Pseudo means false. So, pseudo force is a false or imaginary force. Pseudo force is said to be a force that does not exist, but seems to exist when the frame of reference is accelerating.
It is commonly called fictitious force, or sometimes inertial force, or d’Alembert force. This means that in the frame of reference of every accelerating body, a pseudo force seems to be acting on every Object in the frame.
The local pseudo force always acts in the opposite direction of the acceleration of the frame of reference. The acceleration that the object seems to be acting with in the frame of reference is equal to the motion of the acceleration of the object.
The size of the pseudo force is equal to the mass of the object times the acceleration of the frame of reference. The accelerating frame of reference is referred to as a non-inertial frame of reference and the frame which is not accelerating is referred to as an inertial frame of reference.
What is Pseudo Force?
A Pseudo force (also referred to as an inertial force, fictitious force or a d’Alembert force) is a force that appears to act on all masses whose motion is being described using a non-inertial frame of reference; that is, a frame that is not in inertial motion (rotation is a type of non-inertial motion).
Pseudo force comes into play only when the frame of reference has begun acceleration as compared to a frame that is not accelerating.
The force F does not come from any physical interaction between two bodies, but arises from the acceleration ‘a’ of the non-inertial reference frame.
Since a frame can accelerate in unimpeded arbitrary ways, pseudo forces can be arbitrary (but must only be in response to the acceleration of the frame).
Nevertheless, four pseudo forces are defined for frames accelerating in commonly occurring ways, one is a relative acceleration of the origin in straight line (rectilinear acceleration); two are for rotating frames: Coriolis force and Centrifugal force, and fourth force called Euler force which is the result of a variable rate of rotation.
Example of Pseudo Force
- Railroad Cart
- Pulling a Box with a Brick in it
- A Bob Suspended in a Truck
- Car
- Lift
- Person on a Rotating Playground Disk
#1. Railroad Cart.
A viewer is standing on a railroad cart and another individual is on the ground nearby the railroad tracks.
When the cart accelerates in a forward direction the ground observer sees motion in the cart and no motion in the rocks around him.
But the person on the cart sees the rock on the ground moving with the same acceleration while the railroad cart appears to be stationary.
To the person on the cart, the rocks appear to be moving in the backward direction with no force acting on it. The rocks appear to be in motion owing to the motion of the cart itself.
There is what appears to be a force present in the frame that causes the rocks to move backward in a non-accelerated or at rest at that moment. The force appears to push with acceleration backward. This fictitious force is recognized as a pseudo force.
#2. Pulling a Box with a Brick in it.
If we place a brick in a box, and then set the box in motion with the use of a rope or string, from the perspective of the ground (an inertial frame) the box and brick will show an upward acceleration.
From the perspective of the box (a non-inertial frame), the brick appears as if the brick is at rest and that the box itself is accelerating upwards.
The brick is experiencing a gravitational force directed downward while the normal contact force is acting in the upward direction.
Since the reference frame is accelerating then there must be a pseudo force acting in the opposite direction to the direction of acceleration of the box.
The pseudo force is equal to the mass of the brick multiplied by the acceleration of the frame.
Since the brick is stationary, the net force on the brick must equal zero, or the net upward force must equal the net downward force.
Thus, the net force is equal to the downward force of the pseudo force and the downward force of the gravitational force.
#3. A Bob Suspended in a Truck.
When a bob is hanging in a truck travelling in the forward direction, the suspension string becomes taut, and the bob gets pulled in the backward direction.
If the reference frame is the road, an inertial frame, then there is no pseudo force. If the reference frame in the truck then a pseudo force is considered to be acting in the backward direction.
The gravitational force is acting on the bob in the downward direction; and tension is acting in the string.
The truck is accelerating in the forward direction, and there is a pseudo force acting in the backward direction on the bob with a magnitude equal to mass times acceleration.
The bob appears to be at rest and consequently there net force and acceleration of the bob is equal to 0. The vertical component of tension equals weight and the horizontal component equals the pseudo force. Thus the forces of the bob are balanced.
#4. Car.
When a car is accelerating forward all the other objects present in the frame of reference are all moving in the opposite direction to the observer in the car.
As the car accelerates, the moon, the sun, the tree, or the other objects, and people present outisde the car are all moving in the direction opposite to the acceleration of the car, due to the presence of pseudo force
#5. Lift.
When riding in an elevator going down, a force pulls our body upward. This pseudo force creates the illusion that objects outside the moving elevator have the same amount of acceleration as the elevator, but in the opposite direction. The cause of the force is called pseudo force.
#6. Person on a Rotating Playground Disk
When you are standing on a spinning playground ride (like a merry-go-round) you can feel as though you are being pushed outward, away from the centre.
However, nothing is actually pushing you outward. What is really going on, is that your body wants to go straight; but while your body is spinning an object at a certain rate, it keeps moving in a circle.
As a result, as it is spinning, it seems as if there is a force pulling you outward, even though there is not.
This feeling is a result of a pseudo force, which is not a true force, but feels true because you are standing in something that is spinning. In this sense, it is why you have to hold on tightly or lean into a turn when spinning.