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Forces & Newton’s Laws Ch. 4

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Forces What is a force? –Push or pull one body exerts on another –Units = Newton (N) –Examples: List all of the forces that you can encounter/exert in a day

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Forces Types of forces: 1. Friction- force that opposes motion between two surfaces that are touching. –Air Resistance – Special type of frictional force. Occurs when the object is traveling through the air

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Forces 2. Applied Force – A force that is applied to an object by another object. Example: You, wind, water, etc 3.Gravitational Force – The force of gravity on an object.

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Forces Balanced forces: forces exerted on an object of equal magnitude (size) but in opposite directions. Unbalanced Forces: unequal pulling of forces on an object.

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Forces Example: In a tug of war, the net force causes an acceleration in the same direction. Are the forces balanced or unbalanced in the picture?

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Forces If forces are balanced (all forces cancel) Acceleration = 0 m/s 2 F net on object = 0 N Object is doing 1 of 2 things…

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Forces Object is doing 1 of 2 things At rest v = 0 m/s In motion “constant” velocity stays at rest stays in motion

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Forces Inertia –The tendency of an object to resist any change in motion. –The velocity of an object will remain the same unless a force acts on it. Example:

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Forces Inertia is dependent on mass. Would you expect a bowling ball and a tennis ball to have the same inertia? **Think about resistance to change motion** More mass greater inertia!!

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Forces Newton’s 1 st Law Law of Inertia An object at rest will stay at rest. An object in motion will stay in motion. Unless acted on by an outside force.

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Forces Examples: A rolling basketball would continue to roll forever if an outside force did not act on it. A rock will never move unless an outside force moves it.

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Newton’s 2 nd Law If forces are unbalanced (they do not cancel) The mass accelerates in the direction of the larger force.

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Newton’s 2 nd Law The relationship between F and a F net = ma N kg m/s 2 “net” means total

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Newton’s 2 nd Law Newton’s 2 nd Law Law of Force If a larger force exists, then there must be an acceleration in the direction of that force.

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Newton’s 2 nd Law Helpful Hints: F ma N kg ms2ms2

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Newton’s 2 nd Law Example:

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Newton’s 3 rd Law Equal and Opposite For every action there is an equal and opposite reaction Example : –Imagine a rocket is being launched from the earth. Hot gases are pushed out from the bottom of the rocket as the rocket is thrust upward. The force of the gases pushing against the surface of the earth is equal and opposite to the force with which the rocket moves upward

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Momentum A property a moving object has because of its mass and velocity Momentum depends on both mass and velocity

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Momentum Momentum can be calculated using the following equation –p= mv p= momentum v= velocity(m/s) m= mass (kg) Unit for momentum is kg·m/s

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Momentum Objects that are not moving (at rest) have ZERO momentum Example: –A bicycle and a car are traveling at the same velocity. Which has a greater momentum?

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Which has a greater momentum? WHY? The car would be have a greater momentum, because the car has a greater mass

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Law of Conservation of Momentum The amount of momentum a group of objects does not change unless and outside force acts on them. Example:

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