In today’s world, structures around us such as bridges, buildings, aircraft, robots, cars, factories, etc., have been planned, designed, and constructed by engineers who have used the principles of mechanics to create them. The word “Mechanics” was first invented by Sir Isaac Newton. He defined mechanics as the branch of science that deals with the study of the physical states of bodies at rest and in motion under the action of forces.
Engineering mechanics is divided into two parts: Mechanics of Solids & Mechanics of Fluids.
The mechanics that deal with solid behavior are known as solid mechanics. Examples of solid mechanics are buildings, bridges, and cars.
The study of motion and forces in fluids, which can be liquids or gases, is called fluid mechanics. Examples of fluid mechanics are the air conditioning breeze and the waves produced by water. Ideal fluids such as water in a glass, viscous fluids such as mercury, and incompressible fluids such as any liquid classify fluid mechanics further.
Mechanics of Solids are further divided into two types: Mechanics of Rigid Bodies & Mechanics of Deformable Bodies.
In Mechanics of Rigid Bodies, we study bodies that do not deform even after applying forces or deform insignificantly.
In Mechanics of Deformable Bodies, we study bodies that deform after applying forces.
For example, a solid box is a rigid body, while an ice cube is a deformable body.
Mechanics of Deformable Bodies is further divided into Strength of Material, Theory of Elasticity, and Theory of Plasticity.
Similarly, the Mechanics of Rigid Bodies are further divided into Statics, where the body remains in a state of rest, and Dynamics, where the body is in a state of motion. For example, a building remains in a state of rest, while a moving car is in a state of motion.
Dynamics is also divided into two parts: Kinematics, where we do not consider forces that are responsible for motion, and Kinetics, where we consider forces that are responsible for motion.
Consider a rolling football on a football field. When we only count the rolling football without accounting for forces, it is Kinematics. But when we consider a boy who is kicking the football, applying force, and then the football is set in motion, it is Kinetics.
Next, fundamental concepts of mechanics which come into play are- Length, Time, Mass & Weight, Momentum, Force & its characteristics.
Length is a concept used to measure linear distances. In short, it is the distance between two points, such as A-B.
Time helps to study the occurrences of events. For example, the earth rotates around its axis once a day, i.e., 24 hours.
Mass is the quantity of matter within a body. Mass is independent of gravitational force and remains the same for the body. On the other hand, weight is a force that causes anybody containing mass to be attracted toward the center of the earth due to gravity. Therefore, the weight always acts vertically downwards. In simple terms, Weight = Mass X Gravitational Pull.
The gravitational pull on the earth’s surface is 9.81m/s2. Consider an astronaut whose mass is 120 kg. Its mass will remain the same on earth and in space. But its weight on earth will be 120X9.81 = 117.2N due to gravitational pull, while its weight in space will be zero because there is no gravity, and it will float in the air without weight or gravity.
Momentum is defined when we measure the motion of a moving body. Momentum = Mass X Velocity. Consider two vehicles, a bus and a car, moving toward each other on a road. If both the car and the bus have the same velocity, which vehicle will stop first? Of course, it is more difficult to stop a bus in comparison to a car. But why? Because their speeds were the same, but their masses were different. Therefore, more effort is required to stop a bus.
When we talk about forces, there are five types of forces – Tensile force, Compressive force, Shear force, Bending force, and Frictional force.
Tensile force is an applied force that pulls the object in its direction.
For example, in the game of tug of war when two people pull a rope with their bodies, tensile force is applied.
Compressive force is an applied force that compresses an object. For example, the suspension springs used in automobiles come under compressive force.
Shear force is a force that either separates or permanently deforms an object by acting in a direction parallel to the surface.
Bending force is a force that bends an object by putting tension on one side and compression on the other.
Frictional force is a force that occurs when there is motion or tendency of motion of one body concerning another body involving rubbing of surfaces of contact. Frictional force always opposes the direction of motion. It develops between the tires of a car and the road surface when the tires try to move on the road surface.
Now, let’s talk about the basic laws of mechanics.
The first law is Newton’s First Law, which states that an object at rest remains at rest or an object in motion remains in motion with a constant velocity in a straight line if the net force on it is zero.
The rate of change of momentum of a body is directly proportional to the impressed force and it takes place in the direction of the force acting on it.
Force is directly proportional to the rate of change of momentum.
But momentum = mass x velocity.
Assuming that the mass of the object does not change, we can further express that force is directly proportional to mass x rate of change of velocity. That is, force is directly proportional to mass x acceleration.
Hence, acceleration occurs when a force acts on the mass.
For example, let’s assume that two people are grocery shopping, one with a full cart and the other with an empty one. It is easy for the person with the empty cart to push it, whereas the person with the full cart needs to apply more force. In conclusion, an empty cart can be easily accelerated compared to a non-empty cart.
Now, let’s move on to Newton’s Third Law. Newton’s third law states that for every action there is an equal and opposite reaction.
For example, when we fire a bullet from a gun, the gun moves backward, which shows that there is an equal and opposite reaction to the gun’s action.
After Newton’s first, second, and third law, it’s time to talk about Newton’s Law of Gravitation.
Newton’s law of gravitation states that each mass in the universe exerts a mutual, attractive gravitational force on every other mass in the universe. This force is proportional to the product of two masses and inversely proportional to the distance between two objects.
You can see the mathematical expression of this on the screen. F=Gm1m2/r²
For example, let’s consider the electrical orbit of the moon around the earth. It follows Newton’s law of gravitation because the moon and the earth are constantly trying to attract each other through gravity. However, due to their orbital velocities, they do not deviate from their path and continue to follow it.
This was the complete introduction of engineering mechanics.
