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## FORCE

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**FORCE**The cause of acceleration**Force: a Push or a Pull**Examples: • Muscular forces • Force of gravity • Friction force • Electrical and Magnetic forces • Nuclear force**Force is a Vector**• Force has magnitude and direction F q**Aristotle’s Motion**• Natural Motion is up or down • Down for falling objects • Up for smoke • Circular for heavenly bodies since without end • Violent Motion • Due to imposed forces such as wind pushing a ship or someone pulling a cart • Natural state of motion is rest • A force is needed to keep something moving**Aristotle’s Basic Error**• Friction not understood as a force**Galileo’s Motion**• Force is a push or a pull • Friction is a force that occurs when objects move past each other • Friction due to tiny irregularities • Only when friction is present is a force required to keep something moving**Ball rolling downhill speeds up**Ball rolling uphill slows down He asked about ball on smooth level surface Concluded it would roll forever in absence of friction Galileo’s Inclined Planes**Inertia**• Resistance to change in state of motion • Galileo concluded all objects have inertia • Contradicted Aristotle’s theory of motion • No force required to keep Earth in motion around sun because no friction**Newton**• Built on Galileo’s ideas • Proposed three laws of motion at age of 23**LHP**• Write two letters from Isaac Newton to Aristotle and Isaac Newton to Galileo from Newton’s point of view informing the other two why their ideas were incorrect.**Newton’s First LawThe Law of Inertia**• Every body continues its state of rest or of uniform speed in a straight line unless acted on by a non-zero net force • This tendency of a body is called its inertia Massive bodies like ships have huge inertia**Inertia**• Force – an action capable of accelerating an object • Inertia – resistance to acceleration • ∑F = ma • Net force • Separate equations for x and y**Experiencing Inertia**• Inertia is resistance to move • Which is easier to move, a pen or a person? • Why is it so hard to stop a heavy boat?**Inertia in a Car**• Discuss three examples of inertia in a car • Car hitting a wall • Car hit from behind by a truck • Car going around a corner**Mass**• Amount of inertia depends on amount of mass…or amount of material. • Measured in kilograms • Question: Which has more mass, a kilogram of lead or a kilogram of feathers? • Mass vs. Volume: volume is how much space something occupies**Mass vs. Weight**• Mass is amount of matter • Weight measures gravitational force on an object, usually due to a planet • Weight depends on location of object • Question 1: How does mass of a rock compare when on Earth and on moon? • Question 2: How does its weight compare?**Weight Examples**• What does a 70 kg person weigh? W = mg = 70 kg x 9.80 N/m2 = 686 N • An object weighs 9800 N on Earth. What is its mass? • m = W/g = 9800 / 9.8 m/s2 =1000 kg**LHP**• Explain the physics principles of inertia, velocity, and acceleration in the following situations: • 1. Jumping on a trampoline, (why does inertia change from up to down repeatedly) • 2. Being in a collision and having your head hit the airbag.**Newton’s Second Law**• The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. • The direction of its acceleration is the direction of the net force acting on the object • F = ma a = F/m • Net force is vector sum of forces**Force Units**• SI System – the Newton – one Newton is the force needed to accelerate one kg by 1 m/s/s**Examples**• What force is required to accelerate a 1000 kg car at 2.0 m/s2 ? Answer: F = ma = 1000 kg x 2.0 m/s2 = 2000 N. • What is the acceleration of a 145 g baseball thrown with a force of 20.0 N? a = F/m = 20/0.145kg = 138 m/s2**Newton’s Third LawAction and Reaction**• Whenever a body exerts a force on a second body, the second exerts an equal and opposite force on the first • Example: rocket pushes on gas; gas pushes back on rocket Note: the forces in an action-reaction pair act on different objects. They do not act on the same object and “cancel out.”**Another example**• Skater pushes back on ice. Ice pushes forward on skater**Example: pushing on wall**• What are the forces when you push on a wall? • You exert force on wall • You accelerate in the opposite direction • Wall must have exerted a force on you in the direction you accelerated (by 2nd Law)**Example: person walking**• Foot exerts force backward on ground • Ground exerts force forward on foot**Horse and Cart**• Horse exerts force on cart • Cart exerts equal and opposite force on horse • Net force = zero • Cart can’t move • Huh?**LHP**• Write and solve two word problems related to the F = m a equation solving for two different variables. • Describe two real world examples of Newton’s third law other than what has already been taught.