15. A spanner of length 15 cm is used to
unscrew a nut by applying a minimum force
of 4 N. Calculate the moment of force
required.
0.5 Nm
60 Nm
0.6 Nm
50 Nm
Answers
Answer:
An introduction to moments and mechanical forces of rotation
Forces can cause an object to rotate and the turning effect of the force is called a moment.
If a resultant force acts on an object about a fixed turning point (the pivot) it will cause the object to rotate e.g. turning a nut with a spanner, applying a screwdriver, opening a door fixed on hinges. The pivot might also be called the fulcrum.
The rotational or turning effect, the moment, has a magnitude easily calculated from the formula:
M = F x d, where M = the moment of a force (Nm), F = force applied (N)
and d (m) is the perpendicular distance from the pivot point to the line of action of the force.
The perpendicular distance d is the shortest distance between the pivot point and the line of action of the force.
You get the maximum moment by pushing/pulling the spanner at a right angle to the line (d) between the pivot point and the line of action where the force is applied.
Screwing or unscrewing a nut on a bolt
This is illustrated by the simple diagram of a spanner above. The turning force is F x d.
The pivot point is the central axis of the bolt on which the nut is being turned by the spanner.
To tighten or loosen the nut you apply a force, to best advantage, at 90o to the spanner itself.
Applying the force at any other angle less than 90o reduces d and so reduces the effective moment of the force.
You determine the force F by how hard you push/pull the end of the spanner, but d is a fixed distance for give spanner.
This is one of many situations where you are applying a force to increase the effect of your hand muscles.
The size of the moment increases with increase in distance d or applied force F.
The longer the spanner, the greater is d, hence the greater the turning force generated - the greater the mechanical advantage of the lever system.