Okay, so W=Fdcostheta. I thought that normal force is perpendicular, so theta eq

Question

Okay, so W=Fdcostheta. I thought that normal force is perpendicular, so theta equals 90 degrees. Which equals 0, however, on the first incline problem Fn=0, however, in the second problem of an incline the Fn does not equal 0. If they are both incline problems why do they both not equal zero. The problem does not need to be solved, I just want to understand the concept.

Your pet snail, Speedy, slides down an incline of phi = 25 degree at a constant speed. If Speedy has a mass of 35 g and slides 0.50 m, what is the work done on him by the normal force, friction, and gravity? What is the net work done on him? What is the minimum work needed to push a 950 kg car 810 m up along a 9.0 degree incline? Ignore friction Assume the effective coefficient of friction retarding the car is 0.25

Explanation / Answer

CONCEPT1: Basically, Fn cannot be zero if there is a contact between body and surface.

CONCEPT2: Friction is neglected in case 1, thats why work done is zero......and friction is considered in case 2 and it is parallel to the direction of the motion, that's why its work done is not zero.

CONCEPT 3: Work done in both cases by Fn is zero. because it is always perpendicular to the the motion of the path.

CONCEPT 4: net contact force is contributed by both Fn and friction; is case 1 friction is zero so net work done by contact force is zero.

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