Physics 205A Quiz 5, fall semester 2015

Cuesta College, San Luis Obispo, CA

A crate gradually decreases in speed, as it slides up along a ramp. The ramp is not frictionless. Identify the force(s) that do positive work on the crate.

(A)

**w**.

(B)

**N**.

(C)

*f*_{k}.

(D) (Two of the above choices.)

(E) (All of the above choices.)

(F) (None of the above choices.)

Correct answer (highlight to unhide):

(F)The three forces acting on the crate are:

normal force **N** (perpendicular outwards from the ramp surface);

weight force **w** (vertically downwards);

friction force *f*_{k} (down along the ramp).

The definition of work done is

*W* =

*F*·

*s*·cosθ, where the angle θ is measured between the direction of the force vector and the displacement vector (which points in the direction of motion) when drawn together "tail-to-tail."

The angle θ between the normal force

**N** (perpendicular outwards from the ramp surface) and the displacement vector

**s** (up along the ramp) is 90°, and since cos(90°) = 0, the normal force does zero work on the crate.

The angle θ between the kinetic friction force

*f*_{k} (down along the ramp) and the displacement vector

**s** (up along the ramp) is 180°, and since cos(180°) = –1, the kinetic friction force does negative work on the crate.

The angle θ between the weight force

**w** (vertically downwards) and the displacement vector

**s** (up along the ramp) is an obtuse angle between 90° and 180°, and since cosθ is negative for 90° < θ < 180°, the weight force also does negative work on the crate.

Thus there are no forces that do positive work on the crate.

Sections 70854, 70855, 73320

Exam code: quiz04w04K

(A) : 4 students

(B) : 22 students

(C) : 4 students

(D) : 11 students

(E) : 1 student

(F) : 30 students

Success level: 42%

Discrimination index (

Aubrecht & Aubrecht, 1983): 0.71