Why are my force diagrams wrong?

skeeter

Math Team
Jul 2011
3,360
1,850
Texas
For the paving stone leaning against the wall, you did not place the forces of static friction acting on the corners. Think about leaning a ladder against a wall where the wall and ground are both icy (essentially frictionless) ... wouldn't the ladder slide?

For the snooker ball collision, the force of static friction acts on the balls as they roll ... at the moment of collision, they are not rolling.
 
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skeeter

Math Team
Jul 2011
3,360
1,850
Texas
Static friction?
At the instant of collision, the balls cease rotation (unless “english” is applied). When they separate, static friction applies the torque necessary for rolling without slipping ... once the objects reach a constant angular speed, static friction no longer acts on the object.
 
Feb 2016
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Bedfordshire
For the paving stone leaning against the wall, you did not place the forces of static friction acting on the corners. Think about leaning a ladder against a wall where the wall and ground are both icy (essentially frictionless) ... wouldn't the ladder slide?
What if only the wall or ground was icy... would the ladder still slide?

For the snooker ball collision, the force of static friction acts on the balls as they roll ... at the moment of collision, they are not rolling.
ok, but why is the P arrow on the other side of the ball? I thought you only do that if it's a non-contact force like air resistance.
 

skeeter

Math Team
Jul 2011
3,360
1,850
Texas
more than likely, it would slide if the ground were icy ... not so much if the wall had negligible friction. one would have to conduct an experiment to observe what actually would happen.

translating a vector does not change its magnitude or direction. yes, to be correct, it should be placed at the contact position, but they probably wanted vector P to be more visible in the sketch they made.
 
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skeeter

Math Team
Jul 2011
3,360
1,850
Texas
static friction acts on rolling object if it is accelerating without slipping ...

F_s rolling torque.png




rolling at a constant speed indicates no acceleration $\implies$ no friction force

idealroll.gif

bowling ball dynamics is interesting, kinetic friction exerts the torque to start spinning until rolling w/o slipping is achieved ...

bowling ball.png

concept of rolling resistance ...

rolling_resistance.jpg