Question

1. All of the following show friction as a useful force, except _____.

Answer 1

1. Having to push a rough and heavy box across the floor to move it.
2. The coefficient of sliding friction will decrease.
3. Greater than the friction involved in rolling an object.
4. Drag.
5. Sliding (kinetic) friction. Sliding friction is also known as kinetic friction - force that is needed to keep a surface sliding along another surface.

Answer 2

1. having to push a rough and heavy box across the floor to move it

In this case, friction is not a useful force: in fact, our aim is to push the box across the floor and move it, while the direction of the friction is against the motion of the box, so this means that we have to do more work in order to win the friction and move the box.

2. The coefficient of sliding friction will stay the same.

In fact, the coefficient of sliding friction does not depend on the mass or the size of the object, but it depends only on the two materials which are in contact between each other (so, the material of the object and the material of the floor/surface).

3. the friction involved in sliding an object is greater than the friction involved in rolling an object

This can be explained by thinking that when sliding an object, the area of contact between the object and surface is generally larger than the area of contact when rolling an object (ideally, there is only one point of contact between the object and the surface when rolling an object), so friction has a greater effect.

4. drag

Drag is the frictional force exerted by a fluid (liquid or gas) on a solid object moving through the fluid. In particular, it is due to the contact between the surface of the object and the molecules of fluid: part of the energy of the object is transmitted to the molecules of the fluid, and the object decelerates.

5. kinetic friction

Kinetic friction is the force of reistance that occurs when an object is being pulled over a surface, and its formula is given by

$F_f = \mu m g$

where $\mu$ is the coefficient of friction, m is the mass of the object and g is the gravitational acceleration.