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Transcript
Name________________KEY______________________________ Date________9/18/15________________ Investigation 3 Vectors and Newton’s First and Second Laws of Motion Vectors 1. When two vectors are added together, they produce a resultant that is found using the parallelogram rule. In the diagrams below, a and b are examples of parallelogram rule. Use these as a guide to find the resultant of the vectors in c and d. a d R R 2. c b Below is a top view of an airplane being blown off course by wind in various directions. Use the parallelogram rule to show the resulting speed and direction of travel for each case. c b a Which airplane is traveling the fastest across the ground? ___b_____ d How do you know? b has the longest resultant vector. The length of the vector is proportional to the strength of the vector quantity. 3. Use the parallelogram rule in reverse to find the components of the vector R along the dashed lines. In the diagrams below, a and b are examples. Use these as a guide to draw the components of R in c and d. a c b R R d Newtons’s Laws of Motion The motions we observe in our everyday life follow some simple rules. These rules are called Newton’s Laws and can be expressed as follows: First Law: “Every object continues in a state of rest or of uniform speed in a straight line unless acted upon by a nonzero net force.” Second Law: “The net force on a body equals the mass of that body times its acceleration, and the directions of both are the same.” (net force) = (mass)x(acceleration), and the direction of the net force is the same as the direction of the acceleration Answer the following questions are clearly and concisely as you can. 1. A physics student holds a ball in her hand at rest. a. Name the two forces that are acting on the ball, and indicate the directions of the forces. Gravitational force of the earth, downward; hand holding ball, upward. b. The two forces must have the same magnitude but must be in opposite directions so that the ball is at rest. Which one of Newton’s Laws tells us this? First Law: both forces are acting on the same object (ball). 2. The physics student releases the ball (lets the ball drop). Consider the ball as it falls toward the ground without air resistance. a. Is the ball accelerating? If it is, then in what direction is its acceleration? It is accelerating downward. b. Which one of Newton’s Laws applies to this situation? Second Law: net force = mass x acceleration c. What does Newton’s Second Law say about the direction of the “net force” acting on the object and the direction of the acceleration? The direction of the net force and the acceleration are the same. d. Name the force (or forces) that is (are) acting on the ball as it is falling, and indicate its (their) direction(s). Only the gravitational force downward. 3. A physics student who weighs 800 Newtons (about 170 lbs) stands on a bathroom scale in an elevator that is not moving (at rest). a. What two forces are acting on him, i.e., what forces does he feel? How are the strengths of the two forces related to each other? Which one of Newton’s laws applies here? forces: gravitational force downward; scale pushing upward strengths: the forces have the same strength of 800 Newtons (equal to each other) Newton’s Law: b. First Law: forces are on the same object (physics student) On the diagram of the student in the elevator above, draw two vectors to represent the two forces acting on the student. (Remember that vectors represent both the direction and the magnitude of forces.) In the following questions, you should think first about the type of motion – is the object stationary (at rest) or traveling at a constant velocity (uniform speed in a straight line), or is the object undergoing an acceleration? This will tell you whether Newton’s First Law or Newton’s Second Law applies. 4. Refer to the situation in question 3. For the following motions of the elevator, indicate whether the reading on the bathroom scale is the same as, greater than, or less than the reading when the elevator was stationary, and indicate the directions of the “net force” on the student as up, down or no net force. Also indicate the direction of the acceleration according to Newton’s Second Law as up, down, or no acceleration. bathroom scale reading “net force” direction* acceleration direction* same no net force no acceleration >800 Newtons upward upward moving upward with constant velocity same no net force no acceleration slows as it reaches the top floor less than downward downward starts downward from the top floor <800 Newtons downward downward slows as it reaches the bottom floor greater than upward upward elevator motion not moving begins to move upward *Remember that the direction of the net force and the acceleration are the same. 5. A physics student sitting in a stationary Lamborghini (car), is holding onto the steering wheel, and is strapped in with her seat belt. She knows that in the vertical direction, the gravitational force pulls her downward and the seat pushes her upward and that the net vertical force is zero, thus resulting in no upward or downward acceleration. In the horizontal direction, the back of the seat pushes her forward and the steering wheel pushes her backward. Compare the strengths of the two horizontal forces -- the back of the seat pushing her forward and the steering wheel pushing her backward (which one is greater, or are they the same?) when she accelerates away from a stop sign back of the seat pushing forward is greater she travels on a straight road at 120 mph they are the same she slows because she hears a CHP siren steering wheel pushing backward is greater