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how to find the minimum F

how to get the correct value here?
(edited 3 years ago)
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Reply 1
Avatar for Sinnoh
Sinnoh
22
Can't see the question
Reply 2
Avatar for Sinnoh
Sinnoh
22
Original post by Hedwigeeeee
how to get the correct value here?


Okay I see it now.
You don't need to do arctan to find the angle; conveniently enough, 72 + 242 = 252 .
I think you just got mixed up with the directions; F and the frictional force are acting upwards, gravity is pulling it down. So F + friction is equal to the downwards force.
Original post by Sinnoh
Okay I see it now.
You don't need to do arctan to find the angle; conveniently enough, 72 + 242 = 252 .
I think you just got mixed up with the directions; F and the frictional force are acting upwards, gravity is pulling it down. So F + friction is equal to the downwards force.

why the frictional force is also upward?
Reply 4
Avatar for Sinnoh
Sinnoh
22
Original post by Hedwigeeeee
why the frictional force is also upward?


Friction force just acts opposite to the direction the object would move. It's a resistive force.
Think of it a bit more intuitively - if you've got a box on a slope, friction isn't going to make it accelerate downwards. Without the extra force F, the box would be accelerating down the slope due to gravity.
Original post by Sinnoh
Friction force just acts opposite to the direction the object would move. It's a resistive force.
Think of it a bit more intuitively - if you've got a box on a slope, friction isn't going to make it accelerate downwards. Without the extra force F, the box would be accelerating down the slope due to gravity.

Isee, so if that F let the box to move upwards, then friction is downward in this case right?here is at rest
Reply 6
Avatar for jojicr7
jojicr7
9
To find the maximum or minimum value of friction, you almost always consider the case the particle is about to slip. So:
a) crate is about to slip down the plane (so friction acts up the plane)
Resolving forces up the line of greatest slope of hill:
F μR = mg•sinθ

a) crate is about slip up the plane (so friction acts downwards)
Resolving forces down the line of greatest slope of the hill:

mg•sinθ μR = F

One of these will give you the minimum value of friction, depending on the magnitudes. (This also implies that when the particle is stationary, the friction can not be outside the range imposed by the maximum and minimum value found above.)
Hope this was helpful xD
{been a while since I did mechanics so correct me if I’m wrong}

Edit: sorry, switch theta with alpha. Also, for
tan α = 7/24, you could directly find
sin α = 7 and cos α = 24
Because of the trig rule
(edited 3 years ago)

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