Force and Gravity

Force Force is a push or a pull on an object in a certain direction. Forces can cause an object to start moving or stop, change speed or direction, and even change shape. Forces are usually measured in Newtons (N) Vectors are arrows that represent the strength and direction of a force

Net Force Forces can work together or in opposite directions. The combination of all forces acting on an object is called net force.

Balanced vs. Unbalanced Forces Two or more opposite forces are balanced forces if their effects cancel each other and do not cause a change in the object’s motion.

Balanced vs. Unbalanced Forces If one force is stronger than the other, the forces are unbalanced. Unbalanced forces cause a change in motion; speed and/or direction.

The Force of Friction Friction is a force that opposes motion between two surfaces that are in contact. Friction occurs because the surface of any object is rough, even if it feels smooth. Friction is greater between surfaces that are rough. Kinetic Friction (moving) is caused by sliding, rolling, or fluid resistance (air or liquid). Static Friction (non-moving) occurs when a force applied is not enough to overcome the force of friction.

Slide a book across a table and watch it slide to a rest position Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force - that force being the force of friction - which brings the book to a rest position. Inertia Friction

You can increase friction by making surfaces rougher, or by increasing the force that pushes the surfaces together. You can decrease force by using lubricants (oils, wax or grease) , smoother surfaces, and rolling instead of sliding.

The Force of Gravity Gravity is a force of attraction between two objects that is due to their mass. It can change an object’s motion by changing speed or direction. Gravitational force pulls object toward each other, but the force must be large enough for the objects to actually move. THAT is why objects fall toward the earth… its mass is HUGE!

The Law of Universal Gravitation says that all objects in the universe attract each other through gravitational force, depending on the masses of the objects and the distance between them. The gravitational force increases as masses increase and decreases as distance increases.

Weight is related to mass, but they are not the same. Weight is the measure of gravitational force on an object, which is different on earth than on the moon or other planets. Mass is just the amount of matter in an object, so it never changes.

Gravity and Motion Objects fall to the earth at the same rate because acceleration due to gravity is the same for all objects. Why is this true?

Acceleration depends on both force and mass. While heavier objects have a greater gravitational force than lighter objects, they are also harder to accelerate due to more mass. So a greater mass exactly balances the additional gravitational force. All objects accelerate towards earth at 9.8 m/s2

Acceleration stops at terminal velocity as a result of air resistance, which depends on the size, shape and speed of the object. Free fall is when gravity is pulling down on an object and no other forces are acting on it. So it only occurs when there is no air. Is this really a free fall?

Orbiting Objects Orbiting objects are in free fall. When a space craft orbits the earth, it is moving forward while also free falling toward the earth. The astronauts are also in free fall, that why they float! Is this really “weightlessness”

The moon orbits the earth, and the earth and other planets orbit the sun. These objects move in a circular path due to an unbalanced force called centripetal force, caused by gravity. The orbit is the result of centripetal force combined with forward motion. There is a perfect combination of mass, distance, and speed that causes the moon to move in a circular path around the earth.