Dear Doctor

[The 3rd Law (The Law of Action-Reaction) “For every action force there is an equal and opposite reaction force.”

Anytime something applies a force, there will be an equal and opposite force back in the opposite direction.

● Push on the handle of a lawnmower to make it go forward, and it will push back against you in the opposite direction with just as much force.

● This is the pressure that you feel of the handle against your hand.

There is one ultra important thing to remember when you are looking at action-reaction pairs.

● The two forces that you are looking at are each acting on different objects!

● If you are examining what you think are action-reaction forces, but the forces are both acting on the one object, it is not an action-reaction pair.

● In the above example, you exert a certain force on the lawnmower. The lawnmower exerts an equal force on you. Two objects, two forces.

Some examples of action-reaction forces depend on the objects being in direct contact, meaning that the two objects involved are actually touching each other to exert forces on each other. These are called "contact forces." The following examples explain two objects interacting according to Third Law.

1. A car driving forwards Action: the tires on a car push backwards on the road… Reaction: the road pushes forwards on the tires.

2. You swimming Action: you push the water backwards... Reaction: the water pushes you forward.

3. A rocket takes off from a launch pad Action: a rocket pushes out exhaust backwards... Reaction: the exhaust pushes the rocket forward.

One of the original arguments that flight in the vacuum of space was impossible was that there would be nothing to push against. This action-reaction explains how a rocket can fly in space where there is no air to push against.

action-reaction pairs can also happen without friction, or even with the objects not touching each other, known as "action at a distance" forces …]

Dear Dr. Sundus F Hantoosh

In seismic engineering, Newton's three laws play an important role through mathematical analysis. These are the laws of inertia, acceleration and action-reaction. The bodies around us that remain stationary are, in fact, not free of forces. Various forces are exerted on them, but because of the equilibrium between them, their sum is zero. For example, the weight of a body is counteracted by the reaction of the ground. The weight of the mass combined with the acceleration of the ground determines the force of inertia of the structure. The force of inertia multiplied by the height at which it is applied gives us the moment of inertia.

Consider a six-storey building with an area of 100 m² on each floor, where each floor consists of 35 cubic metres of concrete. The total amount of concrete is 210 m³ for the floors and 50 m³ for the base. The specific weight of the concrete is 2450 kg/m³. The weight of the structure is therefore 514,5 tonnes for the floors and 122,5 tonnes for the base, with a total weight of 637 tonnes of concrete. Adding the floors, the payload, and the walls, the total load is 817 tonnes.

A recent earthquake in Turkey exceeded an acceleration of 2g, while larger earthquakes can reach 3g. Let's consider the 817-ton building under the acceleration of 2.41g, which is 2.41×9.81≈23.6631 m/s² To calculate the force of inertia for a building with a mass of 817 tons and an acceleration of 23.6631 m/s², we follow the following procedure:

Mass to kilograms (kg) formula:

817 tonnes = 817×1,000 kg = 817,000 kg

Calculation of the force of inertia:

Inertia force = Mass × Acceleration

Inertia force=817.000 kg×23,6631 m/s2

Force of inertia=817.000kg×23,6631m/s2 =19.322.622,7 N

Conversion of force to kilo-Newton (kN):

1 kN=1,000 N

Force of inertia = 19,322,622.7 N/1,000 = 19,322.6 kN

The force of inertia is 19,322.6 kN or 1,970 tonnes.

Each floor of the 6 floors has an inertia of 1970/6=328 tons

The moment of inertia for each floor is the height of the floors times the inertia force of one floor. If the height of each floor is 3 meters, the total overturning moment for the six floors is 3+6+9+12+15+18=63 meters x 328 tons = 20664 tons.

Conclusion: A six-storey building with a floor area of 100 m² and a weight of 817 tonnes, under a ground acceleration of 2.41g, will develop an inertia and base moment of 1970 tonnes and generate a overturning moment of 20664 tonnes.

Is it possible for an 817-ton building to withstand a overturning moment of 20664 tons without overturning or damage to the connecting beams and girders? Obviously not, and that is why footing with the ground is required so that the ground can absorb the moment force.

Is it possible for an 817 ton building to withstand a base shear force of 1970 tons? Obviously not, and that is why prestressing is required in the cross section to increase the dynamic mean of the friction of the aggregate.