Physics Conceptual Questions (Exam #2 Practice)
Determine which of the following moving objects obey the equations of projectile motion developed in this chapter. (Select all that apply.) (a) A ball is thrown in an arbitrary direction. (b) A jet airplane crosses the sky with its engines thrusting the plane forward. (c) A rocket leaves the launch pad. (d) A rocket moves through the sky after its engines have failed. (e) A stone is thrown under water.
(a) A ball is thrown in an arbitrary direction. (d) A rocket moves through the sky after its engines have failed. Explanation:There is only one force acting on an object in ideal projectile motion problems- gravity.
A ball is thrown upward in the air by a passenger on a train that is moving with constant velocity. (a) Describe the path of the ball as seen by the passenger. (b) Describe the path as seen by a stationary observer outside the train. (c) How would these observations change if the train were accelerating along the track?
(a) The passenger sees the ball go into the air and come back in the same way he would if he were at rest on the Earth. (b) The passenger on the train would see the ball fall behind the position it would reach in the absence of the acceleration. The stationary observer outside the train would see the ball follow the same path as before (assuming the initial release velocity is the same). (c) If the train were accelerating, the ball would fall behind the position it would reach in the absence of the acceleration.
Suppose you're playing with your niece in the snow. She's sitting on a sled and asks you to move her across a flat, horizontal field. You have a choice of the following. Which option would be easier? (a) attaching a rope to the front of the sled and pulling with a force at 30° above the horizontal (Fig. b) (b) pushing her from behind by applying a force downward on her shoulders at 30° below the horizontal (Fig. a)
(a) attaching a rope to the front of the sled and pulling with a force at 30° above the horizontal (Fig. b) Explanation: it is easier to attach the rope and pull. In this case, there is a component of your applied force that is upward. This reduces the normal force between the sled and the snow. In turn, this reduces the friction force between the sled and the snow, making it easier to move. If you push from behind, with a force with a downward component, the normal force is larger, the friction force is larger, and the sled is harder to move.
As an apple tree is transported by a truck moving to the right with a constant velocity, one of its apples shakes loose and falls toward the bed of the truck. (i) Of the curves shown in the figure above, which best describes the path followed by the apple as seen by a stationary observer on the ground, who observes the truck moving from his left to his right? (a) curved left (b) 180° vertical (c) curved right (d) slanted at a 45° left (e) slanted at a 45° right
(a) curved left; he would see the apple fall the opposite direction from where it was originally
A larger static friction constant would result in a: (Select all that apply.) (a) larger component of gravitational force along the ramp at the maximum angle. (b) larger component of normal force at the maximum angle. (c) smaller maximum angle. (d) larger maximum angle. (e) smaller component of gravitational force along the ramp at the maximum angle.
(a) larger component of gravitational force along the ramp at the maximum angle. (d) larger maximum angle.
A traffic light weighing 102 N hangs from a vertical cable tied to two other cables that are fastened to a support, as in figure (a). Which of these would increase if a second traffic light were attached to the first? Assume the cables do not change their lengths. (Select all that apply.) (a) the tension T2 (b) the angle of the cable with tension T2 (c) the angle of the cable with tension T1 (d) the tension T1
(a) the tension T2 (d) the tension T1
As an apple tree is transported by a truck moving to the right with a constant velocity, one of its apples shakes loose and falls toward the bed of the truck. (ii) Of the curves shown in the figure above, which best describes the path as seen by an observer sitting in the truck?
(b) 180° vertical; the bed of the truck is below his line of vision, so we would see it fall straight down
Which of the following statements are true? (a) An astronaut's weight is the same on the Moon as on Earth. (b) An astronaut's mass is the same on the International Space Station as it is on Earth. (c) Earth's gravity has no effect on astronauts inside the International Space Station. (d) An astronaut's mass is greater on Earth than on the Moon. (e) None of these statements are true.
(b) An astronaut's mass is the same on the International Space Station as it is on Earth. Explanation: mass doesn't change due to gravity
Under what circumstances would a vector have components that are equal in magnitude? (a) The vector lies at a 60 degree angle with two axes along which the component lie. (b) The vector lies at a 45 degree angle with two axes along which the component lie. (c) The vector lies at a 30 degree angle with two axes along which the component lie.
(b) The vector lies at a 45 degree angle with two axes along which the component lie. Explanation: 90/2 = 45
Suppose you are carrying a ball and running at a constant speed, and wish to throw the ball and catch it as it comes back down. Neglecting air resistance, you should do which of the following? (a) Throw the ball at an angle of about 45° with the horizontal and maintain the same speed. (b) Throw the ball straight up in the air and maintain the same speed. (c) Throw the ball straight up in the air and slow down to catch it.
(b) Throw the ball straight up in the air and maintain the same speed. Explanation:B is only correct assuming that he is running in straight line. a) is wrong because the ball will have an additional horizontal component of velocity and will be always ahead of the runner who is traveling at the same speed(velocity) c) is wrong because horizontal component of ball will be larger than that o f the runner
Consider the following controls in an automobile: gas pedal, brake, steering wheel. Which controls in this list cause an acceleration of the car? (Select all that apply.) (a) none of these (b) steering wheel (c) gas pedal (d) brake
(b) steering wheel (c) gas pedal (d) brake Explanation: Turning the steering wheel would cause angular momentum and angular acceleration of the wheel itself, also when you turn the car in any direction you are causing a new acceleration in that direction, where it was previously zero. A gas pedal causes (+) acceleration A brake causes (-) acceleration
A crate is sitting in the center of a flatbed truck. As the truck accelerates to the east, the crate moves with it, not sliding on the bed of the truck. In what direction is the friction force exerted by the bed of the truck on the crate? (a) to the west (b) to the east (c) there is no friction force, because the crate isn't sliding
(b) to the east Explanation: The static friction force between the bottom surface of the crate and the surface of the truck bed is the net horizontal force on the crate that causes it to accelerate. It is in the same direction as the acceleration, toward the east.
A truck loaded with sand accelerates along a highway. The driving force on the truck remains constant. What happens to the acceleration of the truck as its trailer leaks sand at a constant rate through a hole in its bottom? (a) It remains constant. (b) It increases and then decreases. (c) It increases at a steady rate. (d) It decreases at a steady rate. (e) It decreases and then increases.
(c) It increases at a steady rate. Explanation: According to newtons 2nd law of motion, we know that F = ma and can determine that a = f/m Here, the force remains constant but the mass decreases, and we know that mass is inversely proportional to acceleration. So, we can determine that acceleration increases.
As a block slides down a friction-less incline, which of the following statements is true? (a) Its speed increases and its acceleration decreases. (b) Its speed and acceleration remain constant. (c) Its speed increases and its acceleration remains constant. (d) Both its speed and acceleration increase. (e) Both its speed and acceleration decrease.
(c) Its speed increases and its acceleration remains constant. Explanation: Gravity always acts with 9.8m/s^2 of acceleration unless there is an external force acting, and there is no external force, so the acceleration remains constant. Because there is no force to slow it down, the velocity (or speed) increases.
If an object is in equilibrium, which of the following statements is not true? (a)The velocity is constant. (b) The acceleration of the object is zero. (c) The object must be at rest. (d) The speed of the object remains constant. (e) The net force acting on the object is zero.
(c) The object must be at rest. Explanation: This might be true for some cases, but not all cases. For example, an object can be moving and so long as there is no opposing unbalanced force acting on it, it will continue to move and be at equilibrium.
For the child being pulled forward on the toboggan in the figure below, which of the following is true of the magnitude of the normal force exerted by the ground on the toboggan? (a) possibly greater or less than the total weight, depending on the size of the weight relative to the tension in the rope (b) greater than the total weight (c) less than the total weight (d) equal to the total weight
(c) less than the total weight Explanation: The tension in the rope has a vertical component that supports part of the total weight of the woman and sled. Thus, the upward normal force exerted by the ground is less than the total weight.
A large crate of mass m is placed on the back of a truck but not tied down. As the truck accelerates forward with an acceleration a, the crate remains at rest relative to the truck. What force causes the crate to accelerate forward? (a)the normal force (b) the force of gravity (c) the force of friction between the crate and the floor of the truck (d) the "ma" force (e) none of these
(c) the force of friction between the crate and the floor of the truck Explanation: the friction force is responsible for both the forward motion of the truck as m*a in the opposite direction and the acceleration forward of the crate.
If you press a book flat against a vertical wall with your hand, in what direction is the friction force exerted by the wall on the book? (a) into the wall (b) out from the wall (c) upward (d) downward
(c) upward Explanation: The tension in the rope has a vertical component that supports part of the total weight of the woman and sled. Thus, the upward normal force exerted by the ground is less than the total weight.
A projectile is launched at some angle to the horizontal with some initial speed vi, and air resistance is negligible. (b) What is its acceleration in the vertical direction? (Let up be the positive direction.)
-9.8m/s^2; gravity would try to pull it downwards from its upward launched position
A projectile is launched at some angle to the horizontal with some initial speed vi, and air resistance is negligible. (c) What is its acceleration in the horizontal direction?
0 m/s^2; Once it is launched, there are no forces acting in the horizontal plane, therefore there is no acceleration in the horizontal direction.
A sled is tied to a tree on a frictionless, snow-covered hill, as shown in figure. 1. Consider the same scenario on a hill with a steeper slope. Would the magnitude of the tension in the rope get larger, smaller, or remain the same as before? (a) The magnitude of the tension force would be greater. (b) The magnitude of the tension force would be smaller. (c) The magnitude of the tension force would remain the same. 2. How would the normal force be affected? (a) The magnitude of the normal force would be greater. (b) The magnitude of the normal force would be smaller. (c) The magnitude of the normal force would remain the same
1.(a) The magnitude of the tension force would be greater. 2. (b) The magnitude of the normal force would be smaller.
As a projectile moves in its parabolic path, the velocity and acceleration vectors are perpendicular to each other 1. everywhere along the projectile's path 2. at the peak of its path 3.nowhere along its path 4. not enough information given at the peak of its path
2. at the peak of its path Explanation: at the peak, velocity is horizontal so vertical component becomes zero and acceleration is always vertically downwards
If the rope doesn't move (in a game of tug-of-war), what horizontal force does each athlete exert against the ground?
290N Explanation: It's the same; if the force against the ground (usually called the force of Friction or Fs) was not there they would not be able to put the tension upon the rope.
In a tug-of-war between two athletes, each pulls on the rope with a force of 290 N. What is the tension in the rope?
290N Explanation: Whenever the rope is stationary, or when it moves with a constant speed, and when we neglect the mass of the rope, the pull on both sides will be the same. This is equal to the tension of the rope.
A ball is held in a person's hand. (b) If the ball is dropped, what force is exerted on it while it is falling? Identify the reaction force in this case. (Neglect air resistance.)
After the ball leaves the hand, the only external force acting on the ball is the gravitational force exerted by Earth. The reaction is an upward gravitational force exerted by the ball on Earth.
Identify the action- reaction pairs in the following situations. (a) a man takes a step
As a man takes a step, the action is the force his foot exerts on the Earth; the reaction is the force of the Earth on his foot.
If gold were sold by weight, would you rather buy it in Denver or in Death Valley?
Denver Explanation: w = mg and g decreases with altitude. Thus to get a good buy, purchase it in Denver. If it were sold by mass, it would not matter where you bought it.
True or False: In general, the magnitude of the acceleration of an object is determined by the magnitudes of the forces acting on it.
False Explanation: Consider how the components of force vector F 1 and vector F 2 must be related to produce an acceleration with a component in the desired direction. Consider whether the condition can be satisfied for a force vector F 2 that has no component pointing directly across the river. Write the expression for the contributions to the net force component in a direction perpendicular to the river and consider how F1, F2 , and the two angles must be related to produce a force component in the desired direction.
(T/F) (e) If an object isn't accelerating, no external force is acting on it.
False; The external forces can add up to zero to hold it in place. There still are external forces.
(T/F) (f) If the net force acting on an object is in the positive x-direction, the object moves only in the positive x-direction.
False; the object could be moving in the (-) x direction and slowing down due to the NET force in the (+) x direction
(T/F) (b) If an object isn't moving, no external forces act on it.
False; to make this statement true, no NET external forces are acting upon it.
A weight lifter stands on a bathroom scale. (b) Suppose she is strong enough to actually throw the barbell upward. How does the reading on the scale vary now?
If the barbell is moving upward, the lifter can declare she has thrown it, just by letting go of it for a moment. At the top of the motion and as the barbell is allowed to move back downward, the scale reading is less than the combined weights.
(c) What force causes a rowboat to move?
In a rowboat, the rower pushes the water backward with the oars (the action). The water pushes forward on the oars and hence the boat (the reaction).
The figure below shows two vectors lying in the xy plane. Determine the signs of the x and y components of vector A , vector B , and vector A + vector B , and place your answers in the following table. (In 4 quadrants, Vector A is headed along the (+) y axis and (-) axis. Vector B is travelling towards the (-) y axis along the (+) axis. ) (b) Vector B
In the x axis: (+) In the y axis: (-)
The figure below shows two vectors lying in the xy plane. Determine the signs of the x and y components of vector A , vector B , and vector A + vector B , and place your answers in the following table. (In 4 quadrants, Vector A is headed along the (+) y axis and (-) axis. Vector B is travelling towards the (-) y axis along the (+) axis. ) (a) Vector A
In the x axis: (-) In the y axis: (+)
The figure below shows two vectors lying in the xy plane. Determine the signs of the x and y components of vector A , vector B , and vector A + vector B , and place your answers in the following table. (In 4 quadrants, Vector A is headed along the (+) y axis and (-) axis. Vector B is travelling towards the (-) y axis along the (+) axis. ) (c) Vector A + Vector B
In the x axis: (-) In the y axis: (-) Explanation: (the resultant vector is in quadrant IV)
Identify the action- reaction pairs in the following situations. (b) a snowball hits a girl in the back
In this case, the action is the force exerted on the girl's back by the snowball; the reaction is the force exerted on the snowball by the girl's back.
Which is true for a car moving around a circular track with constant speed? (Select all that apply.) It has an acceleration with a direction that cannot be determined from the information given. It has an acceleration component in the direction of its velocity. It has an acceleration directed toward the center of its path. It has zero acceleration. It has an acceleration directed away from the center of its path.
It has an acceleration directed toward the center of its path; This is a circular path, which means velocity's direction changes, and the only way to change velocity is to have an acceleration. Since tangent's speed remains constant, the only way is to have an acceleration perpendicular to the tangent, and thus radial.
A crate remains stationary after it has been placed on a ramp inclined at an angle with the horizontal. Which of the following statements must be true about the magnitude of the frictional force that acts on the crate? (a)It is greater than the component of the gravitational force acting down the ramp. (b) It is at least equal to the weight of the crate. (c)It is equal to the component of the gravitational force acting down the ramp. (d) It is larger than the weight of the crate. (e)It is equal to μsn.
It is equal to the component of the gravitational force acting down the ramp. Explanation: The friction force = (coefficient of friction) * (weight of crate) * (cosine of angle) The friction force is always in the opposite direction of motion. So, the friction force is preventing the crate from sliding down the ramp. The gravitational force acting down the ramp = (weight of crate) * (sine of angle) The gravitational force acting down the ramp is causing the crate to slide down the ramp. If the crate remains stationary, the friction force must greater than or equal to the gravitational force acting down the ramp.
If gold were sold by mass, in which of the two locations would you prefer to buy it, Denver or Death Valley?
It makes no difference. Explanation: If it were sold by mass, it would not matter where you bought it.
A baseball is thrown from the outfield toward the catcher. When the ball reaches its highest point, which statement is true? Its velocity is not zero, but its acceleration is zero. Its velocity and its acceleration are both zero. Its velocity is perpendicular to its acceleration. Its acceleration depends on the angle at which the ball was thrown. None of the above statements are true.
Its velocity is perpendicular to its acceleration; the ball has zero vertical velocity and only a horizontal velocity. Acceleration due to gravity is down which is perpendicular to the horizontal velocity.
If only one force acts on an object, can it be in equilibrium?
No; In order for an object to be in equilibrium, the resultant force acting on it must be zero. Thus, it is not possible for an object to be in equilibrium when a single force of non-zero magnitude acts on it.
(b) As a projectile moves in its path, is there any point along the path where the velocity and acceleration vectors are parallel to each other? (Assume the object is thrown upward and forward, but not straight up.)
No; In order for the velocity and acceleration to be parallel, the x component of the velocity would have to drop to zero. However, velocity of x always remains equal to its initial value; therefore, the velocity and the acceleration can never be parallel.
Two projectiles are thrown with the same initial speed, one at an angle θ with respect to the level ground and the other at angle 90° − θ. Both projectiles strike the ground at the same distance from the projection point. Are both projectiles in the air for the same length of time?
No; When two projectiles are thrown with the same initial speed and they have the same range, then the one thrown with the smaller launch angle with respect to the ground has the shorter flight time.
A ball is projected horizontally from the top of a building. One second later, another ball is projected horizontally from the same point with the same velocity. (d) Can the horizontal projection velocity of the second ball be changed so that the balls arrive at the ground at the same time? Yes No
No; since the vertical component of the motion determines the flight time.
A space explorer is moving through space far from any planet or star. He notices a large rock, taken as a specimen from an alien planet, floating around the cabin of the ship. Should he push it gently, or should he kick it toward the storage compartment?
Push gently; if it has a large mass, it will take a large force to alter its motion even when floating in space. Thus, to avoid injuring himself, he should push it gently toward the storage compartment.
A student throws a heavy red ball horizontally from a balcony of a tall building with an initial speed v0. At the same time, a second student drops a lighter blue ball from the same balcony. Neglecting air resistance, which statement is true? The blue ball reaches the ground first. The balls reach the ground at the same instant. The red ball reaches the ground first. Both balls hit the ground with the same speed. None of the above statements are true.
The balls reach the ground at the same instant; according to the laws of gravitation, all objects accelerate in the same rate under gravity. Whether a heavy object or a lightweight one, if there is no air resistance, both fall with the same speeds if their initial speeds were zero. So, the have the same final speeds before reaching the ground. ***Remember that acceleration due to gravity is independent of mass.
A weight lifter stands on a bathroom scale. (a) As she pumps a barbell up and down, what happens to the reading on the scale?
The barbell always exerts a downward force on the lifter equal in magnitude to the upward force that she exerts on the barbell. Since the lifter is in equilibrium, the magnitude of the upward force exerted on her by the scale (that is, the scale reading) equals the sum of her weight and the downward force exerted by the barbell. As the barbell goes through the bottom of the cycle and is being lifted upward, the scale reading exceeds the combined weights of the lifter and the barbell. At the top of the motion and as the barbell is allowed to move back downward, the scale reading is less than the combined weights.
Suppose you are driving a car at a high speed. Why should you avoid slamming on your brakes when you want to stop in the shortest possible distance? (Newer cars have anti lock brakes that avoid this problem.)
The brakes may lock and the car will slide farther than it would if the wheels continued to roll because the coefficient of kinetic friction is less than the coefficient of static friction. Hence, the force of kinetic friction is less than the maximum force of static friction.
If you push on a heavy box that is at rest, you must exert some force to start its motion. Once the box is sliding, why does a smaller force to maintain its motion?
The coefficient of static friction is larger than that of kinetic friction. To start the box moving, you must counterbalance the maximum static friction force. This exceeds the kinetic friction force that you must counterbalance to maintain constant velocity once it starts moving.
(a) What force causes an automobile to move?
The force causing an automobile to move is the force of friction between the tires and the roadway as the automobile attempts to push the roadway backward.
(b) What force causes a propeller-driven airplane to move?
The force driving a propeller airplane forward is the reaction force exerted by the air on the propeller as the rotating propeller pushes the air backward (the action).
In the photo, a locomotive has broken through the wall of a train station. During the collision, what can be said about the force exerted by the locomotive on the wall?
The force exerted by the locomotive on the wall was the same in magnitude as the force exerted by the wall on the locomotive. Explanation: Newton's 1st law
A passenger sitting in the rear of a bus claims that she was injured as the driver slammed on the brakes, causing a suitcase to come flying toward her from the front of the bus. If you were the judge in this case, what disposition would you make? Explain.
The inertia of the suitcase would keep it moving forward as the bus stops. There would be no tendency for the suitcase to be thrown backward toward the passenger. The case should be dismissed.
Analyze the motion of a rock dropped in water in terms of its speed and acceleration as it falls. Assume that a resistive force is acting on the rock that increases as the velocity of the rock increases.
The net force acting on the object decreases as the resistive force increases. Eventually, the resistive force becomes equal to the weight of the object, and the net force goes to zero. In this condition, the object stops accelerating, and the velocity of the rock stops increasing and stays constant. The rock has reached its terminal velocity.
Which has greater value, a newton of gold on Earth or a newton of gold on the Moon?
The newton of gold on the Moon; the newton is a unit of weight, and the quantity (or mass) of gold that weighs 1 newton is m= (1N)/g . Because the value of g is smaller on the Moon than on the Earth, someone possessing 1 newton of gold on the Moon has more gold than does a person having 1 newton of gold on Earth.
A spacecraft drifts through space at a constant velocity. Suddenly a gas leak in the side of the spacecraft causes it to constantly accelerate in a direction perpendicular to the initial velocity. The orientation of the spacecraft does not change, so the acceleration remains perpendicular to the original direction of the velocity. What is the shape of the path followed by the spacecraft?
The spacecraft will follow a parabolic path. This is equivalent to a projectile thrown off a cliff with a horizontal velocity. For the projectile, gravity provides an acceleration that is always perpendicular to the initial velocity, resulting in a parabolic path. For the spacecraft, the initial velocity plays the role of the horizontal velocity of the projectile. The acceleration provided by the leaking gas provides an acceleration that plays the role of gravity for the projectile. If the orientation of the spacecraft were to change in response to the gas leak (which is by far the more likely result), then the acceleration would change direction and the motion could become quite complicated.
Identify the action- reaction pairs in the following situations. (d) a gust of wind strikes a window
This action is the force exerted on the window by the air molecules; the reaction is the force on the air molecules exerted by the window.
Identify the action- reaction pairs in the following situations. (c) a baseball player catches a ball
This action is the force of the glove on the ball; the reaction is the force of the ball on the glove.
(T/F) (c) If a single force acts on an object, the object accelerates.
True; F = ma and therefore a = F / m
(T/F) (d) If an object accelerates, a force is acting on it.
True; a = F / m
(T/F) (a) An object can move even when no force acts on it.
True; an object in motion will remain in motion unless acted upon by an external force.
A ball is held in a person's hand. (a) Identify all the external forces acting on the ball and the reaction to each.
Two external forces act on the ball. One is a downward gravitational force exerted by Earth. The second force on the ball is an upward normal force exerted by the hand. The reactions to these forces are an upward gravitational force exerted by the ball on Earth and a downward force exerted by the ball on the hand.
Under what conditions is the resultant vector equal to zero?
Vector A and B are equal in magnitude and opposite in direction.
If vector B is added to vector A, under what conditions does the resultant vector have a magnitude equal to A + B?
Vector A and B are in the same direction.
Draw a free-body diagram for each of the following scenarios. (c) an athlete running along a horizontal track
Vertically, we've got weight (down) and normal force (up). Horizontally, we have friction (in direction of motion) and drag (against direction of motion); how these compare determine whether or not the runner is accelerating. If there is no acceleration and no drag, then friction is not required to propel him forward.
In the motion picture It Happened One Night (Columbia Pictures, 1934), Clark Gable is standing inside a stationary bus in front of Claudette Colbert, who is seated. The bus suddenly starts moving forward and Clark falls into Claudette's lap. Why did this happen? (No Response)
When the bus starts moving, the mass of Claudette is accelerated by the force of the back of the seat on her body. Clark is standing, however, and the only force on him is the friction between his shoes and the floor of the bus. Thus, when the bus starts moving, his feet start accelerating forward, but the rest of his body experiences almost no accelerating force (except that due to his being attached to his accelerating feet!). As a consequence, his body tends to stay almost at rest, according to Newton's first law, relative to the ground. Relative to Claudette, however, he is moving toward her and falls into her lap.
A ball is projected horizontally from the top of a building. One second later, another ball is projected horizontally from the same point with the same velocity. (b) Will the first ball always be traveling faster than the second? Yes No
Yes; the first ball will always be moving faster, since its flight time is larger, and thus the vertical component of the velocity is larger.
A projectile is launched at some angle to the horizontal with some initial speed vi, and air resistance is negligible. (a) Is the projectile a freely falling body?
Yes; the only force acting in the horizontal direction would be gravity
(a) As a projectile moves in its path, is there any point along the path where the velocity and acceleration vectors are perpendicular to each other? (Assume the object is thrown upward and forward, but not straight up.)
Yes; velocity and acceleration are parallel when an object travels in a straight line, so velocity and acceleration are perpendicular to each other when an object travels in a circle
An Alaskan rescue plane drops a package of emergency rations to stranded hikers, as shown in the figure. The plane is traveling horizontally at 40.0 m/s at a height of 102 m above the ground. Neglecting air friction effects, what path does the package travel as observed by the pilot?
a vertical line downward
Consider the two situations shown in the figure below, in which there is no acceleration. In both cases, the men pull with a force of magnitude F. Is the reading on the scale in figure (i) in which a man pulls a rope connected to a wall greater than, less than, or equal to the reading in figure (ii), two men pulling a rope in equal and opposite directions?
equal to Explanation: In case (i), the scale records the tension in the rope attached to its right end. The section of rope in the man's hands has zero acceleration, and hence, zero net force acting on it. This means that the tension in the rope pulling to the left on this section must equal the force F the man exerts toward the right on it. The scale reading in this case will then be F. In case (ii), the person on the left can be modeled as simply holding the rope tightly while the person on the right pulls. Thus, the person on the left is doing the same thing that the wall does in case (i). The resulting scale reading is the same whether there is a wall or a person holding the left side of the scale.
A ball is thrown upward from the top of a building at an angle of 30.0° to the horizontal and with an initial speed of 20.0 m/s, as in the figure. If the ball is thrown at half the given speed, then it will land:
less than half as far from the building
A ball is projected horizontally from the top of a building. One second later, another ball is projected horizontally from the same point with the same velocity. (c) What will be the time difference between them when the balls hit the ground? one second It depends on the height of the building. no time difference between one and ten seconds
one second, because it was projected one second later
A ball is projected horizontally from the top of a building. One second later, another ball is projected horizontally from the same point with the same velocity. (a) At what point in the motion will the balls be closest to each other? They stay equidistant from each other throughout the motion. the instant the first ball hits the ground the instant the second ball is projected one second after the second ball is projected
the instant the second ball is projected; note that this is an increasing function. The first ball gets farther away from the second ball linearly. So they are closest at t=1. What happens after they land cannot be determined without more information.
The propeller exerts a force on the air, pushing it backwards behind the boat. At the same time, the air exerts a force on the propeller and consequently on the airboat. Forces always come in pairs of this kind, which are formalized in the next section as Newton's third law of motion. The negative answer for the acceleration in part (c) means that the airboat is slowing down. What other forces act on the airboat? (Select all that apply.)
the upward force of the water on the boat the downward force of gravity on the boat
Draw a free-body diagram for each of the following scenarios. (a) a projectile in motion in the presence of air resistance
weight (down), drag (against direction of motion)
Draw a free-body diagram for each of the following scenarios. (b) a rocket leaving the launch pad with its engines operating
weight (down), thrust (up)
The boat in the figure is heading due north as it crosses a wide river with a velocity of 10.0 km/h relative to the water. The river has a uniform velocity of 5.00 km/h due east. Determine the velocity of the boat with respect to an observer on the riverbank. If the speed of the boat relative to the water is increased, what happens to the angle?
θ decreases.
