In this article, we are going to discuss what is the difference between internal and external forces.
Internal forces are the forces exerted from within the system whereas external forces are the forces imposed on the system from the surrounding.
Difference Between Internal And External Forces
| Internal Forces | External Forces |
| Force experiencing within the system without any external potency is known as internal force. | Force acting on the system from the surroundings due to external agents is called external force. |
| The center of mass of the system is immovable as there is no momentum of the system. | The Center of gravity of mass varies with time as the system gains momentum due to external forces. |
| Energy is sustained in the form of mechanical energy. | Mechanical energy is converted into kinetic or potential energy of the system. |
| Internal force is a conservative force. | The external force is not a conservative force. |
| The internal forces acting within the system are in opposite directions to each other, the forces cancel out and hence there is no net work done on the system. | The external forces act in the direction of the force imposed and the work is done. |
| Some examples of internal forces are gravitational force, magnetic force, electric force, spring force, etc. | Examples of external forces are frictional force, applied force, normal force, tension force, air drag, etc. |
What is Internal Force?
The internal force reacting within the object doesn’t cause the acceleration of the object at rest but there are internal actions that result in a change of energy of the system.
Internal forces are the forces acting within the system, which may be due to dipole moment, the motion of the molecules or charged particles, density, etc. Examples of internal forces are gravitational force, electric and magnetic forces, spring force, etc.
Due to the internal actions, the potential or kinetic energy of the object changes to the mechanical energy of the object which is conserved by the system. Since the acceleration of the object due to internal forces is zero, this implies that there is no momentum of the object and hence the work done by the system is always zero and the mechanical energy is also conserved. Hence, the internal force is a conservative force.
How internal forces act on the system?
The internal forces within the system act in a direction opposite to one another thus canceling out and resulting in zero output.
The internal forces mainly appear to resist the changes caused by external forces or in response to the external agents that may be due to electric, magnetic field interaction, or temperature change.
When the conductor is subjected to the electric field, the charged particle moves in a helix but does not cause any exterior change on the object, or cause the acceleration of the center of mass. The motion of the charged particle originates the magnetic field production due to the spin of the electric particle.
On introducing the material having magnetic characteristics in a magnetic field, the dipoles get arranged in the direction of the field. The magnetic flux lines traversing internally cause the magnetic spin dipoles to get aligned according to the field.
An object always exerts a force due to the gravity of the Earth that relies upon the mass of the object.
What is the External Force?
The external forces are the forces acting on the system due to external agency.
The external forces cause the object to displace or resist the motion of the accelerating object. Some external forces are applied force, air resistance, tensional force, normal force, frictional force, etc.
The object at rest has zero kinetic energy, when the external force is applied on the object, the potential energy is converted into the kinetic energy that is utilized in accelerating the object until it experiences a force to resist its motion due to which the kinetic energy is again converted into potential energy.
The work is done in the direction of the applied force. If the work done is positive, it implies that the system gains energy in the form of potential energy or kinetic energy, and if there is a loss of energy from the system then the work done is negative.
How external forces act on the system?
According to Newton’s First Law of Motion, “the object will be in a state of rest on in a continuous motion at constant speed unless and until some external force is exerted on the body.”
The external force is required to either accelerate the body or to resist the motion of the object. This could be applied force, normal force due to the weight of the system, a force due to air resistance, or frictional force exerted on the body that resists the motion of the body dragging it backward.
The equal and opposite force reacting on the object in a direction opposite to the normal force due to weight and geometry, and across the length of the object is a tensional force. This force is created across the length of the body when a load is applied to the material.
Examples
Let us discuss some examples to understand the internal and external forces exerted on the objects.
Car Climbing Up the Hill
Consider a car climbing on a hill. T is a tensional force, N is a normal force, the force due to friction and air drag acting backward and force due to gravity is acting towards the ground from the center of gravity.
For a car to climb up the steeper hill, more acceleration has to be given to the car. The steeper the slope of the road more you need to provide the acceleration because the internal force due to gravity is pointing backward and also the frictional force and air resistance drag the car down the slope.
The more the force acting backward, the equal force will be required for a car to climb the hill; this will create a tensional force reacting forward that keeps the car accelerating forward.
Man Pushing a Load
Consider a man pushing a load of mass ‘m’, the force due to gravity on the object is ‘mg’. The normal force is against the weight of the load.
When a force is applied to displace the object, a frictional force is applied on the surface of the object simultaneously on rubbing to the ground. The greater the mass of the object more will be the frictional force into action. The friction of the surface depends upon the pattern of the surface, less friction will be produced if the surface is smooth, more the roughness of the surface more will be the frictional force on the object accelerating on the surface.
Contraction and Expansion of Rocks
Contraction and extension of the rock forming the cleavages on the surface of the rocks are due to the thermal agitation and varying weather conditions causing the cleavages and erosion of the rocks.
During the cold weather, the molecular distance constituting the rocks contracts whereas in the hot summers the molecular distance expands which results in the formation of cleavages on the rocks. This is only due to the internal activities going within the composition of the rock due to the absorption and emissivity of sunrays.
Weight Attached on a Pulley
Consider a pulley with masses attached on both the ends of the rope, m2>m1. Since the mass m2 is greater than m1, m2 will accelerate down. Due to the weight imposed on both ends of the rope, the tensional force will be created across the length of the rope.
The force acting on the mass m1 is the sum of the external force due to tension in the rope due to the attached mass and internal force due to gravity acting downward and given by the relation,
F1=T-m1g
m1a1=T-m1g
Since the acceleration of the mass m1 is upward in the direction opposite to the force due to gravity, hence it is negative.
The force acting on the mass m2 is a gravitational force in the direction of the acceleration of the mass and opposite to the tensional force acting across the length of the rope from the pulley.
F2=m2g-T
m2a2=m2g-T
On what factors does the internal and external forces depend?
The internal and the external forces in actuality depend upon the intrinsic as well as the extrinsic factors and amount of force imposed on the object.
The internal forces within the system depend upon the dipole moments, internal heat of the system, emissivity, temperature of the system and the surrounding, composition, weight, density, the separation between the molecules constituting the system, motion of the particles in the system, geometry of the system, molecular constituency, covalent bonds between the atoms, number of free particles, etc.
The external forces depend upon the extrinsic properties influenced on the system like how much is a force applied, the normal force due to weight and configuration, the frictional force due to the surface in contact with the object, the air drag, tensional force, etc.
Read more on Types of External Forces: Exhaustive Insight.
Frequently Asked Questions
What forces act on the object floating on the surface of the water?
The force that causes the object to float on the water is a buoyant force.
The buoyant force is acting upwards on the object due to the volume of water which is the external force, whereas the internal force of the object that is the force due to gravity is always acting downward.
What are the various forces employed on the bullet fired from the rifle?
On firing the bullet, the accelerating force is applied on a bullet equal and opposite to the recoiling force on the gun.
When a bullet is in the air traversing the air column, the air resistance drags the motion of the bullet due to which the frictional force comes into the act as the bullet passes brushing the air while the gravitational force is acting downward.
What forces are associated with the athlete while running?
The athletic is able to run because of the gravitational force acting downward and the frictional force that prevents the athletic from falling.
Most importantly the muscular force is required by the athlete to accelerate his/her body which is an internal force involved by the athlete.
Why the center of gravity of an object doesn’t change on the application of internal forces?
On acceleration of the object the center of mass moves along with it.
The object doesn’t accelerate due to the cause of internal forces hence the center of gravity remains constant.
Why spring force is an internal force?
The spring force helps the spring to regain its original form. Every spring has different spring constants.
When a load is imposed on the spring it is stretched gaining the potential energy that converts into kinetic energy while the spring tries to come back to its original shape and size resulting in harmonic oscillations.
Also Read:
- Centrifugal force vs coriolis force
- Is muscular force a contact force
- Torque vs force
- Electrostatic force and field
- How to find net force with mass and velocity
- Example of static force
- Negative electrostatic force
- Is gravitational force a central force
- Is support force a contact force
- How to find normal force between two blocks
Hi, I’m Akshita Mapari. I have done M.Sc. in Physics. I have worked on projects like Numerical modeling of winds and waves during cyclone, Physics of toys and mechanized thrill machines in amusement park based on Classical Mechanics. I have pursued a course on Arduino and have accomplished some mini projects on Arduino UNO. I always like to explore new zones in the field of science. I personally believe that learning is more enthusiastic when learnt with creativity. Apart from this, I like to read, travel, strumming on guitar, identifying rocks and strata, photography and playing chess.
