We want to slide a 12 kg crate up a 2.5 m long ramp inclined at 30o. A worker, ignoring friction, calculates that he can do this by giving it an initial speed of 5.0 m/s at the bottom and letting it go. But friction is not negligible; the crateslides only 1.6 m up the ramp, stops, and slides back down.

Find the magnitude of the friction force acting on the crate, assuming that it is constant.

33.72N

Explanation:

To find the magnitude of the friction force you take into account all forces over the object:

   (1)

where Mgsin0 is the gravitational force, Ff is the friction force F is the force of the worker, m is the mass and a is the acceleration of the object while it is going up the ramp.

it is necessary to find the acceleration of the crate:

with this value and the values for m, g, the angle and d you obtain, by using (1)  you obtain:

(3)

For the calculation of F:

If the surface would be frictionless you have:

with these values you can calculate the force F:

BY replacing this values in the expression (3) you get:

hence, the magnitude of the friction force is 33.72N

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.

luster is not a good characteristic to use by itself because luster is the property of minerals that indicates how much the surface of a mineral reflects light. the luster of a mineral is affected by the brilliance of the light used to observe the mineral surface.