Unit 3: Dynamics

• Dynamics is the study of the forces that cause changes in the motion of objects.
• Force causes the motion of objects to change, a push or a pull.
• Force is a vector quantity
• Some forces do not require direct contact between objects to have an effect.
• Newton's first law is that objects resist changes in motion. Unless an unbalancing force is applied.
• Constant velocity means that the acceleration is 0, but it's still moving at the same speed.
• Net force is the total of all the forces acting on an object.
• When net force is 0, that doesn't mean there is no force acting on the object, there's just no acceleration.
• Inertia is the tendency to resist changes in motion, which depends on the object's mass.
• An inertial frame of reference where an object seems at rest from your perspective (earth).
• Newton's second law states that acceleration is directly proportional to the net force and inversely proportional to the mass.
• The direction of acceleration is the same as the net force.
• Newton's third law states that every action has an equal, opposite reaction.
• Reaction force is the force that the object acts on you.
• Forces always come in pairs called action-reaction pairs.
• When looking at the forces on an object, they're not all action-reaction pairs, which is why things can move.
• the two types of forces are contact forces and action-at-a-distance forces (both vectors).
• Contact forces are forces that act between two objects that are in contact with each other, such as normal force, frictional force, and tension force.
• Action-at-a-distance forces exert a force with no contact, such as gravitational force.
• Weight is the gravitational force exerted on an object by the earth, measured in newtons.
• g is always positive.
• The applied force is the push/pull that is applied, which usually has a force exerted that is equal in magnitude but opposite in direction.
• Spring force is the applied force that stretches or compresses a spring.
• Restoring force is the counteractive force required for a spring to return to its original state.
• Tension is the force transmitted when it's stretched by weight, usually the same as weight but opposite in direction.
• Tension force is always a pull.
• When looking at tension, always ignore the mass of the actual rope or spring.
• A pulley changes the direction of tension force without changing its magnitude.
• When on a surface, the surface exerts a force on the object that is perpendicular to the surface regardless of the angle of the surface.
• Normal force prevents objects from passing through each other. It acts as the action-reaction pair to gravity/applied force.
• Friction is the force that acts between surfaces that are in contact. This force is parallel to surfaces and opposes their motion.
• The size of the surface area in contact with the surface does not affect the frictional force.
• The two types of friction are static friction and kinetic friction.
• Static friction is used when two objects are at rest relative to each other. It's the force you must overcome to begin moving an object. Static force will match the applied force until the maximum static force is reached.
• Kinetic friction is where two objects are in motion relative to each other.
• The force required to overcome the static frictional force is always greater than the force needed to overcome the kinetic frictional force.
• When there's no acceleration, the applied force is equal to the force of friction.
• The coefficient of friction is a measure of the "roughness of a surface". It does not have a unit of measurement.
• Free-body diagrams show all the forces acting on an object.
• The coefficient of friction is never negative.