4 Newton's Third Law and Vector Addition Introduction Newton's Laws of Motion Newton's First Law Newton's Third Law Newton's Second Law Law of inertia Action-reaction F = ma Moving Objects objects at rest Force of A Equals force keep moving stay at rest on B of B on A Force and bull acceleration are proportional awollot Types of forces When answering the question, "Why do things move the way they do?", it is helpful to draw a diagram to show all the force vectors that act on an object. Take a moment to review the following types of forces: . gravity: the attractive force between two masses; the weight of an object is the gravitational force exerted on it by the planet on which the object is located . normal force: the force exerted on an object pressed against a surface; the normal force is always perpendicular to the surface . tension: the force exerted on an object by a rope or other material suspending it desert Hint - unless you have a bor resting on a surface and being pulled by a rope, it is unlikely that you would have BOTH tension and a normal force acting on an object. . friction: the resistive force against sliding between two surfaces in contact; the friction force is always parallel to the surface and opposite the direction of motion Vectors The graphical addition of two vectors can be done using the parallelogram method, as shown in Fig. 1. The same result can achieved by drawing the vectors "head-to-tail" and then drawing the resulting vector. The trigonometric addition of two vectors involves resolving the vectors into their r and y components. The components in each direction are then added to find the components of the resulting vector.F2 F1 F3 F1 F2 Fig. 1 - illustration of parallelogram method of vector addition The x and y components of each vector are given by: Fix = F1 cos 0 Fly = Fi sin 0 F2x = F2 cos 0 Fzy = F2 sin 0 where Fi and F2 are the lengths of the vectors and 01 and 02 are their angles with respect to the horizontal. These components can then be added to find the resultant vector components as follows: FRI = Fix + F2x FRy = Fly + Fay The magnitude of FR is found using the Pythagorean Theorem FR = VFR + FRy and the angle OR is given by OR = arctan FRy FRI Pre-lab and Discussion Questions Pre-Lab Questions This part of the lab is to be completed prior to class. You MUST show it to your instructor upon arrival or else no credit will be given for this section. 1. Write Newton's Third Law in your own words: [5 pts] 2. Give an example of where you have experienced Newton's Third Law in your daily life. [2 pts]An action-reaction pair involves: TWO FORCES acting on TWO OBJECTS 3. For each scenario described below, identify the action-reaction pair along with their forces. [3 pts] Example: A hammer strikes a nail. I Dele Action = hammer presses down on nail gogol olded onTol Reaction = nail presses up on hammer (a) A weightlifter presses a barbell above their head. Action = Reaction = (b) A person leans against a wall. Action = Reaction = moul Siting ould svart lliw Lod (c) A balloon is inflated from an air tank. Action = Addition (15 pts) Results Reaction = Discussion Questions This part of the lab is to be completed with your group in class. 4. A box is suspended motionless from the ceiling by two ropes. Draw the free-body diagram for this situation: [5 pts] What direction is the net force? 5. A rightward force is applied to a book in order to move it across a desk at an increasing velocity. Do not ignore friction. Draw the free-body diagram for this situation: [5 pts] What direction is the net force? 41