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FORCE & MOTION Why do Newton’s Laws Matter to Me? • Sir Isaac Newton discovered answers to many questions about motion. • Many historians believe Newton’s ideas about motion were the beginning of modern science. • The laws of motion explain the relationships between forces acting on an object, the object’s mass, and its motion. FORCE • A soccer ball sits on the ground, motionless, unless you kick it. • Your binder sits on the table unless you pick it up. • If you hold your pencil above the ground and let it go, gravity pulls it to the floor. • IN ALL OF THESE CASES, THE MOTION OF THE BALL, BINDER, OR PENCIL WAS CHANGED BY SOMETHING PUSHING OR PULLING ON IT. FORCE CONTINUED • An object will speed up, slow down, or turn only if something is pushing or pulling on it. • FORCE is a push or a pull. • A force can be exerted in different ways. Example Imagine throwing a ball. Your hand exerts a force on the ball. After the ball leaves your hand, gravity’s force on it causes its path to curve downward. When the ball hits the ground, the ground exerts a force, stopping the ball. How Do Scientists Measure and Describe Forces? • More than 1 force can act on an object at the same time. • Example: You hold a paper clip near a magnet. You, the magnet, and gravity all exert forces on the paper clip. • The combination of ALL the forces acting on an object is called the net force. • When more than one force is acting on an object, the net force determines the motion of the object. • In our example, the paper clip is not moving, so the net force is zero. Combining Forces • Forces in the same direction add together to form the Net Force. • The net force is the difference between 2 forces that are in the opposite direction. The net force is also in the direction of the larger force. EXAMPLE You and friend are standing on opposite sides of a door. You are both pushing against the door, but the door does not move. The Force that each of you is exerting on the door is equal and in the opposite direction. The Net Force is therefore zero. EXAMPLE You and your friend are on opposites sides of the door again. Only this time the door starts to close. This is because the force you are exerting on the door is greater than the force your friend is exerting to try to keep the door open. The Net Force is the difference between the force that you and your friend are exerting. Yours is the larger force that is why the door is closing. BALANCED & UNBALANCED FORCES • A Force can act on an object without causing it to accelerate if the other forces cancel the push or pull of the force. • EXAMPLE: You and your friend pushing on the door and it didn’t move. The two forces are equal in opposite directions and therefore, cancel each other out. • BALANCED FORCES: Two or more forces exerted on an object that cancel each other’s effects and do not cause a change in the object’s motion. The Net Force is zero. • UNBALANCED FORCES: The forces do not cancel each other and the motion of an object changes. The Net Force does not = zero. EXAMPLE You and your friend pushing on a door. The door starts to close even though your friend is pushing to hold it open. NEWTON’S LAWS OF MOTION • NEWTON’S FIRST LAW OF MOTION: An object at rest remains at rest unless an unbalanced force acts on it and causes it to move. • EXAMPLE: If you stand on a skateboard and someone pushes you, you and the skateboard start moving. You began to move when the force was applied. • Newton’s First Law says if there in NO net force acting on an object the object remains at rest, OR, if the object is already moving, it continues to move in a straight line with constant speed. • A Force does not need to be constantly applied to an object to keep it moving. • The motion of an object does not change until an unbalanced force acts on it. Friction • FRICTION: Rubbing force that acts against the motion between two touching surfaces and always slows an object down. • Friction is the force that brings nearly everything to a stop. • EXAMPLE: A baseball or softball player sliding into a base. The friction between the ground and the player slows them down. • EXAMPLE: When you’re riding your skateboard, you have to keep pushing because of the friction between the road or sidewalk and the wheels. • Friction is why you never see objects moving with constant velocity unless a net force is applied. • Friction always acts on objects that are sliding or moving through air or water.