Physics Notes Chapter 5
Unit 3 - Dynamics
We have examined velocity and acceleration, but what causes this velocity or the change in an objects velocity? The answer is a force causes these motions to occur.
Forces can be defined as "a push or a pull, that starts an object moving, stops it, or changes its direction or speed."
Forces are vector quantities that have a direction associated with them as well as a magnitude.
All the forces in the universe can be grouped into four categories:
Gravitational: described in detail by Newton but still not understood. This is the force that acts between any two masses anywhere in the universe. Acts over the entire universe but is the weakest of the four forces.
Electromagnetic: derived from the property of charge. A moving charge exerts a magnetic force and a static charge exerts an electric force. In an electromagnetic wave the electric field and the magnetic field are perpendicular to each other.
Strong Force: Strongest of all the forces but only acts over distances roughly the size of the nucleus. This holds the nucleus together and prevents the protons from repulsing each other.
Weak Force: Acts to hold the individual parts of the nucleus together. This is responsible for radioactive decay.
Isaac Newton developed the first laws describing how and why objects move.
Newton's First Law: objects at rest or in constant motion will remain at rest or in motion in a straight line until a force acts on them.
Newton's Second Law: The acceleration an object experiences is directly proportional to the applied force and inversely proportional to the mass of the object, a = F/m, ore commonly written as F=ma.
Newton's Third Law: For every action there is an equal and opposite reaction. In other words forces always occur in pairs.
Weight is the result of the force of attraction between any two masses - specifically the mass of the earth and the mass of the object. Since planets have different masses this explains why our weight is different on the moon or mars or some other planet. Remember that mass is how much matter is in an object and that doesn't change.
There are theoretically two types of mass.
Inertial mass, measured by calculating the force required to accelerate the object and
Gravitational mass, measured by the force gravity exerts on a mass.
In actuality these are identical.
One of the most common everyday applications of force is that of friction. Friction is a force that opposes motion so it is always negative compared to the motion. The source of friction is the microscopic hills and valleys of a materials surface on an atomic level. At rest the hills and valleys nestle into each other and to start motion we must "lift' the surface out of those interlocked spots, even if this lift is incredibly small. Once we have achieved this lift it takes relatively little effort to keep it going. That why it is harder to start motion than it is to maintain motion.
Static friction is the frictional force that opposes the initiation of motion.
Kinetic friction is that force that opposes the motion itself.
We measure the amount of friction a particular surface offers in terms of a unit called the coefficient of friction. It compares the force that friction offers to the "normal" force. Now this normal force is a force equal to an objects weight that acts perpendicular to the surface. On a flat surface the weight is equal to the normal force since there is no motion along the y-axis. On an incline we must use our trig functions to calculate the normal force.
Ff = uFn is the equation that allows us to calculate or work with frictional forces.
Ff is the frictional force
U is the coefficient of friction
Fn is the normal force
Friction is nearly independent of surface area. The primary factor in determining frictional force is the type of material the two surfaces that are in contact with each other are made of.
When motion does begin and friction is overcome the acceleration the object experiences is due to the NET force not simply the applied force. For example if we push with 8 N of force but friction opposes with 10 N of force an object will not move. We have an applied force just not enough to overcome friction. It is vitally important to recognize this distinction!
As an object falls through the atmosphere it accelerates up to a point. The air resistance builds up until the force of the resistance equals the force of gravity and a falling body stops accelerating. This is called the terminal velocity and is dependent upon the surface exposed to the air.
