FORCES episode II newton strikes back Physics Mr. Maloney

© 2002 Mike Maloney A new era of physics  Aristotle ( BC) thought that objects were naturally at rest, would always stop themselves and a force was needed for any motion.  Descartes ( ) thought that some unseen vortex was constantly pushing on things to make them stay in motion.  Galileo ( ) realized that the Greeks weren't accounting for forces such as friction.  Newton summarized Galileo's thoughts and others through his three laws of motion.

© 2002 Mike Maloney objectives You will be able to  describe how mass, force and acceleration are related to eachother.  describe the consequences of Newton’s 2 nd Law.  graph data to find a relationship between variables.  Jump to Post Lab Jump to Post Lab

© 2002 Mike Maloney Newton’s 2 nd Law  Newton’s 2 nd law describes how a mass behaves when forces act on it.  We can guess these forces will produce a change in motion but how are they related?  ILD 2.2 (1 st part) ILD 2.2

© 2002 Mike Maloney Newton’s 2 nd Law  How does acceleration relate to force?  Lets do a little experiment together as a class to find out.

© 2002 Mike Maloney Newton’s 2 nd Law  How does this acceleration relate to the force?  The acceleration will be in the same direction as the force.  The magnitude of the acceleration increases as the force increases  The magnitude of the acceleration decreases as the mass of the object increases  Recreate our lab using this sim.sim

© 2002 Mike Maloney N2L with multiple objects  What if there is more than one object applying a force on some mass? What is the effect now? (back to ILD)back to ILD  It is not just one force that determines how an object will change it motion, it is the sum total of all forces acting on the object … what we call NET FORCE.NET FORCE  Demos with 2 fans and 2 weights.

© 2002 Mike Maloney Newton’s 2 nd Law  In mathematical terms  a =  F/m  acceleration = net force / mass or more commonly written  Force (net) = mass ∙ accelerationnet   F = m∙a  And since force and acceleration are vectors, they will be in the same direction.

© 2002 Mike Maloney MEASURING FORCES  The unit of force is the Newton {N}  It is derived from its effect.  F = m*a  [kg]*[m/s 2 ]  1 N = 1 kg m/s 2  1 N is defined as the amount of force required to accelerate 1 kg at a rate of 1 m/s 2.

© 2002 Mike Maloney 2 nd Law Examples  Accelerating a car vs. truck  Slowing down a car vs. truck  Jogging vs. sprinting  Pushing chair alone vs. someone in it  Freefall

© 2002 Mike Maloney objectives Can you  describe how mass, force and acceleration are related to eachother.  describe the consequences of Newton’s 2 nd Law.  graph data to find a relationship between variables.  Jump to Post Lab Jump to Post Lab

© 2002 Mike Maloney APPENDIX

Inertia  The tendency of an object to resist a change in motion is called its inertia.  Objects with greater masses generally have greater inertias.  For our purposes, an object’s inertia is basically measured by it mass.  BACK

© 2002 Mike Maloney Net Force  NET FORCE refers to the vector sum total of all forces acting on an object. It is often expressed as  F  For example, if there were two leftward forces of 10 lb each, the NET FORCE would be 20 lb leftward.  If there were one 10 lb rightward force and one 8 lb leftward force, the NET FORCE would be 2 lb rightward.  What about if the forces were in X and Y?  BACK

© 2002 Mike Maloney Applied Force  APPLIED FORCE refers to a generic force that is acting on an object.  An APPLIED FORCE is basically any force that is not a WEIGHT, NORMAL FORCE, FRICTIONAL FORCE, SPRING FORCE, or other named force.  BACK