time. • He dropped two cannonballs of different masses from the Leaning Tower of Pisa and they hit the ground at about the same time.

3. Acceleration due to Gravity • Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects. • This may seem odd, but is true. • The force of gravity is greater between Earth and an object with a large mass than between Earth and a less massive object, so you may think that acceleration due to gravity should be greater too. • A greater force must be applied to a large mass than to a small mass to produce the same acceleration, so the difference in force is canceled by the difference in mass.

4. Accelerating at a constant rate • All objects accelerate toward the Earth at a rate of 9.8 meters per second per second(9.8m/s/s). • For every second an object falls, it increases its velocity downward by 9.8m/s.

5. Air Resistance Slows down Acceleration • If you drop two sheets of paper, one crumpled and the other flat, one will fall to the ground quicker than the other. • The flat paper falls slower because of fluid friction that opposes motion of objects through air. • It is also known as air resistance. • It occurs between the surface of the falling object and the air that surrounds it.

6. Air Resistance affects some objects more than others • The amount of air resistance acting on an object depends on the size and shape of the object. • The more surface area of the falling object, the more air resistance it encounters. • The force of air resistance works against the force of gravity to produce the net force acting on the falling object. • Some objects have more or less air resistance, causing them to fall quicker or slower than other objects.

7. Acceleration Stops at the Terminal Velocity • The falling object accelerates downward as long as the net force of falling object is not zero. • The air resistance acting on an object increases as the speed of the object increases. • The resistance will increase until it equals the force of gravity. • At this point, the net force is zero, and the falling object is moving at a constant velocity, this is terminal velocity. • Without terminal velocity, objects would accelerate until impact, which could be very damaging to people on Earth.

8. Free Fall • An object in free fall is falling with only the force of gravity pulling down on it. • This means it does not experience any air resistance. • This can only happen where there is no air, like in a vacuum (space) where there is no matter. • Describing a skydiver to be in free fall is an incorrect statement because they fall in air.

9. Orbiting Objects • Why do astronauts float in space? • Some say that they are weightless in space. • In fact, the astronauts in space are in free fall. • They do not experience any air resistance in space.

10. Cause of Orbiting • Objects are said to be in orbit when it is traveling in a circular path around another object. • When objects orbit they are moving forward but are also in free fall. • These two motions work together to make an orbit. • Earth and other planets orbit the sun and do so in a circular path. • The planets orbit this way due to centripetal force, or inward force and gravity provides that force. • The centripetal force felt by the moon points toward the Earth since Earth’s gravity is pulling it near.

11. Projectile Motion and Gravity • Projectile motion is the curved path an object follows when thrown or propelled near the surface of the Earth. • The motion of thrown balls, hopping frogs, and even arrows shot from a bow are projectile motion. • Projectile motion has two components: horizontal and vertical. • Each component has no effect on the other. • They are separate but form a curved path together.

12. Horizontal Motion • When you throw a ball, your hand exerts a force on the ball that makes the ball move forward. • The force gives the ball horizontal motion which is motion that is parallel to the ground. • There are no forces(except air resistance) to change the ball’s horizontal motion once you let go. • Therefore, the ball’s horizontal velocity is constant.

13. Vertical Motion • After you throw an object, gravity pulls it down. • This downward pull is vertical motion and is perpendicular to the ground. • Since objects in projectile motion accelerate down you always have to aim above a target. • Gravity pulls objects in projectile motion down with an acceleration of 9.8m/s/s just like all falling objects.