(2006? Jinshan District simulation) mass M = 2.0kg of small iron block is stationary in the horizontal rail AB end A. The shape and dimensions of the guide rail and bracket ABCD are shown in the figure. The shape and dimensions of the rail and the bracket ABCD are shown in the figure, the mass
When the rail just to be unable to maintain equilibrium, the C end of the force is zero, at this time the rail (and the bracket) is subject to four forces: the pressure of the slider on the rail FN = mg, vertically downward, the friction of the slider on the guideway Ff = μmg = 10N, gravity G = m ′ g, the role of the O point, the direction of the direction of vertically downward, acting on the axis of the D end of the force.
Set the iron block at this time traveled a distance S, according to the conditions of equilibrium of the object with a fixed axis of rotation and the dimensions of the figure, there are:
m′g × 0.1 + mg (0.7-s) = Ff × 0.8 = μmg × 0.8?
Substitute data, there are: 40 × 0.1 + 20 (0.7-s) = 10 × 0.8?
Solve the s = 0.5m?
The iron block is subjected to a frictional force Ff = 10N in the right direction.
According to Newton's second law, we have: F-Ff=ma
Solve: a=1.0m/s2?
∵ S=
at2?
∴t=1.0s
Answer: from the time of the motion of the small iron block, the longest time for which the guide rail (and the bracket) can be held at rest is 1s."