Direction of frictional force Help

[jumptheair]

Direction of frictional force! Help!

Homework Statement

What is the direction of the frictional force for each question? OR is there not enough info given to answer the question?

1) http://www.learning.physics.dal.ca/library/Graphics/Gtype10/block01-19.gif
2) http://www.learning.physics.dal.ca/library/Graphics/Gtype10/block01-34.gif
3) http://www.learning.physics.dal.ca/library/Graphics/Gtype10/block01-17.gif
4) http://www.learning.physics.dal.ca/library/Graphics/Gtype10/block01-07.gif
5) http://www.learning.physics.dal.ca/library/Graphics/Gtype10/block01-01.gif
6) http://www.learning.physics.dal.ca/library/Graphics/Gtype10/block01-35.gif

Homework Equations

frictional force concepts

The Attempt at a Solution

1) Can't tell, because Fnet is negative but magnitude of weight and F is unknown?
2) Friction up, need it to keep block static against weight and F pushing down.
3) Friction to the right?, because it opposes the direction of motion?
4) Friction to the left, because friction is always perpendicular to the normal force?
5) Friction up, need it to keep block static against weight pushing down.
6) Can't tell, because the magnitude of weight and F is unknown.

I find the direction of frictional force really difficult to grasp.
Feel free to correct me at any point. Thanks

 

[learningphysics]

Everything seems good to me except 1 and 4.

For 4 how do you get left?

 

[Vijay Bhatnagar]

Just remember : Direction of frictional force on the object is opposite to the direction in which the object is moving or tending to move.

 

[jumptheair]

1. Friction down, because it opposes motion? Is this correct? If so, what effect does the decreasing velocity have?

4. Common sense tells me friction would act upwards but my book says the frictional force is ALWAYS perpendicular to the normal force, so I am confused.

 

[learningphysics]

1. Friction down, because it opposes motion? Is this correct?

yes.

If so, what effect does the decreasing velocity have?

that means that the acceleration is downwards and the net force is downwards.

4. Common sense tells me friction would act upwards but my book says the frictional force is ALWAYS perpendicular to the normal force, so I am confused.

But the normal force is to the left... so how can the frictional force be to the left?

 

[jumptheair]

Oh right! I was thinking that normal force is in opposite direction to gravity, but it's the surface. Thanks!