Understanding Force, Weight, and Motion: A Physics Primer
Understanding Force, Weight, and Motion
Force is anything that can deform a body or change its state of rest or motion.
Force = mass × acceleration
Representation of Forces
Forces are represented by arrows. The line segments indicate the direction, and the pointed end of an arrow indicates the sense.
Types of Forces
- Instantaneous forces: Act only during a very short period (e.g., kicking a ball).
- Constant forces: Act on an ongoing basis (e.g., the Earth’s gravity).
The unit of force is the Newton (N) in the SI system.
Gravity and Gravitational Acceleration
Gravity causes all bodies falling freely from a certain height to accelerate. This acceleration is called gravitational acceleration (on Earth, terrestrial gravitational acceleration), represented by the letter g. The gravitational acceleration g is greater the greater the severity. On land, the value of g is approximately 9.8 m/s2.
Weight
Weight is the attractive force that gravity exerts on a body.
The weight of a body depends on:
- Its mass
- The gravitational acceleration of the place where it is located.
Mathematically, it can be expressed as:
Weight = mass × gravitational acceleration
P = m · g
Mass invariably depends on the amount of matter in the body and remains constant regardless of location.
Weight, however, is dependent on gravity. A given body’s weight is not the same on Earth as on another planet or in different places on the Earth’s surface.
Weight is measured with a device called a dynamometer; mass is measured with balances.
The unit of measure for weight in the SI system is the kilopond (kp), also called kilopeso. A kilopond is the weight on Earth of a body with a mass of 1 kg.
As we are situated on land, the weight in kp values shown at the same time, the value of the mass in kilograms.
Thrust (Buoyancy)
Thrust = weight of fluid displaced
E = mL · g = V · dL · g
E = V · dL · g
Where V is the volume of liquid displaced, mL is the mass of the liquid, dL is its density, and g is the acceleration of gravity.
Thrust is a force and, in the International System, is measured in Newtons. Therefore, the other quantities in the equation must be expressed in appropriate units.
If the weight of an object is less than the thrust, the body floats on the surface.
If the weight of an object is equal to the thrust, the body, partially submerged, remains in equilibrium.
If the weight of an object is greater than the thrust, it sinks.
Inertia and Motion
For a body to be put in motion, have its speed modified, or be stopped, a force must act upon it.
Leonardo Galileo stated:
A moving body on which no forces act moves with uniform rectilinear motion.
A constant force applied to a body gives it a constant acceleration, which is directly proportional to the force and inversely proportional to the mass.
Acceleration = force / mass
a = F / m
Force = mass × acceleration
F = m · a
Deformable and Non-Deformable Solids
Solid bodies can be deformable or non-deformable.
Deformable Solids
The change in shape or deformation experienced by a solid body under the action of a force is due to the modification of the gaps between the particles constituting the matter.
If this distance does not affect the sound, it is not distorted.
- Plastic bodies are deformed by the action of a force and do not recover their original form when that force ceases to act.
- Elastic bodies regain their original shape when the force that deformed them stops acting.
Non-Deformable Solids (Rigid Bodies)
Deformable solid bodies are also known as rigid bodies.
If the forces acting on a rigid solid are very large, there may be a break or fracture.
Equilibrium
We say that a body is in equilibrium when the resultant of all forces acting on it is zero.
Work
Work is the amount of energy transferred by a force acting through a distance.
W = Fd · d
Where W means work, Fd is the force acting in the same direction as the motion, and d is the distance over which the force acts. The SI unit for measuring work is the joule (J) (N * m).
Kinetic Energy
K = 1 / 2 m · v2
Potential Energy
Ep = P · h