Please help me understand relationship between work and friction/gravity

[HelloMotto]

Homework Statement

Can the work done by the force of kinetic friction on an object ever be positive? If "yes" give and example. If "no", explain why not.

I've said it can never be positive because the force of friction is always opposite in direction of the displacement

the second question is

Can the work done by the force of Earth's gravity on an object ever b e positive? If "yes" give and example. If "no", why not.

I said yes because let's say somethig is free falling, the force of gravity is the same direction as the displacement.

please correct my if i am wrong!

 

[alphysicist]

Hi HelloMotto,

Homework Statement

Can the work done by the force of kinetic friction on an object ever be positive? If "yes" give and example. If "no", explain why not.

I've said it can never be positive because the force of friction is always opposite in direction of the displacement

This is not correct.

the second question is

Can the work done by the force of Earth's gravity on an object ever b e positive? If "yes" give and example. If "no", why not.

I said yes because let's say somethig is free falling, the force of gravity is the same direction as the displacement.

Your answer here is correct, but the wording on your example is not quite right. Something can be in free fall and be moving upwards. If something is moving vertically downwards then the work of gravity is positive (and I think that's what you were meaning to say).

 

[HelloMotto]

can you explain why the friction force can be positive?

And yea i meant to say something moving vertically downwards. But curious, how can something free falling move vertically upwards?

 

[alphysicist]

can you explain why the friction force can be positive?

The work done by the frictional force can be positive if the frictional force is causing the motion. Can you think of ways that the kinetic frictional force might cause an object to move?

And yea i meant to say something moving vertically downwards. But curious, how can something free falling move vertically upwards?

The term "free fall" means that the only force acting is gravity. If you throw a ball into the air (and ignore air resistance), it's in free fall from the time it leaves your hand until it strikes the ground, even though it during that time it moves upwards, stops, and then moves downwards.

 

[elasticities]

The work done by the frictional force can be positive if the frictional force is causing the motion. Can you think of ways that the kinetic frictional force might cause an object to move?

Sorry for bumping an old thread, but I needed help with this too...
I can't think of ways the frictional force could be doing positive work...:S