1 Forces. 2 Energy & Work. GS 104, Exam II Review

Transcription

1 1 Forces 1. What is a force? 2. Is weight a force? 3. Define weight and mass. 4. In European countries, they measure their weight in kg and in the United States we measure our weight in pounds (lbs). Who is actually measuring weight? Both, us, or them? 5. Describe normal force including direction. 6. List one contact and one non-contact force. 7. What causes an initially moving hockey puck to finally come to rest? 8. Describe Newton s 2nd Law in words. 9. Describe Newton s 1st Law in words. 10. Describe Newton s 3rd Law in words. 11. Why do we need a separate law for Newton s 1st Law? 12. How does Newton s 1st Law relate to equilibrium? 13. When riding in a car and someone quickly hits the brake, you fall forward. Which of Newton s laws explain why? 14. I am pushing a box across a carpet. How many forces are acting on the box and what are they called? 15. In a game of tug-of-war, I pull with 100 N and your group pulls with 100 N. If there were a spring scale in the middle, what would it read (in N)? 2 Energy & Work 1. Define work. 2. Define energy. 3. How is energy related to work? 4. List the two main types of energy we have discussed in the course. 5. Define kinetic energy in words and an equation. 6. Define potential energy in words and an equation. 7. How do potential and kinetic energy related to one another? Given a potential energy plot, draw the kinetic energy plot (assuming an isolated system; energy is not lost). 8. On a potential energy graph, describe where a turning point is and what that means physically. 9. Explain stable and unstable equilibrium points. 10. A book falls off a table and free falls to the ground. Is work done? If so, by which force? 11. A teacher applies a force to the wall and becomes exhausted. Is work done? If so, by which force? 1

2 3 Conservation Laws 1. Can energy be created or destroyed? Where does the energy go? 2. Define Conservation of Energy. 3. How does friction play a role in Conservation of Energy? 4. Define the Work-Energy Theorem. 5. Define Conservation of Momentum. 6. Define Conservation of Angular Momentum. Give a real life example. 7. Describe Conservation of Mass. 8. Can the Work-Energy Theorem and Conservation of Energy be used to solve the same problem? 9. Describe a problem that involves the transfer of potential energy to kinetic energy. 10. Set up the physics for the above described problem. (Starting with E i = E f ) 11. Fill in the two types of energy for the above described problem. 12. Solve for the final velocity (v f ) for the above described problem. 13. In the given YouTube video, pause it before the balls leave the start line and predict which color ramp will win. Then finish the video and see if your prediction matches the outcome. Understand why the outcome was the way it was. ( 4 Momentum & Impulse 1. What is inertia? 2. What is momentum? 3. What is the momentum of a 6300 kg truck going 30 m/s (65 mph)? 4. What are the two ways you can change an object s momentum? 5. What is impulse? 6. What is the impulse-momentum principle? 7. When faced with a car crash, what type of object would you look to crash into (with the attempt of keeping yourself unharmed)? Why? 8. What is an elastic collision? 9. What is an inelastic collision? 10. Is energy lost in collisions? 11. You (65 kg) are ice skating with a friend (80 kg) holding hands and going 2.5 m/s. Your friend pushes you forward (letting go) and slows down by 0.2 m/s as a result. How fast are you going? 2

3 5 Circular & Rotational 1. What is the behavioral inward force of circular motion called? 2. What force keeps the moon in orbit around the earth? 3. What force keeps your car driving in a circle around a round-about? 4. When a force stops acting on an object that was previously moving in a circle, the path the object takes is...straight, a circle, an ellipse, all? 5. What is rotational inertia? 6. What is moment of inertia? 7. What is the rotational quantity for position? 8. What is the relationship between a linear length L and a rotational length L? 3

4 1 Forces 1. What is a force? Push or Pull. 2. Is weight a force? Yes. 3. Define weight and mass: Weight = mass x gravity = F g. Weight is an extrinsic (or external) property. Mass is the amount of matter in an object. Mass is an intrinsic (or internal) property. 4. In European countries, they measure their weight in kg and in the United States we measure our weight in pounds (lbs). Who is actually measuring weight? Both, us, or them? In the United States, we are actually measuring weight as pounds are a unit of force. In European countries, they are actually measuring their mass as kg are a unit of mass. It would be more precise in European countries to say they scale themselves. 5. Describe normal force including direction: Force pushing up by a support structure like a table. The normal force is always perpendicular ( ) to the surface the object is being supported by. 6. List one contact and one non-contact force: Contact - applied, normal, friction. Non-contact - gravity, magnetic, electric. 7. What causes an initially moving hockey puck to finally come to rest? Friction. 8. Describe Newton s 2nd Law in words: The sum of the forces of an object indicates how the object will behave. If the object has a net force, the object will accelerate. If the object does not have a net force, the object will remain in its previous state. A constant velocity has no acceleration. So an object with no net force could remain moving at a constant velocity forever until the net force changes. Newton s 2nd Law is also a state of being out of balance, having a changing velocity, or moving with a constant acceleration. 9. Describe Newton s 1st Law in words: A special case of Newton s 2nd law, in which the net force is zero. An object is motion, remains in motion. An object at rest, remains at rest. Unless otherwise acted upon. Newton s 1st Law is also a state of being in balance, stationary, or moving with a constant velocity. 10. Describe Newton s 3rd Law in words: Two objects in contact have an equal and opposite pointing push or pull on each other. It is difficult to tell which object is doing the pushing or pulling. Depends on the system! 11. Why do we need a separate law for Newton s 1st Law? The 1st Law encompasses a state of being in balance, or being in equilibrium. The 2nd Law encompasses a state of being out of balance, or out of equilibrium. Being in equilibrium means being stationary (no velocity) or having constant velocity (no acceleration). 12. How does Newton s 1st Law related to equilibrium? See 11. Newton s 1st Law is the case of equilibrium. 13. When riding in a car and someone quickly hits the brake, you fall forward. Which of Newton s laws explain why? Newton s 1st. Before the brake is engaged, you are moving at the same speed as the car. When the brake is engaged, you continue moving at the same speed as the car was previously moving, which causes you to fall forward. You only stop falling forward once you are caught by the seat belt. 14. I am pushing a box across a carpet. How many forces are acting on the box, what are they called, and in which directions? Four forces. Force due to gravity (F g ) pointing in the negative y-direction, normal force (F N ) pointing opposite to gravity in the positive y-direction, applied pushing force by me 4

5 (F A ) in the positive x-direction, frictional force (F f ) pointing opposite to motion or the applied force in the negative x-direction. 15. In a game of tug-of-war, I pull with 100 N and your group pulls with 100 N. If there were a spring scale in the middle, what would it read (in N)? 100 N. You can think of it as each side pulling 50 N. 2 Energy & Work 1. Define work: W = F net d. Work = net force (in the direction of the distance travelled) x distance travelled. Unit is Joules (J) or Newton*meter (Nm) or (kg*m 2 )/s Define energy: Property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is the property of a system (or object) that enables it to do work. Unit is the same as work. 3. How is energy related to work? Work transfers energy from one place to another or one from to another. 4. List the two main types of energy we have discussed in the course: Kinetic energy and potential energy. 5. Define kinetic energy in words and an equation: Energy of motion; related to the velocity; the faster an object moves, the larger its kinetic energy. K = 1 2 mv2 6. Define potential energy in words and an equation: Energy of position; energy that has the potential to do work; storing energy to use later. U = mgh 7. How do potential and kinetic energy related to one another? Given a potential energy plot, draw the kinetic energy plot (assuming an isolated system; energy is not lost). Potential energy and kinetic energy are opposite of one another. The total energy is made up of potential energy and kinetic energy in an isolated system (no energy lost due to outside forces). As potential energy is lost, kinetic energy is gained. There is a sloshing of energy between potential and kinetic. Where potential is high, kinetic energy is low. 8. On a potential energy graph, describe where a turning point is and what that means physically: A turning point is a boundary at which the object does not have enough energy to surpass the boundary and physically turns around. This usually happens when all of the object s kinetic energy turns into potential energy and then goes back the way it came turning that potential energy back into kinetic energy but with a velocity in the opposite direction as before. The turning point occurs when the kinetic energy is zero. 9. Explain stable and unstable equilibrium points: An equilibrium point occurs when the object can be balanced or when competing influences are balanced. A stable equilibrium is a state of equilibrium in which if the object is pushed slightly off balance then the object would return to the same spot (like a ball in a valley). An unstable equilibrium is a state of equilibrium in which if the object is pushed slightly off balance then the object would not return to the same spot (like a ball on the very top of a hill; you can balance it there, but it has a strong chance of rolling down into the valley if pushed). 10. A book falls off a table and free falls to the ground. Is work done? If so, by which force? Work is done by the force due to gravity (F g ). 11. A teacher applies a force to the wall and becomes exhausted. Is work done? If so, by which force? No work is done since the wall does not have a net force due to the teacher pushing on the wall. Unless the clock falls down... 5

6 3 Conservation Laws 1. Can energy be created or destroyed? Where does the energy go? Energy cannot be created or destroyed, but is transferred into other types of energy (including, but not limited to: chemical energy, thermal energy (usually in the form of heat), mechanical energy). The total amount of energy never changes. 2. Define Conservation of Energy: The process of changing energy from one of its forms into another. For our class, we will only consider kinetic energy and potential energy. Step 1 - Write out energy conservation equation E i = E f Step 2 - Fill in the two types of energy for the initial and final energy K i + U g,i = K f + U g,f Step 3 - Fill in the specific definitions for kinetic and potential energy 1 2 mv2 i + mgh i = 1 2 mv2 f + mgh f Step 4 - Look for kinetic and potential energies that are zero at the beginning or end. Simplify the equation in Step How does friction play a role in Conservation of Energy? For our class, there is no friction when using conservation of energy. In the real world, energy is still conserved, but the energy is transferred into other forms like thermal energy (energy lost through heat). 4. Define the Work-Energy Theorem: The total work done is the change in the object s kinetic energy. K f K i = W 5. Define Conservation of Momentum: In an isolated system (no external forces or friction), the momentum (p i ) before a collision and the momentum (p f ) after a collision are the same. p i = p f 6. Define Conservation of Angular Momentum. Give a real life example. If the net torque (rotational force) acting on a system is zero, the total angular momentum (L) of the system is conserved. L i = L f. Think of an ice skater turning. He starts with his arms as wide as they can go and he is not currently rotating. He brings his arms close to his body very quickly and that movement allows him to begin to rotate. Just by changing his physical position, he can cause himself to spin. 7. Describe Conservation of Mass: Mass is conserved at the beginning and end of a system even if other influences occur (like growth of tree). The additional mass added to a system must come from another source. 8. Can the Work-Energy Theorem and Conservation of Energy be used to solve the same problem? Yes. We did this with the person on the sled going down the ramp. You could also solve the pendulum and razor blade lab problem using both methods. Usually, conservation of energy is easier to apply. 9. Describe a problem that involves the transfer of potential energy to kinetic energy: Consider a pendulum held with the string parallel to the ground. Once it is released, I want to know what the balls kinetic energy is at the balls lowest point. 6

7 10. Set up the physics for the above described problem. (Starting with E i = E f ) E i = E f 11. Fill in the two types of energy for the above described problem: K 0 0 i + U g,i = K f Solve for the final velocity (v f ) for the above described problem: See steps in 2. v f = 2gh i 13. In the given YouTube video, pause it before the balls leave the start line and predict which color ramp will win. Then finish the video and see if your prediction matches the outcome. Understand why the outcome was the way it was. ( Red wins because all the potential energy is converted into kinetic energy before any of the other ramps. In this case, the longer you have kinetic energy, the sooner you get to the finish line. U g,f 4 Momentum & Impulse 1. What is inertia? The tendency of an object to resist changes in motion, or to resist acceleration. 2. What is momentum? Inertia in motion. Mass in motion. Quantity of motion that an object has. Momentum = mass x velocity. p = mv 3. What is the momentum of a 6300 kg truck going 30 m/s (65 mph)? p = mv = (6300 kg)(30 m/s) = 189,000 (kg m)/s 4. What are the two ways you can change an object s momentum? Change the mass or change the velocity. 5. What is impulse? Impulse is defined as the change in the momentum and the force applied for a period of time. Impulse = force x time = change in momentum. J = F t = p 6. What is the impulse-momentum principle? The impulse acting on an object produces a change in momentum of the object that is equal in both magnitude and direction to the impulse. 7. When faced with a car crash, what type of object would you look to crash into (with the attempt of keeping yourself unharmed)? Why? Between a brick wall and a bale of hay, I would choose the hay because you reduce the amount of force but have it over a longer period of time. The brick wall would apply a large force for a shorter period of time. 8. What is an elastic collision? A collision that the object s colliding bounce off one another. 9. What is an inelastic collision? A collision that the object s colliding stick to one another. 10. Is energy lost in collisions? In perfect collisions, no energy is lost. For this class, all collisions are assumed to be perfect collisions. 11. You (65 kg) are ice skating with a friend (80 kg) holding hands and going 2.5 m/s. Your friend pushes you forward (letting go) and slows down by 0.2 m/s as a result. How fast are you going? v you = 2.75 m/s 7

8 5 Circular & Rotational 1. What is the behavioral inward force of circular motion called? Centripetal Force - center seeking force 2. What force keeps the moon in orbit around the earth? Earth s gravitational force. 3. What force keeps your car driving in a circle around a round-about? Frictional force. 4. When a force stops acting on an object that was previously moving in a circle, the path the object takes is...straight, a circle, an ellipse, all? The path the object takes is straight. 5. What is rotational inertia? The tendency of an object to resist changes in rotational motion, or to resist rotational acceleration. 6. What is moment of inertia? Same as rotational inertia. The moment of inertia depends on how mass is distributed around an axis of rotation. It is also defined as the difficulty to change the rotational velocity of an object around a given axis. 7. What is the rotational quantity for position? Theta - θ 8. What is the relationship between a linear length L and a rotational length L? The linear length L can be defined by the circular properties, the angle (θ) and the radius (r). L = rθ 8