(a) What is the coefficient of static friction? (b) What is the maximum force that can be applied upward along the plane on the rope and not move the block? (c) With a slightly greater applied force, the block will slide up the plane. The coefficient of friction is 80% of that for the static case. In its present state, the crate is just ready to slip and start to move down the plane. A massless rope to which a force can be applied parallel to the surface is attached to the crate and leads to the top of the incline. At this point, the person’s velocity remains constant and we say that the person has reached his terminal velocity ( with the horizontal. A zero net force means that there is no acceleration, as shown by Newton’s second law. However, as the person’s velocity increases, the magnitude of the drag force increases until the magnitude of the drag force is equal to the gravitational force, thus producing a net force of zero. The downward force of gravity remains constant regardless of the velocity at which the person is moving. The two forces acting on him are the force of gravity and the drag force (ignoring the small buoyant force). For instance, consider a skydiver falling through air under the influence of gravity. Some interesting situations connected to Newton’s second law occur when considering the effects of drag forces upon a moving object. One consequence is that careful and precise guidelines must be continuously developed to maintain the integrity of the sport. Such innovations can have the effect of slicing away milliseconds in a race, sometimes making the difference between a gold and a silver medal. The relationship looks like this, in km/h: In other words, falling from 50m high is the equivalent of getting hit by a car going 112 km/h, or 70 miles per hour what would happen if you ran out into a busy freeway. Most elite swimmers (and cyclists) shave their body hair. orkm/h Time until impact: s Energy at impact: joules As is probably obvious, the higher you are, the harder you land. Many swimmers in the 2008 Beijing Olympics wore (Speedo) body suits it might have made a difference in breaking many world records ( (Figure)). Australian Cathy Freeman wore a full body suit in the 2000 Sydney Olympics and won a gold medal in the 400-m race. Bicycle racers and some swimmers and runners wear full bodysuits. The dimples on golf balls are being redesigned, as are the clothes that athletes wear. Substantial research is under way in the sporting world to minimize drag. Typical Values of Drag Coefficient C Object For this reason, during the 1970s oil crisis in the United States, maximum speeds on highways were set at about 90 km/h (55 mi/h). The most fuel-efficient cruising speed is about 70–80 km/h (about 45–50 mi/h). At highway speeds, over 50% of the power of a car is used to overcome air drag. Notice that the drag coefficient is a dimensionless quantity. (Figure) lists some typical drag coefficients for a variety of objects. The drag coefficient can depend upon velocity, but we assume that it is a constant here.
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