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# Mechanics - confusing thought experiment Watch

1. I was doing a little thought experiment which got me a bit confused...

Imagine that you hold a block next to a rough, vertical wall, and then let go. The block will fall downwards due to its weight, but because it's in contact with the wall, there will be friction acting upwards (which means the block's acceleration will be less than 9.81 ms^-2). Now, since the block is moving, - however, surely there is no normal contact force exerted by the wall on the block, since the block's weight acts vertically? If so, wouldn't that mean that there is no frictional force...?

Confused - help!
2. Nice problem, got me thinking also!
Perhaps the flow of air around the block sort of creates downforce, acting horizontally in this case. This creates a contact force between the block and wall. I'm no engineer though, what do you reckon?

What's to say that the block even experiences any friction?
3. (Original post by MrLatinNerd)
I was doing a little thought experiment which got me a bit confused...

Imagine that you hold a block next to a rough, vertical wall, and then let go. The block will fall downwards due to its weight, but because it's in contact with the wall, there will be friction acting upwards (which means the block's acceleration will be less than 9.81 ms^-2). Now, since the block is moving, - however, surely there is no normal contact force exerted by the wall on the block, since the block's weight acts vertically? If so, wouldn't that mean that there is no frictional force...?

Confused - help!
The block probably won't remain in contact with the wall throughout its descent. Try an experiment by placing an object against a wall and dropping it.
4. (Original post by MrLatinNerd)
I was doing a little thought experiment which got me a bit confused...

Imagine that you hold a block next to a rough, vertical wall, and then let go. The block will fall downwards due to its weight, but because it's in contact with the wall, there will be friction acting upwards (which means the block's acceleration will be less than 9.81 ms^-2). Now, since the block is moving, - however, surely there is no normal contact force exerted by the wall on the block, since the block's weight acts vertically? If so, wouldn't that mean that there is no frictional force...?

Confused - help!
There are different kinds of frictions.
5. (Original post by Zacken)
There are different kinds of frictions.
OK... So in this case, how does (sliding?) friction behave - how can you calculate the magnitude of the frictional force?

Edit: also, why does friction act differently in this scenario to one where the block is in contact with a plane inclined at an acute angle?

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