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Explanation of Inertial and Real Forces by Ron Kurtus - Succeed in Understanding Physics. Also refer to physical science, fictitious, pseudo, inertia, centrifugal, acceleration, School for Champions. Copyright © Restrictions
Inertial and Real Forces
by Ron Kurtus (16 March 2010)
An inertial force is an apparent force on an object that is result of inertia or a frame of reference acceleration. An inertial force is also called a fictitious or pseudo (false) force.
This is as opposed to a real force, which consists of an agent acting on an object, causing the object to accelerate, decelerate or change directions. With inertial forces, you can see the movement and measure the force, but you cannot define the agent.
An explanation of inertial forces is seen by examining the frame of reference or point of view of the observer.
Questions you may have include:
- What are examples of real forces and their agents?
- What are some inertial forces?
- How does the frame of reference explain the motion?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
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Real forces
A real force applied to an object is caused by some agent acting on the object.
Throw a ball
For example, when you throw a ball, you are exerting a real force on the ball, causing it to accelerate. You are an outside agent applying a force on the ball.
Roll a ball
If the ball is rolling along the ground, there is the resistive force of friction slowing down that motion. In this case, the friction on the ground is an outside agent that is applying a real force on the ball.
Force at a distance
A force acting at a distance, such as magnetism or gravity, are also outside agents causing acceleration, deceleration or a change in direction.
In all these cases, you can define the agent that is applying the force.
Inertial forces
With inertial forces, you can see an object move and even measure its force, but you cannot define a physical agent that is acting on the object and causing the force.
Acceleration in a straight line
The force you feel of being pushed back in your seat as the aircraft you are flying in accelerate and takes off is an example of an inertial force. You can measure the force you are being pushed back, but you cannot define a exactly what is pushing you back.
Note: In high speed jet aircraft or the space shuttle, that force is called a G-force and measured by how much greater it is than the effect of gravity.
Centrifugal force
If you swing a ball around on a rope and then let it go, the centrifugal inertial force will send the ball outward in a straight line. However, you cannot tell what real force accelerated the ball to fly off in the way it did. It was caused by a pseudo force.
View with respect to frame of reference
The concept of an inertial or fictitious force can be explained by the relative point-of-view or frame of reference of the observer.
If your frame of reference accelerates, decelerates or changes direction, you will feel a force acting on you in the opposite direction of the action. Although this force seems real, to an outside observer in a larger frame of reference, your motion is related to the law of inertia.
This can be illustrated with a person in a moving bus and an outside observer.
Constant velocity
If you are wearing roller skates and standing in a bus that is moving at a constant velocity and you cannot see out the windows, you will be unaware of its motion. (Doesn't everyone wear roller skates in a bus?)
However, an outside observer will see you and the bus moving along at that given velocity, with respect to the ground.
Deceleration and acceleration
If the bus driver suddenly slammed on the brakes, decelerating the vehicle, you would experience a force on you that causes you to roll forward. However, the outside observer would see the bus slow down and see you continue at the former velocity. Likewise, if the bus accelerated, you would roll backwards.
Going around a corner
Now, if the bus rapidly went around a corner, you would be thrown toward the side. To the outside observer, you would be continuing in a straight line, while the bus turned.
Greater scheme of things
The frame of reference in the bus is but a subset of a greater frame of reference, such as that of the person observing the bus. However, there are multiple frame of references. Someone on the Moon could see the person watching the bus (through some very, very powerful binoculars) moving along with the rotation of the Earth.
The concept of these different frame of references was a basis for Einstein's Special Theory of Relativity.
Summary
An inertial, fictitious or pseudo force is an apparent force on an object that is result of inertia or a frame of reference acceleration. A real force, which consists of an agent acting on an object, causing the object to accelerate, decelerate or change directions. With inertial forces, you can see the movement and measure the force, but you cannot define the agent. An explanation of inertial forces is seen by examining the frame of reference of the observer.
See side menu for other Force topics
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Resources and references
The following resources provide information on this subject:
Websites
Books
Forces In Nature by Liz Sonneborn Rosen; Publishing Group (2004) $25.25 - Understanding gravitational, electrical and magnetic force
The Science of Forces by Steve Parker; Heinemann (2005) $29.29 - Projects with experiments with forces and machines
Glencoe Science: Motion, Forces, and Energy, by McGraw-Hill; Glencoe/McGraw-Hill (2001) $19.32 - Student edition (Hardcover)
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physics. If you had problems, you had better look over the material again.
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Inertial and Real Forces