Forces in Thermodynamics

In this section, we will review what forces are and see how they connect to Thermodynamics. Let's take a flashback to some Physics 1💡 

If an object of interest interacts with several other objects, the net force is the vector sum of the individual forces. Projectile motion and circular motion are both included in AP Physics 1. 

Newton’s Second Law states, “The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.”

Net Force is the vector sum of all forces acting on a particle or body. Ring any bells yet?

And is it also important to know how pressure relates to force? A system’s pressure tells you the force that the system exerts on the walls of its container and measures the average change in the momentum or impulse of the molecules colliding with the walls of the container. We will come back to the idea of momentum later. 

In this section the forces you will deal with forces you have dealt with before. You will see gravitational force, the buoyant force and the normal force most commonly.

The test may ask you to make a FBD for this unit but it is much more likely that the FBD will deal with fluids or gases or electricity.

For this unit, the idea of pressure is significantly more important than force. Yet is it incredibly important to keep in mind what forces are and what type of forces we will see?

Key points about forces at an AP Physics 2 level:

• A force is a push or a pull that acts on an object. Forces can cause objects to accelerate, change direction, or change shape.
• Forces can be described by their magnitude (size), direction, and point of application.
• There are many different types of forces, including gravitational forces, electromagnetic forces, and frictional forces.
• The magnitude of a force is usually measured in newtons (N). One newton is defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared.
• The direction of a force is usually described using polar coordinates, with the angle measured from a reference line (such as the x-axis).
• The point of application of a force is the location on an object where the force is applied.
• Forces can act in pairs, and the net force on an object is the sum of all the forces acting on it. The net force determines how an object will accelerate.
• Newton's laws of motion describe how forces interact with objects to produce motion. The first law states that an object at rest will stay at rest, and an object in motion will stay in motion with a constant velocity, unless acted upon by a net force. The second law states that the acceleration of an object is proportional to the net force acting on it and inversely proportional to its mass. The third law states that for every action, there is an equal and opposite reaction.

Q. A metal ball 🎳 and a plastic ball 🏀 are dropped from a building. Which ball has a greater acceleration? Which ball experiences a greater force? On which ball does gravity do more work? 

A. Both the balls have the same acceleration, g. The metal ball experiences more force because it has more mass. Both the balls drop the same distance but the metal ball experiences a greater force so more work is done on the heavier metal ball. 🏈 

Q. Can an object exert a force on itself? 

A. No. By definition an object can not exert a force on itself🛑 

Q. What is the relationship between momentum and force? 

A. Multiplying the amount of time a force acts on an object by that force gives us the change in momentum, impulse, for the object. 

Q. What are conservative forces vs non conservative forces? 

A. If the object ends up where it started but experiences a conservative force the entire time, no work is done. If the object ends up where it started but experiences a non-conservative force the entire time, work is done✅

Here are some ways that the concept of force relates to thermodynamics:

• In thermodynamics, forces can be used to do work on a system. For example, a gas in a cylinder can be compressed by applying a force to the piston, or a weight can be lifted by applying a force to a rope. The work done on a system increases the internal energy of the system.
• Forces can also be generated by temperature differences. For example, a temperature gradient in a fluid can produce a buoyancy force, which can cause the fluid to circulate. This type of force is known as a thermodiffusive force.
• The concept of force is closely related to the concept of pressure. Pressure is defined as the force per unit area exerted on a surface. In thermodynamics, pressure is an important variable that determines the behavior of gases and liquids.
• The concept of force is also closely related to the concept of energy. Energy is the ability to do work, and work is the transfer of energy from one object to another through the application of a force. In thermodynamics, energy is often described in terms of heat (the transfer of energy from one body to another as a result of a temperature difference) and work (the transfer of energy from one body to another as a result of a force).