Bell Work: Hannah just finished building a house of cards that stands four-stories high. She is worried that it will fall down. Which of the following statements is definitely true? A house of cards is too fragile and must fall down eventually. If Hannah adds another card to the house of cards, then it will fall down. If no unbalanced force acts upon the house of cards, then it will remain standing forever. As long as nobody touches the house of cards, it will remain standing.

Objectives Identify and explain how Newton’s laws of motion relate to the movement of objects (SPI 0707.11.4).

Forces Pushes or pulls May cause acceleration [changes in motion] May also cause changes in shape

Balanced Forces There is no motion All forces acting on an object are equal

Unbalanced Forces Causes motion!! All forces acting on an object are not equal One or more force is stronger than others

Gravity Force that attracts all objects toward each other The Law of Universal Gravitation Part 1: More mass = more gravity Part 2: Decreases as distance increases Acceleration because of gravity is 9.8 m/s/s for all objects

Friction Force that slows down motion Air resistance creates friction in most situations

Net Force The sum of all forces acting on an object A net force of 0 No motion A net force of more than 0 Motion occurs

Resultant Another term for net force http://www.brainpop.com/science/motionsfor cesandtime/force/

Newton’s First Law of Motion An object at rest remains at rest, and an object in motion remains in motion at a constant speed and in a straight line unless acted on by an unbalanced force. Newton’s first law of motion is sometimes called the law of inertia. Inertia is “want to,” when an object resists change because it “wants to” keep going in a straight line. Inertia depends on an object’s mass. The more mass an object has, the more inertia an object has. Examples A CAR SUDDENLY STOPS AND STRAIN AGAINST THE SEATBELT A DODGE BALL SITS MOTIONLESS UNTIL SOMEONE KICKS IT

Examples of Newton’s 1st Law a) A car suddenly stops and you strain against the seat belt b) When riding a horse, the horse suddenly stops and you fly over its head c) Lawn bowling on a cut and rolled lawn verses an uncut lawn d) A car turns left and you appear to slide to the right

Newton’s Second Law of Motion The acceleration of an object depends on the mass of the object and the amount of force applied. Weight vs. Mass Weight is a measure of gravity on an object. Mass is the amount of matter (what an object is made of) in an object. F (force) = m (mass) x a (acceleration) a (acceleration) = F (force) ÷ m (mass) a = F ÷ m F = m x a

Newton’s Second Law of Motion The acceleration of an object depends on the mass of the object and the amount of force applied. If you want to increase the acceleration of an object, you need to decrease its mass. More mass means more force needed to accelerate at the same rate. Examples An empty shopping cart is easier to push than a full one An engine used less work to move a lighter car than when it moved a heavier car.

What does it mean? If you place a force on an object, 1. It accelerates in the direction…that you push it 2. If you push twice as hard…It accelerates twice as much. 3. If it gets twice the mass…It accelerates half as much.

Newton’s Third Law of Motion For every action there is an equal and opposite reaction. Newton’s third law can be simply stated as follows: All forces act in pairs. Examples: How a helicopter rises vertically because of the reaction to the downward push against the air produced by its propellers A soccer player kicks a ball with their foot and their toes are left stinging.

Example #1: Action: Baseball pushes glove leftwards Reaction: ????

The glove pushes the baseball rightward. Example #1 Answer: The glove pushes the baseball rightward.

Example #2: Action: Bowling ball pushes pin leftwards Reaction: ????

Pin pushes bowling ball rightward. Example #2 Answer: Pin pushes bowling ball rightward.

Some Real-Life Applications of Newton's Third Law 1. An athlete executes a high jump by pushing against the earth which, in turn, pushes him/her up in the air. 2. We swim by pushing against the water, which reacts, propelling us ahead. 3. A helicopter rises vertically because of the reaction to the downward push against the air produced by its propellers. 4. Water of high pressure hoses leaves the nozzle with great speed resulting in a thrust which pushes the hose with such force that one person cannot control it. Most people have experienced this to a much lesser degree with the garden hose.

Momentum Imagine a compact car and a large truck traveling with the same velocity. The drivers of both vehicles put on the brakes at the same time. Which vehicle will stop first? The car because a larger force is needed to stop the truck. The truck has more momentum because it has more mass. The momentum of an object depends on the object’s mass and velocity. The more mass an object has, the more momentum it has.