Forces Forces are push or and represented by vectors. There are contact and field forces. Inertia is the tendency of a mass to resist a force changing its state of motion. Friction is the force that opposes motion. Newton’s three laws are used to understand dynamic motion. Newton’s First Law (Law of inertia) – In the absence of a net external force, an object maintains constant velocity. Newton’s Second Law- If a net force is acting on a mass then the mass will accelerate in the direction of the net force and is directly proportional to the force. Fnet=ma. Newton’s 3rd Law - For every action there is an equal and opposite reaction force. Free body diagrams In circular motion net force is directed towards the center and is referred to as centripetal force. Review Section 4: Center of Mass The center of mass is the point in a system (multiple masses) about which all the mass is balanced. The center of mass point is found by taking the weighted average of all points of mass along each axis: xcm = / ycm = / Review Section 5: Electrostatics The nature of Electric Charges There are two types of electric charge: positive and negative. Electrons are the fundamental carriers of negative charge. Protons are the fundamental carriers of positive charge. Like charges repel, while unlike charges attract. The presence of static charges can be determined by the use of an electroscope. Objects become charged through the transfer of electrons. Coulomb’s law Used to find the electric force between two static charges. The force is either attractive (-) or repulsive (+) and is proportional to the product of the charges and inversely proportional to the square of the distance between them. This is an inverse square law. F = F = qE F =(1/4€o ) k=constant €o = constant, Permittivity of free space
The electric potential difference Equal to the work done per unit charge and measured in Joules/ coulomb or VOLTS. W = Eq in the direction of E ΔU= qEΔV Capacitor A device that stores charge and energy. The capacitance of a capacitor is the amount of charge stored per unit volt, farads (F). C = Q/V = €oA/d Electric field lines Indicate the direction of electric force on positive charges. They come out of a positive source charges and into negative source charges. They never cross and their relative spacing indicates the strength of the field. Electric field strength (E) is found by: E = F/q Units: N/C Equipotential lines Indicate regions of constant electric potential. These lines (surfaces), intersect the electric field lines at right angles. The highest values are near positive source charges and the lowest is near negative source charges. The electron volt (eV) is a unit of electrical energy. 1eV = 1.6 x 10-19j Review Section 6: Electric Circuits Current and Electricity (I) units: amps. The direction is from + to -. Measured with an Ammeter (current flows through the ammeter). I=ΔQ/Δt Electromotive force (emf) – The battery that supplies potential difference needed to maintain continuous flow of charge. Not a force, measured in volts (v). Know your Circuit symbols for wire, switch, battery, Ammeter, voltmeter, capacitor, bulb, resistor. Electric Resistance (R): R = V/I *at constant temperatures Units: Ohms(Ω) Rwire = ρL/A ρ = resistivity of material OHMS LAW: