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Transcript
Interactions • A force is an interaction between 2 objects An Introduction to Forces Forces-part 1 – Touching – At a distance • See the Fundamental Particle Chart (http://www.cpepphysics.org/images/2014-fund-chart.jpg) PHYS& 114: Eyres • Identifying Interactions – Choose a system: circle the object – Name the force and both interacting objects Drawing force diagrams Constructing force diagrams 1. 2. 3. 4. Sketch the situation. Circle the system. Identify external interactions. Place a dot at the side of the sketch representing the system object. 5. Draw force arrows to represent the external interactions. 6. Label the forces with a subscript containing two elements. • Example: a rock sinking into sand © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Four Types • weight: Symbol W – Pulling – Between 2 objects with mass Force Notation • I am pushing down on a book that is lying on the table. • Tension: Symbol T – Pulling – Must have a rope or string or spring attached • Normal: Symbol N or sometimes n – Pushing – Due to contact with a surface • friction: Symbol f – Opposite direction to motion or the direction of potential motion – Rubbing (kinetic friction) or “Stickiness” associated with “trying to move it” (static friction) Force Type By On Symbol Direction Weight Earth Book W E on B Down Normal Me Book NM on B Down Normal Table Book NT on B Up Symbol W 1 Adding forces graphically Adding forces graphically (Cont'd) • Draw the vectors head to tail. • Draw the vector that goes from the tail of the first vector to the head of the second vector. – This is the sum vector, also called the resultant vector. – In this case this vector is the net force (it is not a new force, but rather the combined effect of all the forces being exerted on the object). © 2014 Pearson Education, Inc. Testing the relationship between the sum of forces and the motion of the system object © 2014 Pearson Education, Inc. Reasoning without mathematical equations • Motion and force diagrams and the rule relating motion and force can be used to reason qualitatively about physical processes: – To determine the relative magnitudes of forces if you have information about motion – To estimate velocity changes if you have information about forces © 2014 Pearson Education, Inc. Testing the relationship between the sum of forces and the motion of the system object © 2014 Pearson Education, Inc. Inertial reference frame • An inertial reference frame is one in which an observer: – Sees that the velocity of the system object does not change if no other objects exert forces on it or – Sees no change in the velocity if the sum of all forces exerted on the system object is zero • Make sure the unknown representation is consistent with the known representation. • In noninertial reference frames, the velocity of the system object can change even though the sum of forces exerted on it is zero. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 2 Newton's first law of motion • For an observer in an inertial reference frame, the object continues moving at constant velocity (including remaining at rest): – When no other objects exert forces on an system object or – When the forces exerted on the object add to zero • Inertia is the phenomenon in which an object continues to move at constant velocity when the sum of the forces exerted on it by other objects is zero. Newton’s Laws • Newton’s 1st Law If there is no net Force, there is a constant velocity. What might the x vs t graph look like? What might the v vs t graph look like? What might the a vs t graph look like? © 2014 Pearson Education, Inc. Newton’s Laws • Newton’s 1st Law If there is no net Force, there is a constant velocity. What might the x vs t graph look like? What might the v vs t graph look like? What might the a vs t graph look like? Newton's second law of motion • Observation experiments help us construct the following relationship between the sum of forces on a system object and the system object's motion: • The symbol α means "is proportional to." For example, if the sum of the forces doubles, then the acceleration doubles. © 2014 Pearson Education, Inc. Newton's second law of motion • Observation experiments help us construct the following relationship for the proportionality between the acceleration of a system object and the system object's mass: Cause-effect relationships • The equation we deduced for Newton's second law is: • The right side of the equation (the sum of the forces being exerted on the system) is the cause of the effect (the system's acceleration) on the left side. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 3 Newton’s Laws • Newton’s 2nd Newton’s Laws • Newton’s 2nd Law ΣF = ma Law F = ma What might the x vs t graph look like? What might the x vs t graph look like? What might the v vs t graph look like? What might the v vs t graph look like? What might the a vs t graph look like? What might the a vs t graph look like? 1st Law or 2nd Law? Graph Sketches? 1st Law or 2nd Law? Graph Sketches? X=0 X=0 x vs. t Constant Speed therefore ∆v=0 and a=0 v vs. t therefore 1st Law 1st Law or 2nd Law? Graph Sketches? a vs. t 1st Law or 2nd Law? Graph Sketches? X=0 X=0 x vs. t Speeding up therefore ∆v not 0 and a not 0 v vs. t therefore 2nd Law a vs. t 4 1st Law or 2nd Law? Graph Sketches? 1st Law or 2nd Law? Graph Sketches? X=0 X=0 x vs. t Slowing down therefore ∆v not 0 and a not 0 v vs. t therefore 2nd Law Gravitational force a vs. t Weight • The weight of the object on a planet is the force that the planet exerts on the object. • In everyday language, the normal force that a scale exerts on you (which balances the downward force you exert on it) is your weight. • We will not use the term "weight of an object" because it implies that weight is a property of the object rather than an © 2014 Pearson interaction between two objects. Education, Inc. © 2014 Pearson Education, Inc. Newton’s Laws Newton's third law of motion • Newton’s 3rd Law If Object 1 pushes on Object 2 then Object 2 pushes on Object 1 • When two objects interact, object 1 exerts a force on object 2. Object 2 in turn exerts an equal-magnitude, oppositely directed force on object 1. On 2 By 1 N21 On 1 By 2 N12 • This can help to identify forces • These forces are exerted on different objects and cannot be added to find the sum of the forces exerted on one object. © 2014 Pearson Education, Inc. 5