Newton's Laws
Consider the video tutorial you just watched. Suppose that we duplicate this experimental setup in an elevator. What will the spring scale read if the elevator is moving upward at constant speed?
18 N because since the elevator is not accelerating, the reading on the scale is the same as in the video.
Which of Newton's Laws explains why satellites need very little fuel to stay in orbit?
1st Law; Once a satellite is in orbit, the only net force acting on it is the inward force of gravity. The tangential speed of the satellite is just right so that gravity only has the effect of changing the direction of travel. It is a near vacuum in space so the satellite encounters almost no friction to slow them down.
Consider the video tutorial you just watched. Suppose that we repeat the experiment, but this time we replace the original 56-g magnet with a more powerful magnet of the same mass. As you know from experience, the more powerful a magnet is, the more strongly it attracts or repels other magnets or magnetic objects. You have also probably noticed that magnetic forces fall off sharply with distance—two magnets that interact strongly across a distance of millimeters interact more weakly at a distance of a centimeter. (In fact, the strength of the force falls off with the square of the distance.) With the magnet hovering above the base, what will the scale read? The scale has been zeroed (tared) to subtract the weight of the base.
56 g because In equilibrium, the base must push up on the magnet with a force equal to the weight of the magnet, and the magnet will exert an equal-but-opposite force on the base. The reading on the scale will not change.
A heavy crate is attached to the wall by a light rope, as shown in the figure. Another rope hangs off the opposite edge of the box. If you slowly increase the force on the free rope by pulling on it in a horizontal direction, which rope will break? Ignore friction and the mass of the ropes.
Both ropes are equally likely to break.Since the attached rope doesn't have to support any weight (as it did in the vertical case), the tension is the same in both ropes.
A force pair is produced when a tennis racket strikes a tennis ball. Which of the following best explains why the tennis ball does not have zero net force acting on it?
Each half of the force pair acts on a different object. The forces in the force pair are equal in size, act in opposite directions, and act on different objects. One half of the force pair acts on the tennis ball and the other half acts on the racket and both objects individually experience a non-zero net force
An object's inertia causes it to come to a rest position.
False; Inertia is the tendency of an object to resist change in velocity. It is not a force that causes an object to accelerate or decelerate. A moving object with a lot of inertia (measured by its mass) would actually require more net force to change its velocity in a given amount of time than an object with a low inertia.
An object that is not accelerating or decelerating has no forces acting on it.
False; When you push anything, you are applying a force. Despite this, you still might not be able to change its acceleration. That is because there might be another force that nets against yours. For example, you push on a beached whale. The reason why you can't accelerate it is because the force of friction between the whale and the sand perfectly nets against (same magnitude, opposite direction) the force you are applying. For another example, you're presumably not plummeting to the center of the Earth right now despite the fact that Earth's gravity is pulling on you. This is because your chair is exerting a normal force in the opposite direction (radially outwards from the center of Earth) that perfectly offsets the force of gravity. This statement would have been true if it said "net forces", not just "forces".
The mass of a rocket decreases as it burns through its fuel. If the rocket engine produces constant force (thrust), how does the acceleration of the rocket change over time?
It increases because the constant-force rocket's acceleration is inversely related to its mass. Because its mass decreases over time, its acceleration increases.
Consider the video demonstration that you just watched. Which of the following changes would make it more likely for the ball to hit both the white can and the green can?
None of the above because By Newton's first law, after it has left the circular track, the ball will travel in a straight line until it is subject to a nonzero net force. Thus, the ball can only hit the white can, because that is the only can in the ball's straight-line path.
A ball is moving upwards and to the left. The net forces acting on it are also upwards and to the left.
Not enough information; If we are on Earth, then the net forces are downward due to Earth's gravity (and possibly down and to the right due to air friction). If the ball is traveling through space, it could maintain that constant velocity without any net forces. Remember, the net forces change velocity. The existence of movement alone doesn't tell you anything about the net forces acting on it (unless you knew the magnitude and direction of acceleration) so, for this case, there really isn't enough information.
In general, how does the coefficient of static friction compare to the coefficient of kinetic friction for the same two materials?
The coefficient of static friction is greater than the coefficient of kinetic friction.
In the video, forces acting on the car that are parallel to the direction of motion are analyzed. How are these forces related?
The forces are equal in size, act in opposite directions, and produce a zero net force. This is because the car is subject to two forces acting along the direction of motion. The propelling force acts in the direction of motion, while the force of drag acts opposite the direction of motion. Because the two forces are equal in size, they cancel, producing a zero net force. As a result, the car travels at constant speed.
A packing crate is sitting at rest on an inclined loading ramp. How does the magnitude of the force of static friction compare to the other forces acting on the crate?
The magnitude of the force of static friction is equal to the magnitude of the component of the weight of the crate parallel to the inclined ramp.
An object is hanging by a string from the ceiling of an elevator. The elevator is moving upward with a constant speed. What is the magnitude of the tension in the string?
The magnitude of the tension in the string is equal to the magnitude of the weight of the object.
An object is hanging by a string from the ceiling of an elevator. The elevator is slowing down while moving upward. What is the magnitude of the tension in the string?
The magnitude of the tension in the string is less than the magnitude of the weight of the object.
An object moves in a circular path at a constant speed. What is the direction of the net force acting on the object?
The net force is directed toward the center of the circular path.
An experiment measures the acceleration of an object of known mass subject to a known net force. The experiment is then repeated while varying the mass and the net force. The table below summarizes the conditions used in each trial.
Trial Mass Net force 1 x2 x2 2 x(1/2) not changed 3 x2 not changed 4 x(1/2) x2 Largest acceleration to smallest acceleration 4 2 1 3 Because The accelerations from largest to smallest are as follows: Trial 4 (quadrupled), Trial 2 (doubled), Trial 1 (unchanged) and Trial 3, (halved).
Objects in orbit around the Earth (like a satellite) still have net forces acting on them.
True; An object with no net forces acting on it would not have a change in velocity. If it is stationary, it would stay stationary. If it is in motion, it will stay in motion with a fixed velocity (moving in a straight line). This comes directly out of Newton's First Law of Motion. An object in orbit may have a constant speed, but its direction is constantly changing as it moves in a circle (or ellipse) and, thus, its velocity is also changing (remember, velocity takes into consideration speed and direction). Therefore, there must be a net force acting on it. This is the net force of Earth's gravity acting on the object. For the most part, we can assume a vacuum in "space" near Earth, but it is not actually an absolute vacuum (there is just a very low density of molecules). It isn't the intent of this question, but bumping into those very sparse molecules does apply a very small net force on a satellite as well which would, over time, slow it down.
When an object is stationary, all of the forces acting on it are balanced.
True; When you push anything, you are applying a force. Despite this, you still might not be able to change its acceleration. That is because there might be another force that balances against yours (netting to 0 net force). For example, you push on a beached whale. The reason why you can't accelerate it is because the force of friction between the whale and the sand perfectly nets against (same magnitude, opposite direction) the force you are applying. For another example, you're presumably not plummeting to the center of the Earth right now despite the fact that Earth's gravity is pulling on you. This is because your chair is exerting a normal force in the opposite direction (radially outwards from the center of Earth) that perfectly offsets the force of gravity. If the forces were not balanced, then the stationary object would accelerate.
An object that is not accelerating or decelerating has no net forces acting on it.
True; If all the forces net out, then the object's velocity will not change. This is just a restatement of Newton's First Law of Motion.
The only way to slow down a moving object is to apply a net force to it.
True; Velocity will only change if a net force is applied. In this case, you'll slow the speed of the object if you apply a net force in the direction opposite its direction of motion.
A stationary object has no forces acting on it.
True; When you push on a stationary object, you are applying a force. Despite this, you still might not be able to accelerate it. That is because there might be another force that nets against yours. For example, you push on a beached whale. The reason why you can't accelerate it is because the force of friction between the whale and the sand perfectly nets against (same magnitude, opposite direction) the force you are applying. For another example, you're presumably not plummeting to the center of the Earth right now despite the fact that Earth's gravity is pulling on you. This is because your chair is exerting a normal force in the opposite direction (radially outwards from the center of Earth) that perfectly offsets the force of gravity. This statement would have been true if it said "net forces", not just "forces".
Newton's second law relates an object's acceleration to its mass and the net force acting on it. Does Newton's second law apply to a situation in which there is no net force? Select the best explanation.
Yes. The law applies and it tells us that the object has constant velocity. Newton's second law applies to all situations, whether there is a net force or not. In the case of zero net force, any finite mass object must have zero acceleration, which means that its velocity is constant.
An elevator moves straight upward at a constant speed. Newton's first law predicts that the net force acting on the elevator will _____.
be zero because an object moving in a straight line at constant speed has zero acceleration because the net force acting on it is also zero.
Recall the portion of the video in which the girl pushes her brother on the sled at constant velocity. The pushing force she exerts on the sled is _____ the frictional force the ground exerts on the sled.
equal to because the sled's velocity is constant, the net force must be zero and the two horizontal forces must balance out.
Compared to the magnet in the video, the magnet in Part A will hover at a position
farther above the base. In equilibrium, the magnetic force exerted by the base on the magnet must equal the magnet's weight (or else the magnet would accelerate). Because this magnet is stronger than the one in the video (and because the magnetic forces weaken with distance), this magnet hovers farther from the base
The video identifies the force pair produced when an apple falls through the air. Which force belongs in a free-body diagram of the apple
force of Earth on apple because The video describes a gravitational force pair: the force of the apple on Earth and the force of Earth on the apple. The force of Earth on the apple appears in the apple's free-body diagram.
According to Newton's first law, when the net force acting on an object is zero, the object must _____.
have constant velocity because when the net force is zero, the acceleration is zero, so the velocity must be constant. Note that zero velocity is simply a special case of constant velocity.
What explains the dramatically different magnitudes of accelerations that result when a mosquito collides head on with a moving truck?
unequal masses of the bus and the mosquito because In accordance with Newton's second law, the dramatically different accelerations result when the dramatically different masses are subjected to equal magnitude forces.
Imagine holding a basketball in both hands, throwing it straight up as high as you can, and then catching it when it falls. At which points in time does a zero net force act on the ball? Ignore air resistance.
when you hold the ball still in your hands before it is thrown; when you hold the ball still in your hands after catching it because When an object's velocity is changing, the net force on it is not zero, even if it stops for an instant.
