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Dynamics: Cause of Motion

Dynamics: Cause of Motion. Section 7.1. Reminders. Online Mallard Reading Quiz due by start of class on Tuesday LAB A3-FF: Free Fall lab due by Friday at 4 p.m. We have the 10-question Quiz #3 today. In-class Quiz #4 on Thursday, February 27 th addressing chapter 7, sections 1-3

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Dynamics: Cause of Motion

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  1. Dynamics: Cause of Motion Section 7.1

  2. Reminders • Online Mallard Reading Quiz due by start of class on Tuesday • LAB A3-FF: Free Fall lab due by Friday at 4 p.m. • We have the 10-question Quiz #3 today. • In-class Quiz #4 on Thursday, February 27th addressing chapter 7, sections 1-3 • Begin preparing now for Test #2, Tuesday, March 4, Chapters 4, 6, and 7.

  3. The First Law of Motion • Better known as Newton’s first law of motion • An isolated body at rest will remain at rest, and an isolated body in motion continues its motion with constant velocity along a straight line. • The influence of forces… • External • Frictional • Paraconception – Objects need a constant source of force to keep them in motion. True, but only in the presence of friction.

  4. Where Aristotle Went Wrong • Motion is a natural state, just like rest. • A body in motion will continue in that same state of motion until an unbalance force acts on it. • Demonstration – the dynamic projectile cart • A projectile is not left behind because it has forward motion of its own. • The air is carried along with the spinning Earth. • The state of physics knowledge in early times was comparatively limited.

  5. Momentum & Inertia • Momentum, p, is a vector quantity associated with both mass (arbitrarily defined) & velocity. • p = mv (units typically kg*m/s) • The momentum of an object at rest is 0. • Inertia should not be confused with momentum; inertia is a body’s tendency to resist changes in motion; non-mathematical.

  6. Total Momentum is Conserved • Momentum before = momentum after (pi = pf) • Holds in explosions, collisions, and some +/- of mass • Does not hold for falling bodies UNLESS Earth’s momentum is taken into account • Examples of momentum conservation: • Recoil of a rifle or handgun. • Dropping mass on a moving cart. • Ice skaters pushing off one another. • Astronaut pushing off a space ship. • Rocket in flight.

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