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Transcript
FORCES episode III The return of Newton Physics Mr. Maloney Objectives You will be able to … describe what happens when all forces are balanced out. apply concepts of equilibrium to problems. observe and describe the consequences of Newton’s 1st Law. © 2002 Mike Maloney Inertia questions I put a book on a table and what happens? I slide a puck across the ice what happens? An astronaut gets pushed away from the shuttle out in deep space what happens? A magician pulls a table cloth out from under some plates and glasses, what happens? In all these cases the motion, whether the object is moving or at rest, stays constant. How can this be .. aren’t there forces? © 2002 Mike Maloney Net force is zero? What if the net force acting on an object is zero? This could mean there are no forces acting on the object Or the forces cancel each other out. © 2002 Mike Maloney Newton’s 1st Law (Inertia) An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced NET force. It describes what happens when there is NO NET force acting on something. © 2002 Mike Maloney Newton’s 1st Law MATHEMATICALLY AT REST TENDS TO STAY AT REST Velocity =0 No net outside force [F=0] Acceleration (change in v) = 0 Object stays stationary © 2002 Mike Maloney Newton’s 1st Law MATHEMATICALLY (part2) IN MOTION STAYS IN MOTION … Velocity ≠0 No net outside force [F=0] Acceleration (change in v) = 0 Object is in constant velocity motion (speed and direction) © 2002 Mike Maloney Newton’s 1st Law (Consequences) An object at rest will stay there if you do not disturb it A moving object’s motion will not change (no a, no v and no change in direction) if there is no outside net force acting on it. When the net force acting on something is zero, the object is said to be in equilibrium. Where have you heard that before? © 2002 Mike Maloney Newton’s 1st Law (Consequence Examples) Pulling table cloth off table. Card flick Hammering head with books Car crash into a wall How is Homer’s crash messed up? Dancing Ball-Guy © 2002 Mike Maloney Extra Note on newton’s 1st law Another consequence of Newton’s first law is that there is no difference between constant velocity motion and no motion … they both are cases of 0 acceleration. This seems weird, but think about driving in a car. When you are at rest sitting in the seat you don’t really feel anything. When you speed up you can “feel” the force speeding you up as the seat pushes on you. But when you get to 55 and set the car on cruise control, you do not feel that force anymore. You are moving at a constant velocity and no force is need to keep you moving. In a force sense, 0 velocity is just another type of constant velocity, nothing special about it. © 2002 Mike Maloney Extra Note on newton’s 1st law Another consequence of Newton’s first law is that there is no difference between constant velocity motion and no Newton’s 1st0law is really just a motion… they are both acceleration. This seems about driving a car. specialweird, casebutofthink Newton’s 2nd inLaw, When you are at rest sitting in the seat you don’t really feel anything. When you speed up you can “feel” the force speeding you up as the seat pushes on you. But when you get to 55 and set the car on cruise control, you do not feel that force anymore. You are moving at a constant velocity and no force is need to keep you moving. when the acceleration is equal to 0 and the net force is also 0. In a force sense, 0 velocity is just another type of constant velocity, nothing special about it. © 2002 Mike Maloney Objectives Can you … describe what happens when all forces are balanced out. apply concepts of equilibrium to problems. observe and describe the consequences of Newton’s 1st Law. © 2002 Mike Maloney APPENDIX © 2002 Mike Maloney 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 © 2002 Mike Maloney