Non Conservative Forces Class 11 Physics – Example, Properties

NON-CONSERVATIVE FORCES –  Forces that do not store energy are called nonconservative or dissipative forces. Friction is a nonconservative force, and there are others.

Nonconservative forces can be defined as:

“A non-conservative force is a force whose work done is dependent on the path taken”

Nonconservative forces such as friction, including air friction, If you’re dragging something over a field carpeted with sandpaper, for example, the force of friction does different amounts of work on you depending on your path. A path that’s twice as long will involve twice as much work to overcome friction.

Read What are CONSERVATIVE FORCES

When friction is involved, the path you take matters — a longer path will dissipate more kinetic energy than a short one. For that reason, friction is a nonconservative force.

Comparison of the effects of conservative and nonconservative forces on the mechanical energy of a system

Conversion of macroscopic motion into microscopic motion through non-conservative forces.

Nonconservative forces convert large scale motion that’s big enough to see with the naked eye or optical microscope (macroscopic motion) to motion at the atomic scale (microscopic motion). These forces are called non-conservative forces.

A moving car, bouncing a ball, or crawling insect would all exhibit macroscopic motion. Sound, thermal, and light energy are all examples of microscopic motion. Therefore, a non-conservative force converts macroscopic motion into microscopic motion.

Example of nonconservative forces (non conservative force examples):-

An example of non-conservative forces in a baseball game:

Normal force: When a baseball bat hits a baseball (macroscopic motion), the hit will make a sound (microscopic motion)

Air drag: After a baseball player hits the baseball, the ball moves through the air (macroscopic motion). The ball will make the molecules in the air vibrate faster—creating heat (microscopic motion). This is the same as the mechanical equivalent of heat, which converts the motion of fluid to heat. The more air drag there is, the faster the ball will dissipate energy.

Friction: When the player slides to the base (macroscopic motion), friction will cause the atoms in the ground to shake more, the player’s pants move more, and make a sound (microscopic motion).

All real systems have some non-conservative forces associated with them. For example, when the moon rotates around the Earth it creates tidal forces, which will warm the oceans slightly. However, it’s a small effect compared to the energy in the system.

Mechanical energy and non-conservative forces

All systems lose some mechanical energy over time; this is part of the second law of thermodynamics. It’s important to note that non-conservative forces don’t destroy energy they just change it into a less useful (less ordered) form.

Any friction-type force, like air resistance, is a nonconservative force. The energy that it removes from the system is no longer available to the system for kinetic energy.

What’s really not being conserved around a track with friction is the total potential and kinetic energy, which taken together is mechanical energy. When friction is involved, the loss in mechanical energy goes into heat energy. You can say that the total amount of energy doesn’t change if you include that heat energy. However, the heat energy dissipates into the environment quickly,

so it isn’t recoverable or convertible.

Applying Energy Conservation with Nonconservative Forces

KE_i + PE_i + W_nc = KE_f +PE_f

When no change in potential energy occurs, applying

amounts to applying the work-energy theorem by setting the change in kinetic energy to be equal to the net work done on the system, which in the most general case includes both conservative and nonconservative forces. But when seeking instead to find a change in total mechanical energy in situations that involve changes in both potential and kinetic energy, the previous equation

KE_i + PE_i + W_nc = KE_f +PE_f

says that you can start by finding the change in mechanical energy that would have resulted from just the conservative forces, including the potential energy changes, and add to it the work done, with the proper sign, by any non conservative forces involved.

Properties of Non-conservative forces:

• Work done depends upon the path.
• Energy is dissipated as heat energy.
• Work done is not completely recoverable.
• When the starting and ending points are the same, the total work is not zero.
• There is no potential-energy function for the friction force.
• It is not reversible.

A force is said to be a non-conservative force if it results in the change of mechanical energy, which is nothing but the sum of potential and kinetic energy. The work done by a non-conservative force adds or removes mechanical energy. For example, when work is done by friction, thermal energy is dissipated. The energy lost cannot be fully recovered.

CONCLUSION

A force is said to be non-conservative if the work done by or against the force in moving a body depends upon the path between the initial and final positions. This means that the value of work done is different in different paths. Frictional forces are non-conservative forces as the work done against friction depends on the length of the path moved by the body.The force due to air resistance, viscous force is also non-conservative forces as the work done by or against these forces depends upon the velocity of motion.

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