Dynamics Semester Review
Inertial vs Gravitational Mass
Even though inertial mass and gravitational mass are experimentally identical, we need to know the technical differences between them.
Inertial Mass is mass that is calculated by its resistance to acceleration. This is done using the second law equation \(\Sigma F=ma\).
Gravitational Mass is mass that causes the force of gravity and is calculated with the equation \(F_g =mg\)
Newton’s Laws
Newton’s First Law: An object in motion stays in motion at a constant velocity unless acted on by a net external force.
Newton’s Second Law: Acceleration is directly proportional to the net force and inversely proportional to the mass (inertia). or \(a=\frac{\Sigma F}{m}\).
Newton’s Third Law: Newton’s third law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.
Free Body Diagrams
Free Body Diagrams are important for analyzing forces and the accelerations that they cause. A good practice simulation from The Physics Classroom is linked to here. You should be comfortable creating these free body diagrams.
Incline Planes
Understand how to break forces down into components on an incline plane. A more in-depth look at this can be found here.
Friction
Memorize the equation for static friction, \(f_s \leq \mu_s F_n\) and kinetic friction, \(f_k = \mu_k F_n\). Know the basic facts defining both types of friction and when to apply them. A more in-depth video can be found here.
Steps for Solving Second Law Problems
- Choose the system of objects.(not applicable if only one object exists)
- Define a coordinate system.
- Draw free body diagrams.
- Add or subtract forces to write an expression for net force and set it equal to mass time acceleration.
- Solve
When to Use Second Law
- Any problem linking forces, mass and acceleration is usually a second law problem.
- The forces have to be identifiable.
- The second law is usually used when a constant force is applied to an object.