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FORCE

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  1. FORCE The cause of acceleration

  2. Force: a Push or a Pull Examples: • Muscular forces • Force of gravity • Friction force • Electrical and Magnetic forces • Nuclear force

  3. Force is a Vector • Force has magnitude and direction F q

  4. 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

  5. Aristotle’s Basic Error • Friction not understood as a force

  6. 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

  7. 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

  8. 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

  9. Newton • Built on Galileo’s ideas • Proposed three laws of motion at age of 23

  10. 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.

  11. 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

  12. Inertia • Force – an action capable of accelerating an object • Inertia – resistance to acceleration • ∑F = ma • Net force • Separate equations for x and y

  13. 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?

  14. 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

  15. 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

  16. 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?

  17. 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

  18. 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.

  19. 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

  20. Force Units • SI System – the Newton – one Newton is the force needed to accelerate one kg by 1 m/s/s

  21. 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

  22. 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.”

  23. Another example • Skater pushes back on ice. Ice pushes forward on skater

  24. 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)

  25. Example: person walking • Foot exerts force backward on ground • Ground exerts force forward on foot

  26. Horse and Cart • Horse exerts force on cart • Cart exerts equal and opposite force on horse • Net force = zero • Cart can’t move • Huh?

  27. 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.