Forces. Net force is the combination all of the forces acting on an object. All forces have both size and direction.

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1 Objectives Forces Describe forces, and explain how forces act on objects. Determine the net force when more than one force is acting on an object. Compare balanced and unbalanced forces. Describe ways that unbalanced forces cause changes in motion. Forces Acting on Objects force a push or a pull exerted on an object. All forces have both size and direction. Determining Net Force Net force is the combination all of the forces acting on an object. Measured in units called the newton (N). 1kg m/s/s =1N 1N = 0.225lbs Determining Net Force, continued Forces in the Same Direction are added to determine the net force Forces in Different Directions can be found by subtracting the smaller force from the larger one. 1

2 Balanced and Unbalanced Forces Balanced Forces equal and opposite forces produce a net force of 0 N Balanced forces will not cause a change in the motion of a moving object. Balanced and Unbalanced Forces, continued Unbalanced Forces When the net force on an object is not 0 N, Cause an object to accelerate Objectives Explain why friction occurs. List the three types of friction,and give examples of each type. Explain how friction can be both harmful and helpful. 2

3 The Source of Friction Friction is a force that opposes motion between two surfaces that are in contact. 3 kinds Sliding Rolling Fluid the rougher the surface is, the greater the friction is. Friction: Harmful and Helpful Friction can be both harmful and helpful, Car tires pencil Some Ways to Reduce Friction Lubricants ---motor oil, wax, and grease. rolling kinetic friction--wheels Ball bearings make surfaces smoother 3

4 Some Ways to Increase Friction Making surfaces rougher sand scattered on icy roads increase the force pushing the surfaces together.. Objectives Describe gravity and its effect on matter. Explain the law of universal gravitation. Describe the difference between mass and weight. The Effects of Gravity on Matter Gravity is a force of attraction between objects that is due to their masses. Strength of the force depends on mass and distance. in mass = in distance = in gravity in gravity Newton and the Study of Gravity The Core of an Idea Why do objects fall toward the Earth? What keeps the planets moving in the sky? In 1665, British scientist Sir Isaac Newton, as legend has it, he saw an apple falling from a tree. 4

5 Newton and the Study of Gravity, continued Newton knew unbalanced forces are needed to change the motion of objects. an unbalanced force made the apple fall. an unbalanced force kept the moon moving around the Earth. He proposed that these two forces are actually the same force gravity. Newton and the Study of Gravity, continued The Birth of a Law Newton summarized his ideas about gravity in a law known as the law of universal gravitation. This law describes the relationships between gravitational force, mass, and distance. The law is called universal because it applies to all objects in the universe. The Law of Universal Gravitation Orbiting Planets and Gravity Part 1: Gravitational Force Increases as Mass Increases Gravitational force is small between objects that have small masses. Gravitational force is large when the mass of one or both objects is large. Part 2: Gravitational Force Decreases as Distance Increases Gravitational force is strong when distance between two objects is small. If the distance between two objects increases, the gravitational force pulling them together decreases rapidly. Are you attracted to the person next to you by gravity? Yes you both have mass 5

6 FYI Weight as a Measure of Gravitational Force The Differences Between Weight and Mass Weight is related to mass, but they are not the same. Weight is a measure of the gravitational force on an object. Weight changes whenever gravitational force changes. Mass is the amount of matter in an object. An object s mass does not change if gravitational force changes. 6