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college physics reasoning
Questions and Answers of
College Physics Reasoning
A hockey puck has an initial velocity of 50 m/s and a final velocity of 35 m/s. (a) If it travels 35 m during this time, what is the acceleration? (b) If the mass of the puck is 0.11 kg,
An object with an initial velocity of 12 m/s accelerates uniformly for 25 s. (a) If the final velocity is 45 m/s, what is the acceleration? (b) How far does the object travel during this
An elevator is moving at 1.2 m/s as it approaches its destination floor from below. When the elevator is a distance h from its destination, it accelerates with a - -0.50 m/s2, where the negative sign
Explain why the air bags in a car reduce the forces on a passenger in the event of an accident.
An airplane must reach a speed of 200 mi/h to take off. If the runway is 500 m long, what is the minimum value of the acceleration that will allow the airplane to take off successfully?
For the books in Figure Q3.14, there is a force F from the table supporting the book on the bottom. Identify two actionreaction force pairs for each of the following objects: (a) The
A drag racer is able to complete the 0.40-km course (0.25 mi)in 6.1 s. (a) If her acceleration is constant, what is a? (b) What is her speed when she is halfway to the finish line?
The bacterium in Figure 3.30 experiences a force due to drag from the surrounding fluid. Does the reaction force to Fdrag act on (a) The flagellum, (b) The body of the bacterium,
Consider a sprinter who starts at rest, accelerates to a maximum speed vmax, and then slows to a stop after crossing the finish line. Draw qualitative plots of the acceleration, velocity, and
Two balls of the same diameter are dropped simultaneously from a very tall bridge. One ball is solid lead, and the other is hollow plastic and has a much smaller mass than the solid lead ball. Use a
Draw a qualitative plot of the total force acting on the ball in Figure 3.15 (page 68) as a function of time. Begin your plot while the ball is still in the throwers hand and end it after
Consider a string with one end tied to a tall ceiling and the other end hanging freely. Explain why the tension at the bottom of the string is smaller than the tension at the top.
Pulling g’s. Suppose again you are the astronaut in Problem 9. When most people are subjected to an acceleration greater than about 5 × g, they will usually become unconscious (“black out”).
The lower piece of silk in Figure 3.20 is acted on by two forces, +T2at the upper end and -T2at the lower end. These two forces are equal in magnitude and opposite in direction. Are they an
You are a newly graduated astronaut preparing for your first trip into space. Plans call for your spacecraft to reach a velocity of 500 m/s after 2.4 min. If your mass is 75 kg, what force will be
(a) Suppose a tire rolls without slipping on a horizontal road. Explain the role friction plays in this motion. What two surfaces are involved in this frictional force? Is it static friction or
Your car has a dead battery. It is initially at rest, and you push it along a level road with a force of 120 N, finding that it reaches a velocity of 2.0 m/s in 50 s. What is the mass of the car?
Two balls are thrown from a tall bridge. One is thrown upward with an initial velocity +v0, while the other is thrown downward with an initial velocity -v0. Which one has the greater speed just
A rocket-powered sled of mass 3500 kg travels on a level snow-covered surface with an acceleration of +3.5 m/s2(Fig. P3.7). What are the magnitude and direction of the force on the sled?Figure P3.7 X-
You are riding in a car that starts from rest, accelerates for a short distance, and then moves with constant velocity. Explain why you feel a force from the back of your seat only while the car is
A constant force of 400 N acts on a spacecraft of mass 8000 kg that has an initial velocity of 30 m/s. How far has the spacecraft traveled when it reaches a velocity of 5000 m/s?
Two mountain climbers are suspended by a rope as shown in Figure Q3.5. Which rope is most in danger of breaking? That is, which rope has the greatest tension?Figure Q3.5
A car has a velocity of 10 m/s at t - 7.0 s. It then accelerates uniformly and reaches a velocity of 42 m/s at t - 12.0 s. What is its acceleration during this period?
Two objects are released simultaneously from the same height. The objects are both spherical with the same radius, but their masses differ by a factor of two. (a) Ignoring air drag, which object
An object moves with a constant acceleration of 4.0 m/s2. If it starts with an initial speed of 30 m/s, how long does it take to reach a velocity of 250 m/s?
Give an example of motion in one dimension for an object that starts at the origin at t = 0. At some time later, the displacement from the origin is zero but the velocity is not zero. Draw the
An ice skater moves without friction on a frozen pond. While traveling at 8.0 m/s, she finds that it takes 17 s to travel the length of the pond. How long is the pond?
A hockey puck moves on an icy surface that is friction less, with a constant speed of 30 m/s. How long does it take the puck to travel the length of the hockey rink (60 m)?
A spacecraft that is initially at rest turns on its engine at t - 0. If its mass is m - 3000 kg and the force from the engine is 45 N, what is the acceleration of the spacecraft?
An object is found to move with an acceleration of magnitude 12 m/s2 when it is subjected to a force of magnitude 200 N. Find the mass of the object.
Make a hypothetical sketch of a velocity–time graph in which the velocity is always positive, but the acceleration is always negative. Give a physical example of such motion.
A ball is thrown into the air with an upward velocity of 10 m/s. A short time later, it is caught on its way down, also with a speed of 10 m/s.(a) Draw the velocity–time graph for this
When a car collides with a wall, a force on the car causes it to stop. Identify an action–reaction pair of forces involving the car.
You push on a refrigerator, as in Figure 2.1, but the refrigerator does not move. Hence, even though you are applying a nonzero force, the acceleration is still zero. Explain why this does not
A falling baseball has an acceleration of magnitude 9.8 m/s2. What is its acceleration in feet per second squared?
A car is traveling on an icy road that is extremely slippery. The driver finds that she is not able to stop or turn the car. Explain this situation in terms of the principle of inertia (Newton’s
An elite runner can run 1500 m in 3 minutes and 35 s. What is his average speed? Give your answer to two significant figures.
A car is initially at rest on an icy road that is extremely slippery. The driver finds that he is unable to get the car to drive away because the wheels simply spin when he tries to accelerate.
Consider the motion of a sprinter running a 100-m dash. When it is run outdoors, this race is run along a straight-line portion of a track, so it is an example of motion in one dimension. Draw
In SI units, acceleration is measured in units of meters per second squared (m/s2). Which of the following combinations of units can also be used to measure acceleration?(a) Cm/s(b) Cm/s2(c) m3/(mm2
A jogger maintains a speed of 3.0 m/s for 200 m until he encounters a stoplight, and he abruptly stops and waits 30 s for the light to change. He then resumes his exercise and maintains a speed of
Give an example of motion for which the average velocity is zero, but the speed is never zero.
Give an example of motion for which the average velocity is equal to the instantaneous velocity.
Abracadabra! A magician pulls a tablecloth off of a set table with one swift, graceful motion. Amazingly, the fine china, glassware, and silverware are practically undisturbed. Although amazing, this
A hockey puck that is sliding on an icy surface will eventually come to rest. The (horizontal) force that makes it stop is due to friction between the puck and the ice. Draw qualitative plots of the
Consider a marble falling through a very thick fluid, such as molasses. Draw qualitative plots of the position, velocity, and acceleration as functions of time for the marble, assuming it is released
Give examples of motion matching the following descriptions. (a) The velocity is positive and the acceleration is positive. (b) The velocity is negative and the acceleration is
A person riding on a skateboard is initially coasting on level ground. He then uses his feet to push on the ground so that he speeds up for a few seconds, and then he coasts again. Draw qualitative
A car starts from the origin at t = 0. At some later time, is it possible for the car’s velocity to be positive but its displacement from the origin to be zero? Explain and give an example.
In the drop zone. Consider a skydiver who jumps from an airplane. Suppose she waits for 1 min before opening her parachute and she lands 4 min after leaving the airplane. Draw qualitative plots of
The acceleration of an object that falls freely under the action of gravity near the Earth’s surface is negative and constant. (a) Does the object’s instantaneous acceleration equal its
Consider a skier who coasts up to the top of a hill and then continues down the other side. Draw a qualitative plot of what the skier’s speed might look like.
Consider again Example 2.8. Draw plots of the position, velocity, and acceleration as functions of time, starting from when the ball is released and ending after the ball hits the ground. Indicate
Figure P2.13 shows three motion diagrams, where the dots indicate the positions of an object after equal time intervals. Assume left-to-right motion. For each motion diagram, sketch the appropriate
A typical airplane can fly at a speed of 400 miles per hour. What is its speed in meters per second?
In SI units, velocity is measured in units of meters per second (m/s). Which of the following combinations of units can also be used to measure velocity?(a) Cm/s(b) Cm/s2(c) m3/(mm2 • s2)(d)
Make a qualitative sketch of the position y as a function of time for the center of a yo-yo (the point at the middle of the axle). Also make sketches of the velocity and acceleration as functions of
A bicycle is moving initially with a constant velocity along a level road. The bicyclist then decides to slow down, so she applies her brakes over a period of several seconds. Thereafter, she again
Figure Q2.15 shows a motion diagram for a rocket powered car. The photos are taken at 1.0-s intervals. Make qualitative plots of the position, velocity, acceleration, and force on the car as
Figure P2.15 shows several hypothetical positiontime graphs. For each graph, sketch qualitatively the corresponding velocity time graph.Figure P2.15 Case 1 Case 2 Case 3
A person stands on level ground and throws a baseball straight upward, into the air. (a) Does the person exert a force on the ball while it is in his hand? After it leaves his hand? (b)
The positiontime graphs in Figure P2.15 each describe the possible motion of a particular object. Give at least one example of what the object and motion might be in each case.Figure
Consider the motion of the Moon as it orbits the Earth. (a) Is the Moon’s acceleration zero or nonzero? Explain. (b) If the Moon has a nonzero acceleration, what force is responsible?
Figure P2.17 shows several hypothetical velocitytime graphs. For each case, sketch qualitatively the corresponding accelerationtime graph.Figure P2.17 Case 2 Case 1 Case 3
Consider the motion of a marble as it falls to the bottom of a jar of honey. Experiments show (see also Chapter 3) that the marble moves with a constant velocity. Applying Newton’s first law, does
Figure P2.17 shows several hypothetical velocitytime graphs. For each case, sketch qualitatively a possible corresponding positiontime graph.Figure P2.17 Case 2 Case 1 Case 3
Give examples of objects whose motion might be described by the graphs in Figure P2.17.Figure P2.17 Case 2 Case 1 Case 3
According to Newton’s first law (the principle of inertia), if there is no force exerted on an object, the object will move with constant velocity. Consider the following examples of motion. Is the
Three blocks rest on a table as shown in Figure Q2.20. Identify three actionreaction pairs of forces.Figure Q2.20 3 2.
Figure P2.20 shows several hypothetical accelerationtime graphs. For each case, sketch qualitatively a possible corresponding velocitytime graph.Figure P2.20 Л Case 2 Case 3
Two football players start running at opposite ends of a football field (opposite goal lines), run toward each other, and then collide at the center of the field. They start from rest and are running
Give examples of objects whose motion is described by the plots in Figure P2.20.Figure P2.20 Л Case 2 Case 3 Case 1
A person is riding in a car traveling on a straight level road.What is the direction of the net force on the person if the car is (a) Speeding up, (b) Slowing down, (c) Has a constant
Match each of the following examples of motion to one of the positiontime graphs in Figure P2.22.(a) A person at the beginning of a race, starting from rest(b) A runner near the end of a
Consider the positiontime graph shown in Figure P2.24. Make a careful graphical estimate of the velocity as a function of time by measuring the slopes of tangent lines. What is an
For the object described by Figure P2.24, estimate the average velocity (a) Over the interval from t - 2.0 s to t - 4.0 s and (b) Over the interval from t - 1.0 s to t - 5.0 s.Figure P2.24
Repeat Problem 24 using the positiontime graph in Figure P2.26.Figure P2.26Data From Problem 24Consider the positiontime graph shown in Figure P2.24. Make a careful graphical
Using a graphical approach (i.e., by estimating the slope at various points), find the qualitative behavior of the acceleration as a function of time for the object described by the
For the object described by the velocitytime graph in Figure P2.29, estimate the average acceleration over the interval from t - 0 s to t - 50 s and over the interval from t - 100 s to t
Draw a possible positiontime graph for an object whose velocity as a function of time is described by (a) Figure P2.28 and (b) Figure P2.29.Figure P2.28Figure P2.29 v (m/s) HHt
Draw a graph showing the position x as a function of time for an object whose acceleration is (a) Constant and positive, (b) Constant and negative, and (c) Positive and increasing with
A car travels along a straight, level road. The car begins a distance x - 25 m from the origin at t - 0.0 s. At t - 5.0 s, the car is at x - 100 m; at t - 8.0 s, it is at x - 300 m. Find the average
A squirrel falls from a very tall tree. Initially (at t = 0), the squirrel is at the top of the tree, a distance y = 50 m above the ground. At t - 1.0 s, the squirrel is at y - 45 m, and at t - 2.0 s
A spacecraft takes off from Cape Canaveral in Florida and orbits Earth 18 times before landing back at Cape Canaveral 24 hours and 15 minutes later. During this time, it moves in a circular orbit
Figure P2.35 shows the velocity as a function of time for an object. (a) What is the average acceleration during the interval from t = 0 s to t = 20 s? (b) Estimate the instantaneous
Figure P2.36 shows the acceleration as a function of time for an object. (a) If the object starts from rest at t - 0, what is the velocity of the object as a function of time? (b) If the
A rabbit runs in a straight line with a velocity of +1.5 m/s for a period of time, rests for 10 s, and then runs again along the same line at +0.60 m/s for an unknown amount of time. The rabbit
Figure 2.24 (page 39) shows one of Galileos experiments in which a ball rolls up an incline. A ball that is initially rolling up the incline will roll up to some maximum height and then
A block is sliding at a particular initial speed v when it encounters the following horizontal surfaces: (a) A grass field, (b) An asphalt street, (c) A hardwood floor, (d) An ice
An unsafe way to transport your penguin. A flatbed truck hauls a block of ice on top of which stands a penguin (see Fig. P2.40). Assume the block of ice is frozen solid to the bed of the truck, but
A flatbed rail car is moving at a slow but constant velocity. A man stands in the rail car, facing sideways (perpendicular) to the motion of the rail car. The man holds a baseball at arm’s length
The person shown in Figure 2.1 (page 27) is pushing on a refrigerator that sits on a level floor. Assume the section of the floor underneath the refrigerator is very slippery so that the only
Action and reaction. Consider again the man pushing the refrigerator in Problem 42, but now assume the entire floor, including the portion under the man, is friction less. Use Newtons
In SI units, force is measured in newtons, with 1 N = 1 kg • m/s2. Which of the following combinations of units can also be used to measure force?(a) g • m/s(b) g • cm2/s(c) kg • m4/(s2
In the U.S. customary system of units, mass is measured in units called slugs. Suppose an object has a mass of 15 kg. Use the conversion factors inside the front cover of this book to express the
In the U.S. customary system of units, force is measured in units of pounds (abbreviated lb). Suppose the force on an object is 150 lb. Using the conversion factors inside the front cover of this
A force is found to be 240 g • cm/s2. Convert this value into units of newtons.
A cannon is fired horizontally from a platform (Fig. P2.49). The platform rests on a flat, icy, friction less surface. Just after the shell is fired and while it is moving through the barrel of the
According to Newton’s third law, for every force there is always a reaction force of equal magnitude and opposite direction. In each of the examples below, identify an action–reaction pair of
Tom has two ways he could drive home from work. He could take Highway 99 for 45 miles with a speed limit of 65 mi/h, or he could take Interstate 5, which would take him a bit out of the way at 57
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