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Forces

Forces. (Newton’s Laws of Motion). Forces. A force is a push or pull. Forces are vectors. Force = mass(kg) x acceleration Units: kg*m/s 2 = Newtons (N) 1 Newton is the force needed to accelerate 1 kg at 1 m/s 2 . 1 pound = 4.45 N. Newton’s Laws of Motion. Three laws

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Forces

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  1. Forces (Newton’s Laws of Motion)

  2. Forces • A force is a push or pull. • Forces are vectors. • Force = mass(kg) x acceleration • Units: kg*m/s2 = Newtons (N) • 1 Newton is the force needed to accelerate 1 kg at 1 m/s2. • 1 pound = 4.45 N

  3. Newton’s Laws of Motion • Three laws • Relate force to motion. • In a nutshell: • Law 1: An object’s velocity doesn’t change unless acted upon by a net force. • Law 2: The acceleration of an object upon which a force is acting depends on the amount of net force being applied and on the mass of the object. • Law 3: For every force, there is an equal and opposite counter-force. (Forces come in pairs).

  4. Net Force • Net force means that forces are unbalanced. • Zero net force: Gravity pulls down. Rope pulls up. Crate’s motion does not change. Net force:  Rope snaps. Gravity pulls down. Crate accelerates downward.

  5. The First Law • An object’s velocity doesn’t change unless acted upon by a net force. • Inertia – the natural tendency of an object to resist changes in its motion. • Objects at rest stay at rest. • Objects in motion stay in motion. • Same speed. • Same direction. • UNLESS a net force acts on them. • Newton’s First Law = Law of Inertia

  6. The First Law • NO acceleration if NO net force. • Net force = acceleration. • CAREFUL! • Don’t confuse movement w/ acceleration! • If an object is moving at constant velocity, there is NO NET FORCE acting on it! • There may be forces acting on it, but they are balanced! • Example: An airplane moving at a constant velocity of 200 mph north has many forces acting on it... • But they all cancel out to zero, or the plane would be accelerating!

  7. The First Law Lift Thrust Drag Lift Weight Thrust Drag Weight Net force

  8. The First Law • Inertia explains why: • You feel pushed back into your seat as your car accelerates forward. • You feel thrown forward as your car slows down. • You feel thrown to the side when your car takes a tight curve. • Magicians can yank a tablecloth from beneath a table setting without knocking anything over.

  9. The First Law • David Ginn’s tablecloth stunt demonstration (complete with rabbit!) (YouTube link)

  10. Fnet m The Second Law • The acceleration of an object depends on the magnitude of the net force, and inversely on the mass of the object. • a = • Big force = Big acceleration • Big mass (more inertia) = Small acceleration

  11. The Second Law • Explains why everything falls w/ the same acceleration (neglecting air resistance). • Heavier objects are pulled with more force but have greater mass (inertia). • Lighter objects are pulled with less force but have less mass (inertia). • In both cases, Fnet / m = 9.81 m/s2

  12. The Third Law • For every force, there is an equal and opposite counter-force. • Or, you cannot push something without being pushed back. • Consider a swimmer: • The swimmer pushes back on the water using his hands and feet. • The water accelerates backward. • The water pushes forward on the swimmer. • The swimmer accelerates forward.

  13. The Third Law • Consider walking: • You push back against the Earth with your feet. • The Earth pushes forward on you, causing you to accelerate forward. • Does the Earth accelerate backwards? • YES! But only by a teeny tiny amount. • Why? • Because the magnitude of acceleration depends inversely on the mass (2nd Law) • You  small mass  significant acceleration • Earth  huge mass  infinitessimal acceleration

  14. A Problem for Consideration • A farmer hooks his horse to the cart and says “Pull, horse!” • The horse has studied a bit of physics and says “Nope! I know about Newton’s 3rd Law.” • The farmer says “What about it?” • The horse says “If I pull the cart, the cart pulls right back on me with equal force, and neither of us moves. Therefore, trying to pull the cart is a hopeless task.” • Obviously the horse isn’t totally correct, but why?

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