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Mechanics toppling question

Hey guys

Could you please help me with this toppling question?I have looked at the answer and understand it, but for the second part, why is it not possible to assume that the normal reaction force at vertex P is 550N?

If the object was stationary and in equilibrium, then wouldn't normal reaction equal the weight force (550N) and thus stay that way when toppling? I know this is incorrect but I don't understand why. Can the reaction force change?

Thank you for your help!

Toppling question.png

Reply 1

username2452153

Original post by dyuunn

Hey guys

If the object was stationary and in equilibrium, then wouldn't normal reaction equal the weight force (550N) and thus stay that way when toppling? I know this is incorrect but I don't understand why. Can the reaction force change?

If a body is in equilibrium then net force on it should be zero (translational equilibrium). If normal force were 550N then, the total upwards force would be 550N (normal force) and the vertical component of T (which is obviously non-zero) and downward force would be 550N hence there would be a net upward force, making the object accelerate upwards which is not true.

Reply 2

dyuunn

Original post by tangotangopapa2

Thank you for your answer - so in these sorts of cases, we would not assume anything about the reaction force?

Reply 3

username2452153

Original post by dyuunn

Thank you for your answer - so in these sorts of cases, we would not assume anything about the reaction force?

The reaction force is always equal to the total downward force, here mg(weight) - T sin 60 degrees (upward component of tension). In the microscopic level, object crushes the surface a bit, and the normal force is the force applied by the surface to prevent it from being crushed further (analogous to the restoring force in the spring).

We would assume reaction force to be equal to the total downward force (perpendicular and into the surface to be more general) but in opposite direction.