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Transcript
free-body diagram (FBD) worksheet: Draw in “action-at-a distance” forces (Fgrav, Felec, Fnuke, etc) and, then, ALL other forces MUST be “contact” forces. Newton’s 2nd Law tells us that the vector sum of all of the forces acting on a body equals its mass x acceleration. We can look at the FBD to see how a body’s motion will change AND we can look at how a body’s motion is changing and make assumptions about the forces that must be acting on it. Draw a FBD for a block at rest on a horizontal surface Draw a FBD for a block being pushed on a horizontal, frictionless surface Label the magnitude of the forces for a 10-kg block Label the magnitude of the forces and find the acceleration for a 10-kg block with a push of 50N. Draw a FBD for a block being pushed on a rough, horizontal surface Draw a FBD for a block being pushed on a horizontal surface at a constant velocity. Label the magnitude of the forces and find the acceleration for a 10-kg block with a push of 50N & a 30N friction force. Find the acceleration & label the magnitude of the forces for a 10-kg block with a push of 50N & a constant velocity of 30m/s. Draw a FBD for a block sliding to a stop. Draw a FBD for a block on a frictionless incline. Label the magnitude of the forces for a 5 kg block that is initially traveling at 30 m/s and slows to a stop in 5 s. Find the acceleration of a 10-kg block on a 30o incline. Written by John Burgholzer, Amherst Central HS, 2010 Draw a FBD for a block at rest on an incline. Draw a FBD for a block accelerating down a rough incline. Find the friction force for a 10-kg block on a 30o incline. Find the friction force acting on a 10-kg block on a 30o incline that accelerates at 2 m/s2 The floor is frictionless. The top block moves with the bottom block. Draw a FBD for the two blocks together & for the individual blocks. The big block is 10 kg. Find the magnitude of all of the forces present if the applied force = 36N and the system accelerates at 3 m/s2 Ignoring air resistance, draw a FBD… …for an object dropped from rest. …for an object thrown straight upwards. …for an object thrown at an angle to the horizontal. Ponder these… What is the definition of freefall? How can such different motions result from these FBDs? IF we chose to think about air resistance, in which direction would Fair be in each of these? Draw FBDs for these situations. How do we find Fnet? a If the mass is 5 kg and a = 2 m/s2, what angle does the string make with the vertical? Draw a FBD for a person in an elevator when the elevator is… …moving at constant speed. …accelerating upwards. …accelerating downwards. His mass is 80 kg. What does the scale read when the speed in the 1st diagram is 5 m/s and the acceleration in the other two is 2 m/s2? Which way is the elevator moving in each case? Draw a FBD for the 500-kg jet. If the thrust is 10,000 N and the plane accelerates horizontally at 8m/s2, what is the drag force? Draw a FBD for the 100-kg rocket. If the thrust is 10,000 N, what is the acceleration of the rocket? Fair = zero Al, a BASE jumper, leaps off a cliff. He weighs 1000N so his mass = _______kg. The resistive force due to the air (Fair) varies with his speed and the surface area presented to the air. Using Newton’s 2nd Law, Al’s acceleration (a) is… a = _________ weight = 1000 N 1 a = Fnet = weight - Fair m mass Given the values for weight and Fair for each segment of his trip, calculate his acceleration at each point. Fair = 600 N a = _________ (Note: The answers below are important, but knowing WHY they are correct is much more important!) weight = 1000 N 2 Circle the correct answers: 1. When Al’s speed is least, his acceleration is… [least] [most] [the same]. Fair = 1000 N 2. In which position(s) does Al experience a downward acceleration? [1] [2] [3] [4] [5] 3. In which position(s) does Al experience an upward acceleration? [1] [2] [3] [4] [5] 4. When Al’s acceleration is upward, his velocity is… [directed upwards] [directed downwards] AND his speed is… [decreasing] [increasing] [constant]. 5. In which position(s) is Al’s velocity constant? [1] [2] [3] [4] [5] 6. In which position(s) does Al’s experience terminal velocity? [1] [2] [3] [4] [5] 7. In which position(s) is Al moving at the greatest speed? [1] [2] [3] [4] [5] 8. If Al were heavier, his fastest (terminal) speed would be… [greater] [less] [the same]. a = _________ weight = 1000 N 3 Fair = 1600 N a = _________ weight = 1000 N 4 Fair = 1000 N a = _________ weight = 1000 N 5