Lecture 03 Dynamics Forces and Motion Along A Line
Dynamics: Forces and Motion Along A Line<br />
Force ( )<br /> - a vector quantity that basically affects the motion of the object<br /> - commonly defined as “push” or “pull”<br />Units of Force: <br /> SI Unit: newton (N)<br /> Gaussian: dyne<br /> English: pound (lb)<br />Net Force ( )<br />- resultant force or sum of all forces acting on the object. <br />
Net force<br />Physicists who contributed to the development of force and motion relationship:<br />Aristotle (384-322 BC)<br />Galileo (2000 years later, approx. 1678)<br />Isaac Newton (1687)<br />- Principia (book published)<br />- “three laws of motion”<br />
Newton’s First Law<br />Law of Inertia:An object at rest or an object in motion at constant velocity will remain at rest or at constant velocity in the absence of a net force.<br /><ul><li>The body remains at rest.
The body moves at constant velocity. </li></li></ul><li>Discuss what the driver experiences when a car accelerates from rest and then applies the brakes.<br />(a) The driver is forced to move forward. An object at rest tends to remain at rest. <br />Understanding the First Law:<br />(b) Driver must resist the forward motion as brakes are applied. A moving object tends to remain in motion.<br />
mass (m)<br />- is a measure of inertia of a body.<br /> - synonymous to quantity of matter. This means that it is constant!<br /> * The more mass a body has, the harder it is to change its state of motion.<br />
Newton’s Second Law:<br />Law of Acceleration:Whenever a net force acts on an object, it produces an acceleration with a direction the same to the direction of the net force - an acceleration that is directly proportional to the net force and inversely proportional to the mass.<br />Unit Conversion:<br /> 1 N = 1 kg m/s2; 1 dyne = 1 g cm/s2<br />1 lb = 1 slug ft/s2<br />
Pushing the cart with twice the force produces twice the acceleration. Three times the force triples the acceleration.<br />Acceleration and Force With Zero Friction Forces<br />
F<br />F<br />a/2<br />a<br />Pushing two carts with same force F produces one-half the acceleration. The acceleration varies inversely with the amount of material (the mass).<br />Acceleration and Mass Again With Zero Friction<br />
Newton’s Third Law<br />Force of Man on Floor<br />Force of Ceiling on Man<br />Force of Wall on Hands<br />Force of Floor on Man<br />Force of Hands on Wall<br />Force of Man on Ceiling<br />Law of Interaction:Toevery action force there must be an equal and opposite reaction force. <br />Action and reaction forces act on different objects.<br />
Action<br />Reaction<br />Action<br />Reaction<br />Newton’s Third Law<br />Two More Examples:<br />Action and Reaction Forces Act on Different Objects. They Do Not Cancel Each Other!<br />
Weight, Normal Force, and Tension<br />Weight (w) – gravitational force<br />w = mg<br />where m = mass of the object<br />g = acceleration due to gravity<br /><ul><li> For practical purposes on the earth, 1 kg = 2.2 lb since g is relatively constant near the surface.
Weight is not constant while mass is!</li></li></ul><li> The force of gravity acts on an object when it is falling. <br /> When the body is at rest on the earth, the gravitational force on it does not disappear.<br />Why, then, doesn’t the object move?<br />There must be another force to balance the gravitational force.<br />
Normal force (FN)<br /> - contact force that acts perpendicular to the common surface of contact. <br />FN<br />T<br />w<br />w<br />Tension (T )<br /> - force of a string, cable, wire running on the body<br />