Quiz 1 practice problems

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Physics

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Jan 9, 2024

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Phys 1203: Quiz 1 Practice Problems 1. Mehmet is on a spaceship moving close to the speed of light and shines a light from the oor to the ceiling. A mirror on the ceiling reflects the light back down to a detector on the oor. Gowri is watching from Earth. Which of the following is true about the time each thinks it takes for the light to travel from the oor, to the ceiling, and then back to the A. Mehmet (on the moving ship) measures a longer time than Gowri (on Earth). B. Mehmet (on the m oving ship) measures a shorter time than Gowri (on Earth). C. Mehmet (on the moving ship) measures the same time as Gowri (on Earth). 2. A common saying goes, It's not the fall that hurts you; it's the sudden stop." How do each of Newton's Laws of Motion help make sense of this statement? a. Newton's first law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest, unless another force acts on them. b. Newton's second law: The acceleration of an object is proportional to the net force on the object. c. Newton's third law: Two objects exert equal and opposite forces on each other. 3. Below are two scenarios about objects moving, also depicted in the images. For each scenario, qualitatively sketch the shape of the graphs of each object's position, velocity, acceleration, and force over time. a. A cart glides down a frictionless ramp from the top to the bottom. b. A cart is given a push from rest at the bottom of a ramp and then glides up the ramp, turns around, and comes back down to the bottom. Textbook problems: Chapter 1: ● Q. 3 ● Q. 4 ● Q. 29 ● Q. 30 3. When the Sun was directly overhead in Syene, why wasn’t it directly overhead in Alexandria? The earth is a sphere 4. Earth, like everything else illuminated by the Sun, casts a shadow. Why does this shadow taper? The sun is much larger than the earth 29. If Earth were smaller than it is, but the Alexandria-to- Syene distance were the same, would the shadow of the vertical pillar in Alexandria be longer or shorter at noon during the summer solstice?

If the the Earth were smaller than it is and the Alexandria-to-Syene distance were the same, the shadow of the vertical pillar in Alexandria would be longer. Since the circumference of the Earth would be smaller and the distance between the two towns the same, the fraction of the circumference that the distance takes would be larger, so the angle created between Alexandria and the pole's shadow must be bigger. Therefore, the shadow must be longer to create a bigger angle.c Chapter 2: ● Q. 4 4. What did Galileo discover in his legendary experiment on the Leaning Tower of Pisa? Galileo found that a heavier stone does not fall significantly faster than a lighter one. 5. What did Galileo discover about moving bodies and force in his experiments with inclined planes? In the absence of a retarding force, a body will keep moving at a constant speed in a straight line forever. 8. What type of path does a moving object follow in the absence of a force? It continues to move in a straight line at a constant speed. 20. When you stand at rest on a bathroom scale, how does your weight compare with the support force by the scale? Your weight is equal in magnitude and opposite in direction to the support force from the scale 21. A bowling ball at rest is in equilibrium. Is the ball in equilibrium when it moves at constant speed in a straight-line path? A bowling ball is at equilibrium both when at rest and when its moving at a constant velocity 23. If you push on a crate with a force of 100 N and it slides at constant velocity, how great is the friction acting on the crate? Consider a crate being pushed horizontally across a factory floor. If it moves at a steady speed in a straight - line path, it is in dynamic equilibrium. This tells us that more than one force acts on the crate. Another force exists likely the force of friction between the crate and the floor. The fact that the net force on the crate equals zero means that the force of fricition must equal and opposite to our pushing force. 25. A bird sitting in a tree is traveling at 30 km/s relative to the faraway Sun. When the bird drops to the ground below, does it still move at 30 km/s, or does this speed become zero? It continues to move at 30km/s relative to the Sun, but it has a speed of 0 km/s relative to the ground.

● Q. 5 ● Q. 8 ● Q. 20 ● Q. 21 ● Q. 23 ● Q. 25 Chapter 3: 6. What is the main difference between speed and velocity? Speed is the time rate at which an object is moving along a path, while velocity is the rate and direction of an object's movement. Put another way, speed is a scalar value, while velocity is a vector. 7. If a car moves with a constant velocity, does it also move with a constant speed? Yes the car moves at a constant speed. 8. Yes it maintains constant speed, but no it does not maintain a constant velocity because a constant velocity is both constant speed and constant direction. ● Q. 6 ● Q. 7 ● Q. 8 ● Q. 22 Air resistance slows the acceleration of falling objects . An object falls at its terminal velocity when the upward force of air resistance equals the downward force of gravity. ● Q. 88 ● ● Only on the middle hill does the acceleration along the path decrease with time,

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for the hill becomes less steep as motion progresses. When the hill levels off, acceleration will be zero. On the left hill, acceleration is constant. On the right hill, acceleration increases as the hill becomes steeper. The speed increases in all cases as the ball rolls downhill. ● Q. 91 If air drag is not a factor, an objects acceleration is the same 10 m / s2 regardless of its initial velocit Chapter 4: ● Q. 2 - 4 When you push horizontally on a crate on a level floor that doesn’t slide, how great is the force of friction on the crate? The friction force is equal and opposite to your push. As you increase your push, will friction on the crate increase also? Yes, the friction also increases. Once the crate is sliding, how hard do you push to keep it moving at constant velocity? You push with a force equal to and opposite the dynamic friction force. ● Q. 18 - 20 ● Q. 54 If the net force acting on a sliding block is somehow tripled, what happens to the acceleration? The acceleration triples. If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change? The acceleration is reduced to 1/3 of its original value. If the mass of a sliding block is somehow tripled at the same time the net force on it is tripled, how does the resulting acceleration compare with the original acceleration? The acceleration remains the same. Chapter 5: ● Q. 1 - 2

1. When you push against a wall with your fingers, they bend because they experience a force. Identify this force. You exert an action force and the wall exerts an equal and opposite reaction force 2. A boxer can hit a heavy bag with great force. Why can’t he hit a piece of tissue paper in midair with the same amount of force? The boxer can only hit the tissue paper with a force as large as the tissue paper can exert on the boxer, and the low-mass tissue can only exert a weak force. ● Q. 11 If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why do the cannonball and cannon have very different accelerations? ● Q. 50 Suppose that two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. What is the acceleration of the heavier cart relative to that of the lighter cart as they start to move apart? They have the same speed, but because they have different masses their acceleration is different. The bigger the mass, the smaller the aceleration. So the 2m-less acceleration and speed while the "m" will have greater speed and acceleration. ● Q. 51 ● Q. 55 Chapter 9: ● Q. 8 8. How does the force of gravity between two bodies change when the distance between them is doubled? 9. How does the thickness of paint sprayed on a surface change when the sprayer is held twice as far away? Q. 9 ● Q. 11 Would the springs inside a bathroom scale be more com- pressed or less compressed if you weighed yourself in an elevator that was accelerating upward? Downward?

● ● Q. 45 45. The planet and its moon gravitationally attract each other. Rank the forces of attraction between each pair, from greatest to least. ● Q. 46 ● Consider the light of multiple candle flames, each of the same brightness. Rank the light that enters your eye from brightest to dimmest for the following situations: ● Q. 61 If Earth somehow expanded to a larger radius, with no change in mass, how would your weight be affected? How would it be affected if Earth instead shrunk? ( Hint: Let the equation for gravitational force guide your thinking.) ● ● Chapter 35: ● Q. 7 - 8 7. Inside the moving compartment of Figure 35.4, light travels a certain distance to the front end and a certain distance to the back end of the compartment. How do these distances compare as seen in the frame of reference of the moving rocket? The distances travelled by light are the same. ● Q. 12 12. Time is required for light to travel along a path from one point to another. If this path is seen to be longer because of motion, what happens to the time it takes for light to travel this longer path? ● Q. 27 If you were traveling in a high-speed rocket ship, would metersticks on board appear to you to be contracted? Defend your answer. Meter stick would appear normal to an observer on board. It would appear shorter seen by someone on the earth ● Q. 48 To an Earth observer, metersticks on three spaceships are seen to have these lengths. Rank the

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speeds of the space- ships relative to Earth, from highest to lowest. ● Q. 49 49. If you were in a smooth-riding train with no windows, could you sense the difference between uniform motion and rest? Between accelerated motion and rest? Explain how you could m Only accelerated motion, not uniform motion, can be sensed. You could not detect your motion when traveling uniformly, but accelerated motion could be easily detected by observing that the surface of the water in a bowl is not horizontal.

● Q. 57 Event A occurs before event B in a certain frame of refer- ence. How could event B occur before event A in some other frame of reference? For example, you know that when you are watching a man hammer a nail into a board, but you are standing quite a distance from the man/board, you hear the nail strike a few moments after you see him hit the nail/board. That's a result of the speed of light being so much faster than the speed of sound. Likewise, the speed of light is not infinite. As a result, there will be a time difference between when an event happens, and when the light containing the information about the event reaches different points in space at different distances from the event. For example, if a star is 1 light year away from you and the star explodes, it will be 1 year before the light containing the information that the star exploded will reach you. Likewise, that means that the light you're currently seeing is what the star looked like 1 year ago. Apply this to your question: consider two planets, A and B which are 1 light year apart. Now, both planets explode at exactly the same time. An observer near A will see A explode almost immediately, but will have to wait 1 year before seeing B explode. Conversely, an observer near B will see B explode almost immediately, but will have to wait 1 year before seeing A explode. Depending which reference frame you're in (A or B) will determine which planet exploded first (even though both planets exploded at exactly the same time.) ● Q. 58 ● Q. 68 Because of time dilation, you observe the hands of your friend’s watch to be moving slowly. How does your friend view your watch: as running slowly, running rapidly, or neither? slowly

Quiz 1 practice problems

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