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THE MCAT PHYSICS-3
WORK, ENERGY, MOMENTUM
| Question | Answer |
|---|---|
| What are the 3 types of potential energy? | gravitational, elastic, electric |
| What does potential energy depend on? | Difference between initial and final heights |
| Stiff springs have smaller / larger "k" value | larger k |
| open system | energy and mass are exchanged with surroundings |
| closed system | energy is exchanged with surroundings but mass is not |
| isolated system | neither energy or mass is exchanged with surroundings |
| Give examples of conservative forces. | gravity, spring force, electromagnetic |
| Define conservative force | -energy removed is same as energy recovered -force produces same amount of work regardless of path taken |
| What are 2 ways to see if it is a conservative force? | 1.) Sum of work to move particle in a round trip = 0 2.) Sum of work needed to move a particle between two points is same regardless of path taken |
| Formula for conservative forces | KE + PE = KE + PE |
| Work done against a conservative force is conserved in? | Potential Energy |
| What kind of system is the universe? | Isolated system |
| Examples of nonconservative force | friction and air resistance |
| Define nonconservative force | Energy removed cannot be recovered and is no longer available for kinetic energy |
| Is non-conservative force path-dependent? | YES! |
| Does NC / C forces do work? When is mechanical energy conserved? | -Conservative forces do no work -- ME conserved -Non-conservative forces do work -- ME not conserved |
| Work done against NC force is conserved as? | nothing |
| Conservation of Energy Equation | KE + PE + W = KE + PE + W |
| What does friction affect? | KE only |
| What is work done on a system without friction? | W = Fdcostheta |
| What is work done on a system with friction? | Use Energy Conservation equation |
| Why does conservative force do no work? | Neither energy nor mass is gained or lost by the system |
| What is mechanical energy? | -energy possessed by an object due to its motion/ position -can either be KE or PE |
| What are kinetic frictional forces? | Friction takes KE of a moving object and transfers it into mvmt of individual molecules on surface of objects in contact with each other |
| How to relate work and friction? | W = -f (friction) * x |
| What are 2 types of energy transfer? | work and heat |
| What is work? | -energy is transferred from one system to another via a force -system --> surroundings and vice versa |
| What is heat by definition and in relation to KE? | -energy transfer by natural flow from warm --> cold body -sum of all KE of molecules in an object |
| Is work the same as energy? | NO! --> Work is done on object whenever force acts upon it to cause it to be displaced |
| What do frictional forces change? | change internal and mechanical energy |
| Do perpendicular forces do work? | NO |
| Does work depend on distance traveled? | NO --> only displacement --> path independent |
| 2 equations for power | P = W/t (rate of doing work) P = Fvcostheta |
| What are the 2 ways for energy to escape a system? | work and heat |
| If system is same temperature as surroundings, where does energy change come from? | work |
| What does it mean to do more work? | greater "a" , greater "v", greater KE, less time to achieve displacement |
| What is positive work? | force and distance applied in same direction |
| What is negative work? | force and distance applied in opposite directions |
| Positive work done on an object | object gains energy |
| Positive work done by object | object loses energy |
| negative work done on object | object loses energy |
| negative work done by object | object gains energy |
| How does work compare between sliding down a frictionless inclined plane and free fall from same height? | SAME |
| Any object with velocity must have? | momentum |
| What is momentum? | measure of a moving object's tendency to continue along its present path |
| What are 2 important points about momentum? | -initial and final momentum always conserved -vector: momentum is constant in direction and magnitude |
| How can you make it more difficult to change path of a moving object? | increase velocity and mass |
| Conservation of momentum | With no external forces like friction, total momentum before and after collisions are constant |
| Compare momentum and mechanical energy | Momentum is ALWAYS conserved but mechanical energy is NOT ALWAYS conserved (i.e. KE) |
| Compare elastic and inelastic collisions | -Elastic: energy and momentum conserved / no deform -Inelastic: energy not conserved / momentum conserved / deform |
| What is impulse? | Impact force during collision to change object momentum --> change in momentum --> force of collision multiplied by duration of collision |
| Impulse equation | Fdeltatime = change in momentum |
| Do objects always experience the same magnitude of impulse? | YES when momentum is conserved |
| What happens when balls hit each other and lose speed? | lose kinetic energy and thus have inelastic collision |
| What happens when you pull spring away from equilibrium position? | 1.) Restoring force pulls to left --> object accelerates and builds momentum 2.) decelerates once it passes equil. position 3.) force now pulls in opposite direction |
| What is dampened harmonic motion? | lose energy from friction -energy and amplitude diminish -frequence and period remain constant |
| What does loss in energy in dampened harmonic motion affect? | KE and PE |
| Why does period stay constant in dampened harmonic motion? | Over time, object will cover less distance, but because its doing it at a reduced average speed, time to complete cycle will remain constant |
| What happens when spring is pushed away from equil position? | PE = max, KE = 0, a = max, v = 0 |
| What happens when spring is at equil. position? | PE = min, KE = max, a = 0, v = max |
| When is the tension for a pendulum the greatest? | Bob points down |
| What force opposes tension in a pendulum? | mgcostheta |
| What is tangent to the arc of a pendulum? | mgsintheta |
| Potential energy for a pendulum | PE = -mgL(1 - costheta) |
| What are extrinsic properties? | depends on amount and quantity of material -mass and energy |
| What are intrinsic properties? | does not depend on amount and quantity of material -density and pressure |
| A 15eV photon collides with a proton with ionization potential = 13.6 eV. Describe what happens in relation to conservation of energy. | 15eV photon first provides I.E. to unbind electron --> any excess energy becomes electron's KE |
| Work done by gravity is + / - when it moves down hill and +/- when it moves uphill. | positive and negative |
| Going from PE --> KE, what does object gain? | velocity --> gains momentum |
| Relate frequency and period. | f = 1 / T |
| For work, in what direction must the force be directed? | MUST BE PARALLEL TO MOTION |
| Where must the applied force be directed to change an object's KE? | directed in direction of displacement |
| Does PE depend on the path taken? | NO --> only difference btwn initial and final heights |
| How does work differ in terms of cos/sin between work done on object on ground and one on inclined plane? | -ground = cos -inclined plane = sin |
| Why do lighter objects experience greater changes in velocity than heavier objects upon collisions? | -Impulse equation: Ft = change in momentum -same impulse -->greater change in velocity |
| Impulse = | momentum |
| Whether a car comes to rest abruptly or extended over time, how does momentum change? | same change in overall momentum in both |
| When velocity of two objects after a collision decreases, is collision elastic or inelastic? | inelastic --> lose speed and KE |
| Drop a ball and bounces up to same height - what type of collision? | perfectly elastic collision |
| throw a ball at wall and it bounces back with same speed - what type of collision? | perfectly elastic collision |
| Torque | t = r*Fsintheta |
| In torque, what is the optimal positioning for radius and force? | perpendicular!! |
| Trick to solving torque | -indicate direction force is applied -radius is always perpendicular |
| Mechanical advantage | M.A. = weight / applied force needed to support object |
| Objects with momentum always have...? | KINETIC ENERGY |
| In a collision, what causes a change in momentum? | impulse |
| does impulse = momentum? | NO --> IMPULSE = MOMENTUM CHANGE |
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