Physics (from "Physics 101")

Question 1

Define:

instantaneous velocity (v_o).

Answer 1

Instantaneous velocity is the direction and magnitude of the rate of change for distance per unit time.

v_o = **Δx / t**

or: v_o = p/m

Where p is the object’s momentum and m is its mass.

Question 2

Define:

average velocity (v_ave)

Answer 2

Average velocity is the total distance and direction traveled from an initial position, divided by the total time.

v_ave = Δx_total / Δt_total

or: v_ave = (v_f + v_i)/2

Note: the second equation assumes constant acceleration, which is also assumed on the AP exam unless specified otherwise.

Question 3

Define:

acceleration

Answer 3

Acceleration is the rate at which velocity changes per unit of time.

**a = Δv / **t

or: a = Δx / t²

Question 4

What is the acceleration always associated with an object in free fall?

Answer 4

9.8 m/s²

This is the acceleration due to gravity, commonly called g. On the AP exam, any falling object is assumed to be in free fall (no air resistance) unless told otherwise.

Note: while it’s convenient to use 10 m/s² for calculations, the AP exam expects you to still choose the correct answer mathematically.

Question 5

Describe Newton’s first law of motion.

Answer 5

aka the Law of Inertia: An object in motion will continue with constant velocity unless acted on by a net force.

Similarly, an object at rest will continue to remain at rest until acted on by a net force.

Question 6

What must be true about the acceleration of an object, if all forces acting on it cancel?

Answer 6

The object has zero acceleration.

Since all forces cancel to be zero, there is not a net force and there will not be a change in velocity. If there is no change in velocity, that is the same as no acceleration.

Question 7

What is the relationship between force, mass, and acceleration in Newton’s second law of motion?

Answer 7

F_net = ma

Note: net force and acceleration are both vectors, and must be pointing in the same direction.

Question 8

What is the proportional change in force to make an object move with twice its original acceleration?

Answer 8

Twice the original force must be applied.

From Newton’s second law, F=ma. Force and acceleration are directly proportional.

Question 9

How does Newton’s third law of motion describe the forces between two objects?

Answer 9

F_1on2 = -F_2on1

For every force from one object on a second, there is an equal and opposite force from the second back on the first.

Question 10

What magnitude of force must exist from apple to an orange in free space, if it’s found that there is a force from the orange to the apple of 5N.

Answer 10

5N

From Newton’s third law, every force excerted must have an equal and opposite force. The negative sign is already factored in, since the question specified direction.

Question 11

Define:

Weight

Answer 11

Weight is explicitly the force on an object due to gravity. W=mg

Weight if often confused with mass; an object with one weight on Earth will have a different, lesser, weight on the moon, but its mass will remain constant.

Question 12

Define:

mechanical advantage

Answer 12

Mechanical advantage is the multiplication of a force using a mechanical device. A small force exerted over a large distance is transformed into a larger force over a smaller distance.

On the AP exam, mechanical advantage appears primarily in problems including levers.

Question 13

Define:

the kinetic energy of an object

Answer 13

An object’s kinetic energy is the energy resulting from its motion.

Kinetic energy is defined as:

KE = ½mv²

where m is the object’s mass and v is its speed. The units of kinetic energy are Joules, just like all other forms of energy.

Question 14

Define:

fulcrum and lever arm

Answer 14

The fulcrum is the point of an object that remains fixed during rotation.

The lever arm is the distance from the fulcrum where a force is applied, to create rotation around the fulcrum.

Question 15

Define and give units for:

torque

Answer 15

Torque is the ability of a force to create a change in the angular orientation of an object, by rotation about a fulcrum point. Torque can also be thought of as the component of work perpendicular to a lever arm.

The SI unit of torque is the N-m.

Question 16

Briefly describe the Bohr Theory of the atom.

Answer 16

The Bohr Theory states:

• Electrons can only exist in fixed orbits or energy levels.
• These energy levels are at specific distances from the nucleus.
• Any energy emitted/absorbed from/by an atom will be the result of an electron jumping from one energy level to another.

Question 17

Define:

the atomic number, Z, of an element

Answer 17

The atomic number of an element is the number of protons contained within the element’s nucleus.

Atomic number is a characteristic property of an element. For instance, all atoms with 1 proton in their nucleus are hydrogen atoms, regardless of how many neutrons exist in the nucleus.

Question 18

Define:

buoyancy

Answer 18

Buoyancy is the tendency of an object to weigh less when partially or fully submerged in a liquid.

Buoyancy is caused by the liquid displaced by the object. The liquid pushes up on the object with a force equal to the weight of the liquid displaced.

Question 19

Define:

surface tension of a fluid

Answer 19

Surface tension is the ability of a fluid’s surface to resist an external force.

Surface tension is caused by attractive forces between the molecules of the fluid. It is responsible for the spherical shape of soap bubbles and the ability to skip a stone off the surface of a lake.

Question 20

What is the formula for converting between temperature in Centigrade and Fahrenheit?

Answer 20

T_F = (9/5)*T_C + 32

T_F = Fahrenheit Temperature
T_C = Celsius Temperature

Some standard conversions to memorize:

• 32º F = 0º C
• 77º F = 25º C
• 212º F = 100º C

Question 21

Define:

a conductor

Answer 21

A conductor contains many electrical charges within the medium, and they are relatively moveable.

Common conductors are usually metals with high atomic weight, such as silver, copper, and gold.

Question 22

Define:

an insulator

Answer 22

An insulator has few free electrical charges within the medium, and those are difficult to move.

Common insulators are as glass, quartz, rubber and teflon.

Question 23

Define:

a battery

Answer 23

An electrical battery (or electrochemical cell) is a device that produces a flow of electrons from anode to cathode.

The necessary electrical energy is created by undergoing redox chemical reactions in the cell.

Question 24

Define:

voltage

Answer 24

Voltage (electric potential) is the electrical energy per unit charge necessary to move a test charge against the electric field of a fixed charge.

Question 25

Define:

resistance

Answer 25

Resistance is a measure of how difficult it is to pass current through some object. Many factors affect resistance, including: cross-sectional area, length, and general resistivity.

Though all materials have some resistance at room temperature, super-cooled superconductors have a resistance of zero.

Question 26

What must have happened to the voltage supplied to a fixed resistance circuit, if the current suddenly drops to half original?

Answer 26

The voltage must have also dropped to half.

From Ohm’s law: V=IR

Since R is held constant, V and I are directly proportional. Halving voltage means current must be halved also.

Question 27

What must have happened to the resistance in a fixed voltage circuit, if the current suddenly doubles?

Answer 27

The resistance must have dropped to 1/2 original.

From Ohm’s law: V=IR

Since V is held constant, R and I are inversely proportional. Doubling current means resistance must be halved.

Question 28

What does this symbol stand for in a circuit diagram?

Answer 28

This is a battery. Batteries will always have a larger positive terminal (labeled +) and a smaller negative terminal (labeled –).

Conventional current always flows from + to – around the circuit.

Question 29

What does this symbol stand for in a circuit diagram?

Answer 29

This is a resistor. Resistors impede current flow and have units of ohms.

Question 30

What does this symbol stand for in a circuit diagram?

Answer 30

This is a capacitor. Capacitors store charge and have units of farads.

Question 31

What does this symbol stand for in a circuit diagram?

Answer 31

This is a switch. Switches can be open (as shown) so that no current flows around the circuit, or closed so that both dots are connected and current flows.

Question 32

What does this symbol stand for in a circuit diagram?

Answer 32

This is the symbol for ground. Ground is the physical end of the circuit path and represents zero current or zero voltage.

If no ground symbol is given, the negative terminal of the battery is assumed to be ground.

Question 33

What is the difference visually between resistors in parallel vs. series in a circuit?

Answer 33

Resistors in parallel are each on a diverging circuit pathway. A single charge will pass through either one or the other to complete the circuit. (first image)

Resistors in series follow each other on continuous circuit pathway. A single charge must go through both to complete the circuit. (second image)

Question 34

What has happened to the capacitance of a fixed voltage circuit, if a dielectric material is inserted between plates of a capacitor and the charge on those plates doubles in order for current to continue to flow?

Answer 34

Capacitance will double.

From the formula: C = Q/V

Since V is held constant, Q and C are directly proportional. Doubling charge must mean that capacitance doubles also.

Question 35

Define:

Conductivity

Answer 35

Conductivity is the measure of how well a material is able to conduct electric current through it, and is represented by σ (sigma) or κ (kappa).

Conductivity is the inverse of resistivity and will have units of S/m, where S is the SI unit of siemens.

Question 36

Define and give the SI units for:

Power

Answer 36

Power is the rate at which energy is transferred per unit time and has SI units of Watts (W), which represents J/s.

Question 37

Define:

AC current

Answer 37

AC current (alternating current) periodically reverses direction of flow, creating both positive and negative values.

This is different from DC (direct current), which flows in only one direction and remains always positive.

Note: to make an AC current value into a DC current equivalent, divide the peak AC current by √2, this is known as the RMS factor.

Question 38

Define:

AC Voltage

Answer 38

AC voltage (alternating voltage) periodically reverses sign, having both positive and negative values, due to AC current changing direction and sign.

This is different from normal voltage (direct voltage), which remains always positive, due to supposing that current flows in one direction.

Note: to make an AC voltage value into a DC voltage equivalent, divide the peak AC voltage by √2, this is known as the RMS factor.

Question 39

Define and give the formula for:

AC Power

Answer 39

AC power (alternating power) is the energy transferred by an alternating current.

This is similar to normal power, the only difference is that all equation variables are the RMS equivalents.

P_RMS = I_RMS*V_RMS
P_RMS = I²_RMS*R

Question 40

What is the law that governs the reflection of a light ray off a flat surface?

Answer 40

θ_i = θ_r

The law of reflection states that: light incident on a reflective surface at some angle will bounce off at an equal angle of reflection, as measured from the normal.

Question 41

Define:

refraction

Answer 41

Refraction is when a light ray crosses a boundary from one transparent medium to another.

When the light ray changes media it bends either towards or away from the normal, depending on the properties of the respective media.

Question 42

When white light is separated by a prism into its component colors, in what order do they appear?

Answer 42

The separated colors are, in order, red, orange, yellow, green, blue, indigo, violet.

This is easily remembered by the mnemonic ROYGBIV (Roy Gee Biv).