- #1

Karagoz

- 52

- 5

## Homework Statement

We kick a box upwards a ramp. The box slides upwards, then slides backwards again. We ignore the air drag and other factors. We only look at the gravity, normal forces and friction forces.

Forces on the box when it's moving upwards:

Weight of the box is 0.123 KGs.

Angle α is 24°.

Gravitational acceleration is -9.81 m/s^2

Acceleration of the box when it's moving upwards is: -6.9 m/s^2.

Acceleration of the box when it's moving downwards is -1.16 m/s^2

The question is to find the R (frictional force) when the box is moving upwards and downwards.

## Homework Equations

G_x = mg*sin(α)

## The Attempt at a Solution

I have attemted to solve the problem, and I got almost equal values for R (ca. 0.35N). But in the "answer key" of the problem this is how the problem is solved:

Friction force when box is moving upwards:

F_x = ma

-G_x - R = ma

R = -ma -G_x

R = -ma -mg*sinα

R = -m(a + g*sinα)

R = -0.123kg(-6.9m/s^2 + 9.81m/s^2 * sin24)

R = -0.41N

Friction force when force is moving downwards:

F_x = ma

-G_x + R = ma

R = ma -G_x

R = ma +mg*sinα

R = -m(a + g*sinα)

R = 0.123kg(-1.16m/s^2 + 9.81m/s^2 * sin24)

R = 0.35N

Is the blueprint wrong or?