The biomechanics of weightlifting are heavily driven by force; therefore, weightlifters must have a basic rudimentary understanding of force in physics. The formula for force is mass times acceleration (F = ma). Force is essentially a push or pull and is measured in Newton's. The formula for force is derived from Newton's second law of motion, which states that the force acting on an object is equal to its mass multiplied by its acceleration. Thus, force is mass times acceleration (F = ma).
Force is a vector because it has both magnitude (a unit of measurement to specify the size or intensity of an event or object having distance or quantity) and direction. When one force is stronger than another, it can cause objects to:
Accelerate
Decelerate
Change directions
Change shape
Force is a quantity that causes an object to change its motion, whereby motion is the change in the position of an object concerning its surroundings at a given interval of time and a change in direction. Force, mass, and acceleration are all related! Here’s why - The more mass an object has, the more force you need to accelerate it, and the greater the force, the greater the object's acceleration.
This means that an object's acceleration is directly proportional to the total force acting on it and inversely proportional to its mass. In other words, a larger net force acting on an object causes greater acceleration, and objects with larger masses require more force to accelerate them.
Newton's Second Law of Motion states that an object's acceleration is directly proportional to the net force applied and inversely proportional to its mass. For example, a large force on a small object gives it considerable acceleration, but a small force on a large object gives it very little acceleration. Newton's second law of motion states that the acceleration of an object is dependent upon two variables—the net force acting upon the object and its mass.
The acceleration of an object depends directly upon the net force acting upon the object and inversely upon the object's mass. As the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased.
I always like to use the example below from World's Strongest Man 2014, whereby Hafþór Björnsson set a blistering event record with the Keg Toss event. One can see that the lighter mass kegs (starting at 40 lbs) at the beginning of the time limit required little net force to clear the bar.
However, the heavier kegs towards the end at 55 lbs required Hafþór to exert a larger net force to generate sufficient acceleration to clear the bar, which was minimal in the last keg. This is because as the weight of the kegs (objects) increased, the acceleration of the keg tossed decreased as the force acting upon the keg object increased.
Internal and External Forces
Internal Forces
In physics, an internal force is a force inside an object or system. It arises from the interactions between the parts of the system itself, such as the tension in a rope or the compressive forces in a structural element. Its purpose is to maintain equilibrium.
Internal Forces are forces that the system's particles exert on each other. They are any forces that act on a structure from within. There are generally three types of internal forces in physics:
Axial Forces - (normal force) is a compression or tension force acting aligned with the extension of a structure member. Combats external compression and tension.
Shear Forces – a force acting in a direction perpendicular to the alignment of the member. A vertical force that acts upon the entire object.
Moment forces – the turning result of a force multiplied by the distance from its acting location to the turning point. Navigates the movement of certain parts of an item up and down.
Internal Forces in Biomechanics
Internal forces in biomechanics are generated by ourselves, by muscular contraction. They are forces exchanged by the objects in the system. These are forces generated by the body tissues, such as muscles, bones, and joints, which affect our movement. They are a result of muscle actions by the cross-bridges formed by the cross-bridge theory (CLICK HERE to learn more about this theory).
The illustration below illustrates internal forces acting to create the momentum for an athlete to run. One can see from the knee joints that internal forces such as compressive force, joint reaction and fear force within the system create movement.
These internal forces are transmitted through the tendons to the bones of the skeletal system, causing angular motion around a joint. Internal forces produce torques or rotation about the joints and tension, compression, torsion, or shear within the body's anatomical structures.
External Forces
External forces are forces that act on a body from outside its system, influencing its movement and performance. An external force can be a contact force (like friction or air resistance) or a field force (such as gravitational or magnetic). Examples of the external loads are:
Friction Force: force between two surfaces that are sliding, or trying to slide, across each other. The resistance is offered by the surfaces in contact when they move past each other.
Gravitational Force: The pull of an object toward the Earth. A larger mass object pulls a smaller mass object. Two objects closer together have a stronger gravitational force than two objects further away.
Normal Force: The normal force is the force that surfaces exert to prevent solid objects from passing through each other. It is a contact force. If two surfaces are not in contact, they can't exert a normal force on each other.
Air Resistance (Drag) Force: the force that acts on an object moving through the air, slowing it down in the opposite direction of its motion. Air resistance is a type of friction, a force opposing motion. It occurs when an object collides with air molecules, creating friction that slows the object down.
External Force in Biomechanics
In biomechanics, External forces that act against the human body can be produced by external objects or the body’s voluntary exertion of force, such as kicking a football, throwing a javelin, squatting, deadlifting, or bench pressing a barbell.
The centre of gravity is the location where the body's mass is concentrated. Like the Center of Mass (COM), it is the point within an object where the entire body is balanced with respect to gravity. External forces can significantly impact the body's centre of gravity (CG), influencing its stability, movement, and behaviour.
External Contact and Non-Contact Forces
There are two types of external forces: contact and non-contact; a contact force is any force that occurs due to two objects making contact. Examples of contact force are:
Spring Force
Drag Force
Frictional Force
Normal Force
Applied force
A non-contact force is a force that acts on an object without physical contact with it. They can cause objects to change shape, movement, or change direction. The types of non-contact forces are:
Gravitational Force
Electric Force
Magnetic Force
We will consider only one non-contact force—the gravitational force. Then, for contact forces, we focus on Applied forces. Therefore, let’s talk about both these forces in greater detail.
Gravitational Force
As you might already know, gravity is that invisible force that pulls objects toward each other. It’s the earth’s gravity that keeps us on the ground and what makes things fall, like the apple from the tree! Anything that has mass also has gravity. Objects with more mass have more gravity. Gravity also gets weaker with distance. So, the closer objects are to each other, the stronger their gravitational pull is. Every object, including humans, is pulling on every object in the entire universe. This is Newton's Universal law of gravitation.
The formula for calculating this force isF=GMm / r2, whereby F is the force of gravity, G is the gravitational constant, capital M is the mass of one object, m is the mass of the other object, and r2 is the distance between the two objects.
The force between most objects is so weak it’s not noticeable. What makes a football fall to earth is our planet's immense mass compared to the ball. The large mass of the earth means the force between the earth and the ball is very strong. Because the ball's mass is so small, it is pulled towards Earth. Every object in the universe attracts every other object.
Therefore, we can summarise that the gravitational force depends on two factors:
the masses of the objects
their distances from one another
Gravitational Force Example
Objects with greater mass attract one another more than those with less mass. Objects experience greater forces of gravity when they are nearer to one another. As long as the distance between objects doesn’t change, the force of gravity is constant even if objects are moving.
The more mass, the stronger its gravitational force. Because the Earth is massive, it generates a huge attractive force, which is why when we jump, we fall backwards towards the Earth rather than any other object surrounding us.
Using the earth again, to not feel its effects, you would have to go further than the moon, which is 240,000 miles away and still feels the effects of the earth's gravitational forces.
The force of attraction between any two bodies is directly proportional to the product of their masses and is inversely proportional to the square of the distance between them.
Applied Force
The applied force is the force exerted by one object on one or more other objects that causes it to change in motion, shape, or state of rest. An applied force is a force applied to an object by a person or another object. An applied force is a contact force applied to an object by external means. One can see from the illustration and example of applied force below that a man is applying force to a box by pushing it.
Examples are opening a door, kicking a football, swinging a bat to hit a ball, and weightlifting, whereby muscles exert force to perform movements and overcome resistance. The weights are lifted by applying a force caused by the action of the person's muscles. The example below shows an example of an athlete pushing-pressing a weighted barbell overload by applying force to the barbell to press it overhead.
An applied force is a contact force that is applied to an object by external means. As a result of applied force, the object either moves or deforms. The object generally moves in the direction of the applied force.
An applied force is a force applied to an object by a person or another object. If a person pushes a desk across the room, an applied force is acting upon the object. The applied force is the force exerted on the desk by the person.
Work & Work Done
Work is defined as the force applied over a given distance, and the equation for work is:
Work (W) equals the force (f) times the distance (d) W = f ⋅ d
Work done on an object is equal to the force applied times the distance travelled by the object, whereby both the force and distance are in the same direction. In physics, for something to be quantified as work, the object has to move; holding a bowling ball in one hand for 1 hour doesn’t qualify as work. However, a bowler using applied force to throw the bowling ball down a bowling alley lane would constitute as Work done.
Both force and distance are vector quantities as they have magnitude and direction, but Work is a scalar quantity. The work done due to displacement caused by a force is a scalar quantity, not a vector. Work is considered a scalar quantity because it only has magnitude and no direction. This is because Work is defined as the product of the force applied to an object and the displacement of that object in the direction of the force.
The illustration to the right shows a lady applying force against the wall but pushing against it. Pushing against a wall doesn't count as work in physics because of the definition of work. Work is defined as the product of force and displacement in the direction of that force. Put another way, work is only done when a force causes an object to move a certain distance.
When you push against a wall, you exert a force, but the wall doesn't move. Since there is no displacement, the work done is zero. This is why, in physics, while you may be expending energy in terms of effort or fatigue, it does not qualify as work in the classical mechanical sense. Therefore, force might have been applied to the wall, but the displacement was in a zero direction. Consequently, it does not qualify as work done.
The father pulling his daughter in a trolley below is an example of work done in physics. The father applied 50N (newtons) of force, pulling the object (the trolley with his daughter inside) at a 30-degree angle for 30m (meters). Therefore, the total work done is 50N * 30m = 1500 Joules.
Work is done in weightlifting by increasing the weight on the barbell, dumbbell, or machine or by increasing the distance over which the weightlifter pushes the barbell or weights. When lifting a load, work is done against the force of gravity. The amount of work done equals the force applied multiplied by the distance lifted. This can be expressed mathematically as W = F ⋅ d, where W is the work done, F is the force applied, and d is the distance lifted.
Let's take an example: a weightlifter clean presses a 65-kilogram (kg) barbell (25kg plates on each side and 15kg barbell weight) from the ground by 2 meters (m). The work done on the barbell is against the earth's gravity, which is 9.8 meters per second squared m/s2. Therefore, the work done upon the weight against gravity can be calculated as follows:
Work= 65 x 9.7 = 490 Newtons
Work Done = 490n x 2m = 980 J (Joules)
Work Done = (Mass × acceleration due to gravity) × Displacement
We use joule, a unit of work or energy in the International System of Units (SI), equal to the work done by a force of one newton acting through one meter expressed as Newton • meter (N • m).
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