chap5 - Forces • Newton’s Laws of Motion • Weight •...

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Unformatted text preview: Forces • Newton’s Laws of Motion • Weight • Free fall • Force and motion problems in 1-D • Normal force • Tension • Free body diagrams • Atwood device • Static and kinetic friction • Coefficients of friction • Air resistance • Terminal velocity Examples of Forces • A force is just a push or pull. Examples: – an object’s weight – tension in a rope – a left hook to the schnozola – friction – attraction between an electron and proton • Bodies don’t have to be in contact to exert forces on each other, e.g., gravity. Fundamental Forces of Nature • Gravity – Attraction between any two bodies w/ mass – Weakest but most dominant • Electromagnetic – Forces between any two bodies w/ charge – Attractive or repulsive • Weak nuclear force – responsible for radioactive decay • Strong nuclear force – holds quarks together (constituents of protons and neutrons) Newton’s Laws of Motion 1. Inertia: “An object in motion tends to stay in motion. An object at rest tends to stay at rest.” 2. F net = m a 3. Action – Reaction: “For every action there is an equal but opposite reaction.” 1 st Law: Inertia • A moving body will continue moving in the same direction with the same speed until some net force acts on it. • A body at rest will remain at rest unless a net force acts on it. • Summing it up: It takes a net force to change a body’s velocity . “An object in motion tends to stay in motion; an object at rest tends to stay at rest.” Inertia Example 1 An astronaut in outer space will continue drifting in the same direction at the same speed indefinitely, until acted upon by an outside force. Inertia Example 2 If you’re driving at 65 mph and have an accident, your car may come to a stop in an instant, while your body is still moving at 65 mph. Without a seatbelt, your inertia could carry you through the windshield. 2 nd Law: F net = m a • The acceleration an object undergoes is directly proportion to the net force acting on it. • Mass is the constant of proportionality. • For a given mass, if F net doubles, triples, etc. in size, so does a . • For a given F net if m doubles, a is cut in half. • F net and a are vectors; m is a scalar. • F net and a always point in the same direction. • The 1 st law is really a special case of the 2 nd law (if net force is zero, so is acceleration). What is Net Force? When more than one force acts on a body, the net force (resultant force) is the vector combination of all the forces, i.e., the “net effect.” F 1 F 2 F 3 F net Net Force & the 2 nd Law For a while, we’ll only deal with forces that are horizontal or vertical. When forces act in the same line, we can just add or subtract their magnitudes to find the net force....
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This note was uploaded on 11/17/2011 for the course PHYS 121 taught by Professor Burgeson during the Fall '11 term at BYU.

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chap5 - Forces • Newton’s Laws of Motion • Weight •...

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