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# General Physics Q&A's

By bigdog1870 Sep 29, 2013 1817 Words
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Textbook Sections that I read: 2.1-3
Important Concepts:
An interaction between two objects can be described and measured in terms of two forces. The force is a push or either a pull. There are two types of forces. #1 is a long range force and this force does not require the objects involved to be touching each other. An example of this is when you are holding a magnet away from a refrigerator and you are able to feel the magnetic pull. #2 is a contact force and this occurs only when the objects are touching one another. A vector in physics is another name for a force. All vectors have a direction in space and magnitude as well. Both direction and magnitude are of equal importance. Any vector can be expressed as the sum of vectors that are parallel to the x-, y- axes (if needed the z-axes as well). These components indicate the magnitude and direction of the three vectors along the axes. Reading Assignment questions:

8. (a) Is it possible for the sum of two vectors to be smaller in magnitude than the magnitude of either vector? Yes it is possible if the scalar product of the two vectors is negative. (b) Is it possible for the magnitude of the sum of two vectors to be larger than the sum of the magnitudes of the vectors? It isn’t possible because the sums would only be able to be equal at best. 18. (a) If two vectors have the same magnitude, are they necessarily equal? If not, why not? It depends on the direction they are in. They could be unequal if they are in different directions. Can two vectors with different magnitudes be equal? It would not be possible for them to be equal.

Reading Assignment #2: Newton’s First Law
Textbook sections that I read: 2.4
Important concepts:
When there is more than one force that acts on an object, the subsequent motion of the object is then determined by the net force acting on the object. The net force is the vector sum of all the forces that are acting on the object. Newton’s first law says that an object acted on by zero net force moves in a straight line with constant speed or it remains at rest if it was at rest. Velocity vector is a measurement of both speed and the direction of an objects motion. There is no force required to keep an object in motion as long as there are no forces that oppose its motion. Inertia means resistance to changes in velocity. Newton’s first law can also be called the law of inertia. Reading Assignment Questions:

Explain the need for automobile seat belts in terms of Newton’s first law. Newton’s first law is an object in motion tends to stay in motion unless acted on by an opposing force. A car and seatbelts are connected together where the car goes the seatbelts go. The people in the car are not connected to the car. That is why they must be strapped in by the seatbelts. If the car had an accident of slammed on brakes really hard everyone would stay in motion that the car was in. Because the seatbelts are there to keep people in the same motion in the car it keeps a lot of people from being slung through the windshield. A freight train consists of an engine and several identical cars on level ground. Determine whether each of the statements is correct or incorrect and explain why. (a) If the train is moving at constant speed, the engine must be pulling with a force greater than the train’s weight. False the engine would have to be pulling with the same force as the trains’ weight. (b) if the train is moving at constant speed, the engine’s pull on the first car must exceed that car’s backward pull on the engine. True, if the engines pull did not exceed the backward pull of the first car the weight of the other cars would cause the train to not be able to maintain constant speed. (c) If the train is coasting, its inertia makes it slow down and eventually stop. False, the friction of the trains wheels and tracks would be what caused it to slow down. Reading Assignment #3: Newton’s Third Law

Textbook Sections that I read: 2.5
Important Concepts:
Every force is part of an interaction between two objects and each of those objects exerts a force on the other. These forces are called an interaction pair; each of these forces is partners with one another. Newton’s third law of motion says that interaction partners always have the same magnitude and are in the opposite direction. Interaction partners are always of the same type.

All the parts of an object make up the system.
All the interactions that affect the system must be either internal or external. To find the net force on the system, ignore all the internal forces and add the external forces. Reading Assignment Questions:

If a wagon starts at rest and pulls back on you with a force equal to the force you pull on it, as required by Newton’s third law, how is it possible for you to make the wagon start to move? Explain. The wagon will always pull back on you with the same amount of force that you are pulling on it. However, as you move forward the wagon will move forward as well. The net forward force on the wagon must be greater than the net backward force that is pulling on you. So to make it move the friction between your feet and the grand must be greater than the wagon and its wheels. An SUV collides with a Mini Cooper convertible. Is the force exerted on the Mini by the SUV greater than equal to, or less than the force exerted on the SUV by the Mini? Explain. The force that each one applies is going to be equal. The SUV however has a lot more mass. It is not going to be bothered as much as the Mini Cooper. It is like the example that the Professor used in class by sitting in the chair and kicking off the wall. The Professor had very little mass compared to the wall. So the wall did not move but the Professor did.

Textbook Sections that I Read – 2.6-9
Important Concepts:
Newton’s law of universal gravitation – Any two objects exert gravitational forces on each other that are proportional to the masses (m1-m2) of the two objects. Even though masses may be highly different, the forces exerted on each will always be the same (opposite direction). Since the earth slightly flattened at the poles, the gravitational field at sea level is greater at the poles and slightly smaller at the equator (the same principal works for altitude/as you get farther above sea level, the gravitational field decreases. The value of g is not a universal constant; it is rather a function of position. Gravity has an unlimited range, it will never drop to zero (no matter how far apart objects get).

11. (a) Does a man weigh more at the North Pole or at the equator? He will weigh more at the North Pole because the Earth is not a perfect sphere; it is flattened at the pole. He therefore will be closer to the center of the earth which means gravity would be slightly stronger. The total weight would be a very small difference because the force of gravity at the poles is (9.832 N/kg) and at the equator is (9.814 N/kg) (b) Does he weigh more at the top of Mt. Everest or at the base of the mountain? The same concept applies with this question as well, the greater the distance from the center of the earth would mean the less force of gravity that there is. He would weigh slightly less at the top of Mt. Everest than at the base of the mountain.

22. A bird sits on a stretched clothesline, causing it to sag slightly. Is the tension in the line greatest where the bird sits, greater at either end of the line where it is attached to poles, or the same everywhere along the line? Treat the line as an ideal cord with negligible weight.

Reading Assignment #6: Displacement and Velocity
Textbook Sections that I Read – 3.1-2
Important Concepts:
When a nonzero net force acts upon an object, the velocity changes. The magnitude of the displacement vector isn’t necessarily equal to the distance traveled. Even though vectors are subtracted in order to find displacement, in order to find total displacement, successive displacements must be added. Average velocity is the product of a vector, the displacement, and a positive scalar. (It also depends on the time interval being considered) Instantaneous velocity is a vector quantity in which the magnitude is the speed and the direction is the direction of motion. When the word velocity is said it should be assumed that it means instantaneous velocity. Reading Assignment Questions:

1. Explain the difference between distance traveled, displacement, and displacement magnitude. Displacement is the change of the position vector –final vector minus the initial position vector. 2. Explain the difference between speed and velocity

Reading Assignment #7: Newton’s Second Law
Textbook Sections that I Read – 3.3

Important Concepts:
Newton’s second law says when a none zero net force acts on an object the rate of change of the velocity is proportional to the net force and inversely proportional to the mass of an object. The direction of change in velocity is not necessarily the same as either the initial or final velocity direction. Acceleration- rate of change of velocity.

To find instantaneous acceleration, calculate the average acceleration during a very short time interval. In straight-line motion the acceleration is always either in the same direction as the velocity or in the opposite direction of the velocity (For motion that changes direction, the acceleration is not along the same line as the velocity). The mass of an object is a measure of its inertia.

Weight is the magnitude of the gravitational force acting on it.

14. If an object is acted on by a single constant force, is it possible for the object to remain at rest? Is it possible for the object to move with constant velocity? Is it possible for the objects speed to be decreasing? Is it possible for it to change direction? In each case that is possible describe the direction of the force.

17.You are bicycling along a straight north-south road. Let the x-axis point north. Describe your motion in each of the following cases. Example: ax>0 and vx>0 means you are moving north and speeding up. (a) ax>0 and vx

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