Conceptual Physics Past Paper Practice Page PDF

Chapter 5 Newton’s Third Law of Motion Action and Reaction Pairs 1. In the example below, the action-reaction pair is shown by the arrows (vectors), and the action-reaction is described in words. In a through g, draw the other arrow (vector) and state the reaction to the given action. Th...

Chapter 5 Newton’s Third Law of Motion Action and Reaction Pairs 1. In the example below, the action-reaction pair is shown by the arrows (vectors), and the action-reaction is described in words. In a through g, draw the other arrow (vector) and state the reaction to the given action. Then make up your own example in h. Example: Fist hits wall Head bumps ball Windshield hits bug Wall hits fist a. b. Bat hits ball Hand touches nose Hand pulls on flower c. d. e. Student drawing (open) Athlete pushes bar upward Compressed air pushes balloon surface outward f. g. h.. 2. Draw arrows to show the chain of at least six parts of action-reaction forces below. 27 28 Chapter 5 Newton’s Third Law of Motion Interactions 1. Nellie Newton holds an apple weighing 1 newton at rest on the palm of her hand. The force vectors shown are the forces that act on the apple. a. To say the weight of the apple is 1 N is to say that a downward gravitational force of 1 N is exerted on the apple by [Earth] [her hand]. b. Nellie’s hand supports the apple with normal force N, which acts in a direction opposite to W. We can say N [equals W] [has the same magnitude as W]. c. Since the apple is at rest, the net force on the apple is [zero] [nonzero]. d. Since N is equal and opposite to W, we [can] [cannot] say that N and W make up an action-reaction pair. The reason is because action and reaction always [act on the same object] [act on different objects], and here we see N and W [both acting on the apple] [acting on different objects]. e. In accord with the rule, “If ACTION is A acting on B, then REACTION is B acting on A,” if we say action is Earth pulling down on the apple, reaction is [the apple pulling up on Earth] [N, Nellie’s hand pushing up on the apple]. f. To repeat for emphasis, we see that N and W are equal and opposite to each other [and comprise an action-reaction pair] [but do not comprise an action-reaction pair]. g. Another pair of forces is N (shown), and the downward force of the apple against Nellie’s hand (not shown). This force pair [is] [isn’t] an action-reaction pair. h. Suppose Nellie now pushes upward on the apple a force of 2 N. The apple [is still in equilibrium] [accelerates upward], and compared to W, the magnitude of N is [the same] [twice] [not the same, and not twice]. with i. Once the apple leaves Nellie’s hand, N is [zero] [still twice the magnitude of W], and the net force on the apple is [zero] [only W] [still W – N, which is a negative force]. 29 Chapter 5 Newton’s Third Law of Motion Force-Vector Diagrams In each case, a rock is acted on by one or more forces. Using a pencil and a ruler, draw an accurate vector diagram showing all forces acting on the rock, and no other forces. The first two cases are done as examples. The parallelogram rule in case 2 shows that the vector sum of A + B is equal and opposite to W (that is, A + B = –W). Do the same for cases 3 and 4. Draw and label vectors for the weight and normal support forces in cases 5 to 10, and for the appropriate forces in cases 11 and 12. 1. Static 2. Static 3. Static 4. Static 5. Static 6. Sliding at constant speed without friction 7. Decelerating due to friction 8. Static 9. Rock slides (Friction prevents sliding) (No friction) 10. Static 11. Rock in free fall 12. Falling at terminal velocity 30  thanx to Jim Court 31 Chapter 5 Newton’s Third Law of Motion More on Vectors 1. Each of the vertically suspended blocks on the left has the same weight W. The two forces acting on Block C (W and rope tension T) are shown. Draw vectors to a reasonable scale for rope tensions acting on Blocks A and B. 2. The cart is pulled with force F at angle  as shown. Fx and Fy are components of F. a. How will the magnitude of Fx change if the angle  is increased by a few degrees? [More] [Less] [No change] b. How will the magnitude of Fy change if the angle  is increased by a few degrees? [More] [Less] [No change] c. What will be the value of Fx if angle  is 90°? [More than F] [Zero] [No change] 3. Force F pulls two blocks of equal mass across a friction-free table below. Draw vectors of appropriate lengths for the rope tensions on each block. 4. Consider the boom supported by hinge A and by a cable at B. Vectors are shown for the weight W of the boom at its center, and W/2 for vertical component of upward force supplied by the hinge. a. Draw a vector representing the cable tension T at B. Why is it correct to draw its length so that the vertical component of T = W/2?. b. Draw component Tx at B. Then draw the horizontal component of the force at A. How do these horizontal components compare, and why?. 5. The block rests on the inclined plane. The vector for its weight W is shown. How many other forces act on the block, including static friction? Draw them to a reasonable scale. a. How does the component of W parallel to the plane compare with the force of friction? 32 b. How does the component of W perpendicular to the plane compare with the normal force?. 33 Chapter 5 Newton’s Third Law of Motion Vectors and the Parallelogram Rule 1. When two vectors A and B are at an angle to each other, they add to produce the resultant C by the parallelogram rule. Note that C is the diagonal of a parallelogram where A and B are adjacent sides. Resultant C is shown in the first two diagrams, a and b. Construct resultant C in diagrams c and d. Note that in diagram d you form a rectangle (a special case of a parallelogram). 2. Below we see 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. In which case does the airplane travel fastest across the ground?. Slowest?.. 3. To the right we see the top views of 3 motorboats crossing a river. All have the same speed relative to the water, and all experience the same water flow. Construct resultant vectors showing the speed and direction of the boats. a. Which boat takes the shortest path to the opposite shore? b. Which boat reaches the opposite shore first? c. Which boat provides the fastest ride? 34 Chapter 5 Newton’s Third Law of Motion Velocity Vectors and Components 1. Draw the resultants of the four sets of vectors below. 2. Draw the horizontal and vertical components of the four vectors below. 3. Nellie tosses the stone along the dashed path. The velocity vector, complete with its horizontal and vertical components, is shown at position A. Carefully sketch the appropriate components for positions B and C. a. Since there is no acceleration in the horizontal direction, how does the horizontal component of velocity compare for positions A, B, and C? b. What is the value of the vertical component of velocity at position B? c. How does the vertical component of velocity at position C compare with that of position A?. 35 Chapter 5 Newton’s Third Law of Motion Force and Velocity Vectors 1. Draw sample vectors to represent the 2. Draw sample bold vectors to represent the force velocity of the ball in the positions shown of gravity on the ball in the positions above. With lighter vectors, show the shown above (after it leaves the horizontal and vertical components of thrower’s hand). Neglect air velocity for each position. resistance. 3. a. Which velocity component in the previous question remains constant? Why? b. Which velocity component changes along the path? Why? 4. It is important to distinguish between force and velocity vectors. Force vectors combine with other force vectors, and velocity vectors combine with other velocity vectors. Do velocity vectors combine with force vectors? 5. All forces on the bowling ball, weight (down) and support of alley (up), are shown by vectors at its center before it strikes the pin a. Draw vectors of all the forces that act on the ball b when it strikes the pin, and c after it strikes the pin. 36  thanx to Howie Brand Chapter 5 Newton’s Third Law of Motion Force Vectors and the Parallelogram Rule 1. The heavy ball is supported in each case by two strands of rope. The tension in each strand is shown by the vectors. Use the parallelogram rule to find the resultant of each vector pair. a. Is your resultant vector the same for each case? b. How do you think the resultant vector compares to the weight of the ball?. 2. Now let’s do the opposite of what we’ve done above. More often, we know the weight of the suspended object, but we don’t know the rope tensions. In each case below, the weight of the ball is shown by the vector W. Each dashed vector represents the resultant of the pair of rope tensions. Note that each is equal and opposite to vectors W (they must be; otherwise the ball wouldn’t be at rest). a. Construct parallelograms where the ropes define adjacent sides and the dashed vectors are the diagonals. b. How do the relative lengths of the sides of each parallelogram compare with rope tension?. c. Draw rope-tension vectors, clearly showing their relative magnitudes. 37 3. A lantern is suspended as shown. Draw vectors to show the relative tensions in ropes A, B, and C. Do you see a relationship between your vectors A + B and vector C? Between vectors A + C and vector B?.. 38 Appendix D More About Vectors Vectors and Sailboats (Please do not attempt this until you have studied Appendix D!) 1. The sketch shows a top view of a small railroad car pulled by a rope. The force F that the rope exerts on the car has one component along the track, and another component perpendicular to the track. Draw these components on the sketch. a. Which component is larger?. b. Which component produces acceleration?. c. What would be the effect of pulling on the rope if it were perpendicular to the track? 2. The sketches below represent simplified top views of sailboats in a cross-wind direction. The impact of the wind produces a FORCE vector on each as shown. (We do NOT consider velocity vectors here!) a. Why is the position of the sail above b. Sketch the component of force parallel to useless for propelling the boat along the direction of the boat’s motion (along its forward direction? (Relate this to its keel), and the component Question 1c above. Where the train perpendicular to its motion. Will the boat is constrained by tracks to move in move in a forward direction? (Relate this one direction, the boat is similarly to Question 1b above.) constrained to move along one direction by its deep vertical fin—the keel.) 39 40 Appendix D More About Vectors Vectors and Sailboats—continued 3. The boat to the right is oriented at an angle into the wind. Draw the force vector and its forward and perpendicular components. a. Will the boat move in a forward direction and tack into the wind? Why or why not? 4. The sketch below is a top view of five identical sailboats. Where they exist, draw force vectors to represent wind impact on the sails. Then draw components parallel and perpendicular to the keels of each boat. a. Which boat will sail the fastest in a forward direction? b. Which will respond least to the wind? c. Which will move in a backward direction? d. Which will experience less and less wind impact with increasing speed? 41