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At constant speed, the only friction acting on the car is that needed to keep it traveling on the curved path. Therefore, the friction force acts normal to the path.
I got F, but now I need a_n. When I summed the forces in the normal direction I got: -F = m*a_n. Then, a_n = -F/m. Then the units cancel because the force is in lb and the mass is in lb, and therefore the answer is unitless and that is not correct. What am I doing wrong?
This textbook uses a set of units that is based on "slugs" as units of mass. That is, when the units of "pounds" are used, this refers to force (weight), not mass.
For this problem, you need to divide pounds (weight) by "g" to get the mass in slugs. This will give m*a_n in pounds force with a_n in ft/sec^2.
you can fine a_n first using (v^2)/p. make sure you convert v to ft/s. and then you can make ratio with g which is 32.2 ft/s. for F, you simply use F = m*a_n. 3000 lb is weight so you need to divide it by g.
At constant speed, the only friction acting on the car is that needed to keep it traveling on the curved path. Therefore, the friction force acts normal to the path.
3 comments:
I got F, but now I need a_n. When I summed the forces in the normal direction I got: -F = m*a_n. Then, a_n = -F/m. Then the units cancel because the force is in lb and the mass is in lb, and therefore the answer is unitless and that is not correct. What am I doing wrong?
This textbook uses a set of units that is based on "slugs" as units of mass. That is, when the units of "pounds" are used, this refers to force (weight), not mass.
For this problem, you need to divide pounds (weight) by "g" to get the mass in slugs. This will give m*a_n in pounds force with a_n in ft/sec^2.
you can fine a_n first using (v^2)/p. make sure you convert v to ft/s. and then you can make ratio with g which is 32.2 ft/s.
for F, you simply use F = m*a_n. 3000 lb is weight so you need to divide it by g.
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