Unit 2: Dynamics

Force (F)

• a push or a pull on an object due to an interaction with another object

• SI unit of force: Newton (N)

• a vector quantity with both magnitude and direction

Contact Forces

require physical contact to apply a force to an object

Field Forces

act at a distance and do not contact the object physically

Weight

• how heavy that stuff is on a given planet

• the weight of an object is determined y the gravitational acceleration of the planet on which it resides

• W = F sub g = mg

Mass (m)

how much stuff exists in an object

Applied Force

• a contact force F sub p, where p stands for the push or pull applied to the object

Tension Force

• by string or rope, denoted by F sub t or just capital T

• the force of tension in a rope or string is the same at any point along the medium

• the force of tension felt at one end of a rope in a tug-o-war is the exact same magnitude but opposite direction as the force felt at the other end of the rope

Spring Force (F sub sp)

• the force a spring exerts in opposition to a stretch or compression

• the spring force always opposes the applied force

Hooke’s Law

• F sub sp = -kx

• k is the spring constant

• x is the stretch or compression distance of the spring

• the negative sign represents the spring force’s opposition to the stretch or compression of the spring

Normal Force (F sub n or N)

• a surface force that is perpendicular (normal) to the surface of interaction between two objects

• only exists when a mass is in contact with a surface and it is always perpendicular to the surface

Frictional Forces (F sub f or lowercase f)

• due to the contact between two rough surfaces, where friction always opposes the motion of an object

• 2 forms: static or not moving (f sub s), and kinetic or moving (f sub k)

• mew = f/N —> mew is the slope of friction graphed as a function of the normal force, where mew compares friction to the normal force

• coefficient of static friction will always be greater than the coefficient of kinetic friction

• the pushing force that to apply to the object is proportional to the friction before and after the object starts to move

Newton’s First Law

• an object at rest will remain at rest unless acted on by an unbalanced force. an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force

• F sub net = 0 or Sigma F = 0

• Sigma is the sum of the forces acting on an object

Inertia

• a mass’s resistance to a change in its motiom

The Principle of Superposition of Forces

• combined effect of the forces acting on a mass is the sum of effects of each individual force

Newton’s First Law Equations

• Sigma F sub x = F sub p - f = 0

• Sigma F sub y = N - mg = 0

• when you add up all the forces acting on an object at rest or moving at constant speed, they total to 0

Newton’s Second Law

• acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass

• acceleration = net force/mass

• Sigma F = ma

• Sigma F is the vector sum of the forces acting on the mass

• a = Sigma F/m

Acceleration

a change in velocity, and velocity is a vector with both magnitude and direction, so a change in either would cause acceleration

First Law Application

the mass is not moving in the given dimension or if it is moving at a constantn speed

Second Law Application

the object is accelerating or decelerating

Newton’s Third Law

• for every action there is an equal and opposite reaction

• Force sub A on B = Force sub B on A

• for every force there is an interaction pair