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You are going to toss a rock straight up into the air by placing it on the palm of your hand (you're not gripping it), then pushing your hand up very rapidly. Rank the three times below from the largest to the smallest number of forces acting on the stone. a. the rock at rest on your palm b. As your hand is moving up but before the rock leaves your hand c. Instantly after the rock leaves your hand. d. After the rock has reached its highest point and is now falling straight down. 1. A=D<C<B 2. A<D<C<B 3. A=B<C=D 4. C=D<A=B 5. A<B<C<D Phys 1021 -- 2 1 You are going to toss a rock straight up into the air by placing it on the palm of your hand (you're not gripping it), then pushing your hand up very rapidly. Rank the three times below from the largest to the smallest number of forces acting on the stone. a. the rock at rest on your palm b. As your hand is moving up but before the rock leaves your hand c. Instantly after the rock leaves your hand. d. After the rock has reached its highest point and is now falling straight down. 1. A=D<C<B 2. A<D<C<B 3. A=B<C=D 4. C=D<A=B 5. A<B<C<D For A and B, there are two forces, gravity and the normal force from the hand. For C and D there is only one for, from gravity. Phys 1021 -- 3 Phys 1021 -- 4 2 Phys 1021 -- 5 Phys 1021 -- 6 3 Phys 1021 -- 7 Phys 1021 -- 8 4 Phys 1021 -- 9 New Topic Phys 1021 -- 10 5 amoon g R where G = 6.67 x 10 -11 m3 kg-1 s-2 Direction: attractive along the line between the centers of the two masses Phys 1021 -- 11 Which is stronger, the Earth’s pull on the Moon, or the Moon’s pull on the Earth? Phys 1021 -- 12 6 Which is stronger, the Earth’s pull on the Moon, or the Moon’s pull on the Earth? Phys 1021 -- 13 1 2. 2 Earth 3. 3 4. 4 d 5. 5 2d 1. 1 2 5 4 2m 3 m Phys 1021 -- 14 7 2d 2m 5 4 d 2 1 3 m Phys 1021 -- 15 Near the earth’s surface... $ =g Force of attraction between an object and an astronomical body (Earth, Moon, etc.) Weight = Force of gravity: W = mg Weight (like any force) is a vector. scalar units: newtons (N) units: kilogram (kg) Phys 1021 -- 16 8 Suppose you stand on a spring scale in six identical elevators. Each elevator moves as shown below. Let the reading of the scale in elevator n be Sn. Rank in order, from largest to smallest, the six scale readings S1 to S6. Some may be equal. Give your answer in the form A>B = C > D. Phys 1021 -- 17 New Topic Phys 1021 -- 18 9 Phys 1021 -- 19 Rank order, from largest to smallest, the size of the friction forces in these five different situations. The box and the floor are made of the same materials in all situations. A. fc > B. fb > C. fb > D. fa > E. fa = fd > fc = fc > fc = fb > fe > fd = fd > fd = fc = fb > fa. fe > fa . fe > fa. fe > fb. fd = fe. Phys 1021 -- 20 10 Rank order, from largest to smallest, the size of the friction forces in these five different situations. The box and the floor are made of the same materials in all situations. A. fc > B. fb > C. fb > D. fa > E. fa = fd > fc = fc > fc = fb > fe > fd = fd > fd = fc = fb > fa. fe > fa. fe > fa. fe > fb. fd = fe. Phys 1021 -- 21 1. 2. 3. 4. 5. 6. 1. 2. 3. 4. By Group Number: of a name block on a whiteboard? of an eraser on a whiteboard? of an upside-down eraser on a whiteboard? of a book on a whiteboard? of a calculator on a whiteboard? of a pen (not rolling) on a whiteboard? Steps: Brainstorm how to do this Make calculations, estimate the value Get your materials -- ask me or Deepa for help here Make measurements Phys 1021 -- 22 11 A 2.0 kg wood box slides down a vertical wood wall at a constant speed while you push upward on it at a 45° angle. For wood on wood, the coefficient of kinetic friction is µk = 0.20. What is Fpush? What magnitude of force should you apply to cause the box to slide down with a 0.1 g acceleration? Phys 1021 -- 23 A 2.0 kg wood box slides down a vertical wood wall at a constant speed while you push upward on it at a 45° angle. For wood on wood, the coefficient of kinetic friction is µk = 0.20. What is Fpush? What magnitude of force should you apply to cause the box to slide down with a 0.1 g acceleration? Phys 1021 -- 24 12 Myosin motors move along actin fibers in living things as seen in these videos made by atomic force microscopy (AFM): Newton’s Third Law at work to a) Contract muscles b) Carry cargo c) Move cilia 6.8 fps, Scan area, 150 × 75 nm2 Phys 1021 -- 25 New Topic Phys 1021 -- 26 13 A B Is boxing glove A hitting glove B, or is glove B hitting glove A? The gloves hit each other For every action there is an equal and opposite reaction. For the boxing gloves: Action: A pushes on B Reaction: B pushes on A FA on B = – FB on A Action and Reaction forces: • have the same magnitude • are in opposite directions • act on different objects Phys 1021 -- 27 Action: force of the Earth on the apple (gravity) Reaction: NOT force of the table on the apple BUT force of the apple on the Earth How can a table exert a force on an apple? No apple, no force With the apple, the table bends ever so slightly so there is a force upward (like a diving board) Phys 1021 -- 28 14 ice Phys 1021 -- 29 This is why you need to use a Free Body Diagram and consider the net force on the block… only Fman on block Phys 1021 -- 30 15 Phys 1021 -- 31 Phys 1021 -- 32 16 Two blocks of masses 2m and m are in contact on a horizontal frictionless surface. If a force F is applied to mass 2m, what is the force on mass m ? Phys 1021 -- 33 Two blocks of masses 2m and m are in contact on a horizontal frictionless surface. If a force F is applied to mass 2m, what is the force on mass m ? Phys 1021 -- 34 17 A block of mass m rests on the floor of an elevator that is accelerating upward. What is the relationship between the force due to gravity and the normal force on the block? a m Phys 1021 -- 35 A block of mass m rests on the floor of an elevator that is accelerating upward. What is the relationship between the force due to gravity and the normal force on the block? N m a>0 mg Phys 1021 -- 36 18 T mg Phys 1021 -- 37 Phys 1021 -- 38 19 You tie a rope to a tree and you pull on the rope with a 1. 1 of 100 N. What is the force 2. 2 tension in the rope? 3. 3 4. 5. 4 5 Phys 1021 -- 39 You tie a rope to a tree and you pull on the rope with a force of 100 N. What is the tension in the rope? Phys 1021 -- 40 20 Two tug-of-war opponents each pull with a force of 100 N on 1. 1 opposite ends of a rope. What 2. 2 is the tension in the rope? 3. 4. 5. 3 4 5 (1) 0 N (2) 50 N (3) 100 N (4) 150 N (5) 200 N Phys 1021 -- 41 Two tug-of-war opponents each pull with a force of 100 N on opposite ends of a rope. What is the tension in the rope? (1) 0 N (2) 50 N (3) 100 N (4) 150 N (5) 200 N Phys 1021 -- 42 21 Take 5 rubber bands to your table. You will stretch them, measure the force needed, then combine them end to end in two ways: To make a 5 - band - wide configuration To make a 5 - band - long configuration What is the force needed to stretch a single rubber band by 1 cm, 2 cm, 4 cm? What force do you think is needed to stretch the rubber band by 1 m? This is the spring constant in Hooke’s Law. Calculate and then measure: What is the force needed to stretch each of the configurations of rubber bands by 1 cm? Phys 1021 -- 43 A 200.0-N sign is suspended from a horizontal strut of negligible weight. The force exerted on the strut by the wall is horizontal. Draw an FBD to show the forces acting on the strut. Find the tension T in the diagonal cable supporting the strut. Phys 1021 -- 44 22 In an electricity experiment, an electrically charged plastic ball (mass = 100 g) is suspended on a 60 cm long string. When a charged rod is brought near the ball, the rod exerts a horizontal electrical force Felec on it, causing the ball to swing out to a 20° angle and remain at rest there. What is the magnitude of the electric force Felec? What is the tension in the string? Phys 1021 -- 45 The terminal speed of a Styrofoam ball is 15 m/s. Suppose a Styrofoam ball is shot straight down with an initial speed of 30 m/s. Which velocity graph is correct? Phys 1021 -- 46 23 The terminal speed of a Styrofoam ball is 15 m/s. Suppose a Styrofoam ball is shot straight down with an initial speed of 30 m/s. Which velocity graph is correct? Phys 1021 -- 47 Gravity Normal Friction Tension Weight Phys 1021 -- 48 24