9702/Oct Nov/13/2016/Q16

An air-conditioning unit is supported by a rigid beam PQ, as shown.

 Which diagram shows the directions of the horizontal and vertical forces acting on the ends of the
beam?

Solution:

Answer: B.

The resultant force is the tension which is point to opposite direction as shown in figure below.
Follow the vector concept, only choice B is make sense and fulfill the condition.

Since the air-conditioning unit is in equilibrium, the resultant force on the system is zero.

The weight of the unit acts on the beam PQ. Since the beam is inclined the force appears along the beam, from P to Q.

Breaking down this force results in a component to the right and a component downwards.

Now, end Q of the beam is in contact with the wall. From Newton’s 3^rd law, the wall would also exert a (contact) force on the beam at Q. This force is from Q to P (opposite in direction of the initial force).

Resolving this force gives a component upwards and a component to the left.

We can still observe that the overall force on the system is zero.

Reference: PYQ - Oct/Nov 2016 Paper 13 Q16