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AHSGE Review J. Pollock Spring 2007 Physics Concepts What are metric units for distance? Distance is measured in meters. Prefixes can be used for larger or smaller numbers. Metric conversions of distance can be accomplished using a chart to show decimal movement. Click for metric conversion chart. What are units of time? Time is measured in seconds. Prefixes can also be used to adjust for larger and smaller times. Time is a factor in many physical science calculations, including speed, velocity, and acceleration. What is velocity? Velocity is the speed and direction of motion. The formula is velocity = distance time. Velocity is important for reporting changes in position. How is velocity related to distance, time, and direction? Velocity describes the length of time of motion, how far an object has traveled, and in what direction. An example follows on the next slide. Velocity is inversely proportional (opposite) to distance and directly (same) proportional to time. How do I read a velocity or distance vs. time graph? A velocity graph will show time as the independent variable (x-axis). Distance will be the dependent variable (y-axis). Velocity can be read as the slope of the graph where the two axes create points on a line. Velocity v. Time Graph Distance (m) Velocity Graph 15 10 5 0 Series1 Series2 0 1 2 3 Time (s) 4 5 How are velocity problems solved? Velocity problems are solved by identifying the distance traveled and the time it takes to travel that distance. The values for time and distance are substituted into the equation for density. See next slide for example. Velocity Example Calculation Metal stakes are sometimes placed in glaciers to help measure a glacier’s movement. For several days in 1936, Alaska’s Black Rapids glacier surged as swiftly as 89 m per day (86400 s) down the valley. Find the glacier’s velocity in meters per second. Remember; velocity includes the direction of motion. V = d/t V = 89 m / 86400 s V = 1.0 X 10-3 m/s down the valley What is acceleration? Acceleration is any change in velocity. It can reflect a change in speed or direction. The formula is acceleration = (final velocity – initial velocity) ÷ time. It can be written a = v/t. How is acceleration related to velocity and time? Acceleration is inversely proportional to velocity. It is directly proportional to time. Velocity can be positive or negative, depending on whether the object speeds up or slows down and the direction in which it is moving. How are acceleration problems solved? Acceleration problems are solved by determining the values for velocity and time. The values are then substituted into the equation for acceleration. A = v/t Velocity Example Calculation A flowerpot falls off a second-story windowsill. The flowerpot starts from rest and this the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot. A = v/t A = (14.7-0) / 1.5 A = 9.8 m/s2 What is force? Force is the cause of acceleration, or change in an object’s velocity. Basically, it is what causes an object to move. Force is related to the mass of an object. Balanced forces mean that all forces acting on an object combine to produce a net force of zero. Unbalanced forces mean that there is a greater push or pull in one direction. How is force related to mass and acceleration? Force is the product of mass and acceleration. The equation is force = mass X acceleration. An increase in either variable will increase force. Newton’s three Law of Motion apply to forces. How are force problems solved? The acceleration and mass of an object must be determined and substituted into the equation. F = ma See the next slide for an example. Force Example Calculation Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s upward acceleration is 0.657 m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and the stretcher? F = ma F = 175 X 0.657 F = 115 N What is gravity? Gravity is the attraction between two particles of matter due to their mass. Every object exerts a gravitational force on every other object. The force of gravity changes according to mass and the distance that separates the objects. What are action-reaction forces? Action-reaction forces are applied to different object. They occur in pairs. Actions are always equal and opposite to reaction forces. It is not balanced forces because two different forces are involved. What is inertia? Inertia is the tendency of an object at rest to remain at rest or, if moving, to continue moving with a constant velocity. Every object has inertia because objects resist changes in motion. More massive objects have greater inertia. What is momentum? Momentum is a quality hat results from an object’s mass and velocity. The equation is momentum = mass X velocity. Momentum also has a direction. Greater mass means greater momentum. Momentum Example Calculation Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley. P = mv P = 6.00 X 10.0 P = 60.0 kgm/s down the alley What is friction? Friction is the force between two objects in contact that opposes the motion of either object. Friction is always opposite to the motion of the object. Because of friction, constant force must be applied to maintain an object’s motion. Acceleration required unbalanced forces, while constant velocity requires balanced forces. What is friction? Different surfaces provide different amounts of friction. For the purpose of machinery, you want friction to be minimal. Air resistance is a form of friction that results between the object falling and the air molecules that surround it. What are Newton’s three laws of motion? Newton’s First Law: An object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force. Newton’s Second Law: The unbalanced force action on an object equals the object’s mass times its acceleration. Newton’s Third Law: For every action force, there is an equal and opposite reaction force. What are Newton’s three laws of motion? You may have heard these laws stated in a more common way. 1st: An object in motion tends to stay in motion. 2nd: Force equals mass times acceleration. 3rd: For every action, there is an equal and opposite reaction. What is efficiency? Efficiency is the ratio of useful work output to work input. It is generally expressed as a percentage. The equation is efficiency = useful work output work input. Efficiency Example Calculation A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? E = Wo / Wi E = 140 / 180 E = 0.78 0.78 (100 %) = 78% What is mechanical advantage? Mechanical advantage is a quantity that measures how much a machine multiplies force or distance. Simple machines are designed to make work easier. The equation is mechanical advantage = output force input force. There are no units. How do I calculate mechanical advantage? Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. MA = Fo / Fi MA = 5.0 / 1.5 MA = 3.3 How is force related to work? Work is the energy transferred by a force when it is applied to a body and causes that body to move in the direction of the force. The equation is work = force X distance. A force causes an object to move, in order for work to be done. How are work problems solved? The distance traveled by the object and the force applied to the object are determined. The values of force and distance are substituted into the equation. The units for work are Joules. Work Example Calculation Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? W = fd W = 190 X 2.0 W = 380 J What is energy? Energy is the ability to do work, so it is very closely related to work. However, it can be present whether an object is in motion or at rest. The units are Joules, just like for work. Energy can be potential or kinetic. What is the law of conservation of energy? The law of conservation of energy states that energy cannot be created or destroyed. It can only change forms. Complicated calculations have been completed to prove this law. The conservation of energy depends on whether or not a system is open (energy flowing in and out easily) or closed (energy exchange with outside limited). How do I calculate potential and kinetic energy? Determine which form of energy is present by deciding whether the object is in motion. Substitute given values into the correct equations. Potential Energy Example Calculation A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff? PE = mgh PE = 65 X 9.8 X 35 PE = 22000 J Compare potential and kinetic energy. Potential Stored energy of object at rest Equation: PE = mass X gravity X height Depends on mass and height Kinetic Energy of an object in motion Equation: KE = ½ X mass X velocity squared Depends on mass and speed Kinetic Energy Example Calculation What is the kinetic energy of a 44 kg cheetah running at 31 m/s? KE = ½ mv2 KE = 0.5 X 45 X 312 KE = 21000 J What is thermal energy and how is it related to temperature? Temperature is the measure of the average kinetic energy of the molecules of an object. As molecules move and collide, they give off energy in the form of heat. This heat is measured as temperature change. Describe the flow of energy through matter. Energy can be transmitted using conduction, convection, or radiation. Particles within objects are in constant motion, so they are constantly coming into contact with other particles. Energy is transferred with or without matter and in different phases of matter. Compare the three forms of energy transfer. Convection Conduction Radiation Objects in direct contact Movement of warm fluids Electromagnetic waves Objects at unequal temperatures Unequal densities Infrared radiation, visible light, ultraviolet rays Heating a wire directly in a fire Heat pump Sun warming Earth How is work related to force and energy? Energy is the ability to do work, so it is very closely related to work. When calculating total amounts, work is equal to energy. The equations for the two can be interchanged. Force is what causes work and energy. What is power? Power measures the rate at which work is done. The equation is power = work ÷ time. The unit for power is watts. Power can change if the work done or the time needed to do the work changes. How are work and power related? The length of time it takes to do work is expressed in power. Greater work requires greater power. Speeding up or slowing down changes the amount of power used. P = W / t Work Example Calculation It takes 100,000 J of work to lift an elevator 18 m. If this is done is 20 s, what is the average power of the elevator during the process? P = W / t P = 100000/ 20 P = 5000 watts What are electromagnetic forces? Electromagnetic forces involve the properties of electricity and magnetism. An electromagnet is a strong magnet created when an iron core is inserted into the center of a current-carrying solenoid. Explain the relationship between electricity and magnetism. With electricity, the exchange of electrons creates a force between objects. Electric currents produce magnetic fields. The flow of electrons creates a north and south pole. What is the difference between induction and conduction? Induction is the production of a current by changing the strength, position, or orientation of an external magnetic field. Conduction is the transfer of energy as heat between particles as they collide within a substance or between two objects in contact. Conduction requires objects to be in contact. Induction does not. What is Ohm’s Law? Ohm’s law says that in an electrical circuit, current is directly proportional to the potential difference (voltage drop) and inversely proportional to the resistance. This means that when current is increased, voltage increases and resistance decreases. Resistance = voltage current Resistance Example Calculation The headlights of a typical car are powered by a 12 V battery. What is the resistance of the headlights if they draw 3.0 A of current when turned on? R = V / I R = 12 / 3 R = 4 Ohms How do electrical circuits work? An electrical circuit is a device connected so that it provides one or more complete paths for the movement of charges. Electrons move from the power source through the wires to power a bulb and move on to return to the power source. An Electrical Circuit Identify methods for creating electrical charge with mechanical, magnetic, and chemical methods. Mechanical Magnetic Chemical Charge created by motion Charge created by changing magnetic field Charge created by breaking of bonds How is energy transferred by mechanical waves? Waves can do work because they carry energy. The particles move up and down, but they never leave their horizontal positions. Think of people doing the wave in a stadium. They stand up and sit down, but they do not leave the location of their seats. As the energy moves forward, the next molecules are lifted and set back down. The particles do not actually leave their original locations. Compare transverse and longitudinal waves. Transverse Motion of particles perpendicular to motion of entire wave Light waves Longitudinal Motion of particles parallel to motion of entire wave Sound waves How is energy transferred by electromagnetic waves? A charged particle moving in a magnetic field will experience a force due to the magnetic field. The force causes energy to move forward. Electromagnetic waves are transverse, so particles move up and down as energy passes. They do not move horizontally. What are the physical properties of waves? The top of a wave is called the crest, and the bottom is the trough. The measurement from crest to crest or trough to trough is the wavelength. The distance the particles in a medium move when a wave passes is the amplitude. The length of time necessary for a wave to pass is the period, and the number of waves to pass in a given time is the frequency. Wave Properties How do the properties of waves create sound and light? Light waves are transverse, and sound waves are longitudinal. Light acts as a wave and as a particle. The particles of light move in the manner of a wave to create luminescence. They travel in straight lines. Sound waves are caused by vibrations. They move out in all directions from the source. The waves cause the eardrum to vibrate. What are nuclear forces? Nuclear forces are short-range attractions between neutrons and protons within the nuclei of atoms that cause atoms to be held together. They are caused by the mass of the nuclear particles. The Nucleus What is the structure of an unstable isotope? An unstable nucleus is called a radioactive nuclide, and it is capable of undergoing radioactive decay. This process is spontaneous, meaning it happens without warning. The nucleus disintegrates into a slightly lighter nucleus and emits particles, electromagnetic radiation, or both. All elements with an atomic number higher than 83 are radioactive. The nucleus itself has a number of protons and neutrons that causes it to be unstable. Comparison of Nuclei Compare the three types of nuclear radiation. Alpha Beta Gamma Two protons and two neutrons bound together (helium nuclei) Electron High energy electromagnetic waves Stopped by paper Stopped by lead Stopped by concrete Only very heavy nuclei Atomic number increased by 1 Immediately after other types of decay, while nucleus is still excited Compare fission and fusion. Fission Nucleus splits into two smaller nuclei Tremendous amount of energy released Used in power plants Fusion Nuclei of two smaller atoms join together to make larger nucleus Requires extremely high temperature Energy of sun and stars What are the uses of nuclear technology? Nuclear power Medical tagging within body X-rays Radiation therapy What are some possible side effects of nuclear technology? Cancer Mutation Radiation sickness