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Download Class #14 - Department of Physics | Oregon State University
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Starting from rest on a level, horizontal, frictionless table surface, a 3.00-kg object is subjected to a single horizontal force F, which has a magnitude of 12.0 N and is directed at an angle of 30° above the positive x-axis (i.e. 30° north of east). What is the object’s y-displacement at the moment when F has been acting on it steadily for 5 seconds? 10/28/15 1. 16 m 2. 20 m 3. 25 m 4. 25 km 5. None of the above. Oregon State University PH 211, Class #14 1 Three forces act on an object as shown. In which direction does the object accelerate? 1 10/28/15 2 Oregon State University PH 211, Class #14 3 4 5 2 Three forces act on an object as shown. In which direction is the object moving? v v 1 10/28/15 2 Oregon State University PH 211, Class #14 v 3 v v 4 Not enough info. 5 3 Force Myths • Force of hand (or bat or racket or…) “stays with” a thrown (or hit or…) object after contact has ceased (i.e. while it’s in flight). • Force “transmits” through an intermediate object. • An object’s velocity is always in the direction of the net force. • There is a force of motion. • Force is required to keep an object moving. • The force required to push an object along at a constant speed is greater than the resisting (friction) forces. • Inertia is a force. • Acceleration is a force. • A force is a property of an object. • Air doesn’t exert a force. • Single forces can exist in isolation. • Greater speeds require greater net forces for equal accelerations. 10/28/15 Oregon State University PH 211, Class #14 4 A horse is (400 kg) is pulling a cart (300 kg) at a constant speed (3.50 m/s) along a straight level road. What is the net force on the cart? 10/28/15 1. 0.00 N 2. 350 N 3. 1050 N 4. 1400 N 5. None of the above. Oregon State University PH 211, Class #14 5 Newton’s Laws are often misquoted, misunderstood and misapplied. Consider the cautionary tale of Sir Edward Horse… 10/28/15 Oregon State University PH 211, Class #14 6