Gravity and Motion

Standards P.8.B.1 Students know the effects of balanced and unbalanced forces on an object’s motion. E/S P.8.B.3 Students know every object exerts gravitational force on every other object, and the magnitude of this force depends on the mass of the objects and their distance from one another. I/S

Syllabus Objectives 4.2 The student will describe the relationship between mass and gravity. 4.3 The student will describe the following interactions: speed, velocity, and acceleration. 4.4 The student will draw and interpret diagrams and graphs representing an object’s motion. 4.5 The student will describe the following forces: gravity, magnetism and friction.

Syllabus Objectives Continued 4.6 The student will provide evidence to demonstrate the relationship between force and motion. 4.7 The student will identify the forces acting on moving and stationary objects 4.8 The student will determine the net force on an object given a diagram. 4.9 The student will describe the relationship between distance and gravity The student will explain that every object exerts gravitational force on every other object.

“In Question” What are the forces that you experience when you go down a slide at a park?

Gravity Gravity: A force of attraction between objects that is due to their masses. All matter has mass. Gravity is a result of mass. Therefore, all matter experiences gravity (a pull toward all other objects). The mass of most objects is too small to cause an attraction large enough to move objects toward each other.

Earth’s Gravitational Force Compared with all the objects around you, Earth has an enormous mass. Therefore, Earth’s gravitational pull is very large. You must apply forces to overcome Earth’s gravitational force any time you lift objects.

The Law of Universal Gravitation 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 force increases as mass increases. Gravitational force increases as distance between the objects decreases.

Weight is a Measure of Gravitational Force Weight is a measure of the gravitational force exerted on an object. Weight can also be a measure of the gravitational force exerted on objects by the moon or other planets.

Weight and Mass Weight and mass are not the same thing. Weight changes when the gravitational force changes. Check out your weight in places where the gravitational forces are different. Mass is the amount of matter in an object, and its value does not change.

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 All objects accelerate toward Earth at a rate of 9.8 meters per second per second. (9.8m/s²)

Air Resistance Air resistance slows down acceleration. Air provides a frictional force to falling objects. 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

Acceleration Stops at the Terminal Velocity As long as the net force on a falling object is not zero, the object accelerates downward. The amount of air resistance on an object increases as the speed of the object increases.

As an object falls, the upward force of air resistance continues to increase until it exactly matches the downward force of gravity. When this happens, the net force is zero, and the object stops accelerating.

Terminal Velocity When an object stops accelerating as it falls, (falls at a constant velocity because the net force is zero), we say that the object has reached Terminal Velocity.

Free Fall An object is in free fall only if gravity is pulling it down and no other forces are acting on it. Can a skydiver truly be in free fall?

Answer: No, a sky diver is not really in free fall. Air resistance is acting on them whether their parachute is open or not. Free fall can only truly occur in a vacuum (where there is no air resistance).

Orbiting Objects are in free fall Astronauts “float” in orbiting space-ships because of free fall. Astronauts are constantly falling around the Earth without wind resistance.

Orbit: Traveling in a circular (or nearly circular) path around another object. The space shuttle is constantly being pulled toward the earth as it moves forward. Therefore it is moving both down and forward constantly.

“In Question” What are the forces that you experience when you ride on a Ferris wheel or a merry-go-round?

Centripetal Force The unbalanced force that causes objects to move in a circular path. The word “centripetal” means “toward the center”

“Out Question” Think about the forces we have just talked about. Describe how you would experience these forces in a regular day.