Ch. 4 Check Your Understanding Flashcards
Which of the following statements can be explained by Newton’s first law?
A) when your car suddenly comes to a halt, you lunge forward.
B) when your car rapidly accelerates, you are pressed backward against the seat
C) neither
D) both
B
The net external force acting on an object is zero. Which one of the following statements is true?
A) the object can only be stationary
B) the object can only be traveling with a constant velocity
C) the object can be either stationary or traveling with a constant velocity
D) the object can only be traveling with a velocity that is changing
C
In case A an object is moving straight downward with a constant speed of 9.80m/s, while in case B an object is moving straight downward with a constant acceleration of magnitude m/s^2. Which of the following is true?
A) a nonzero net external force acts in the object in both cases
B) a nonzero net external force acts on the object in neither case
C) a nonzero net external force acts on the object in case A only.
D) a nonzero net external force acts on the object in case B only.
D
Newton’s second law indicates that when a net force acts on an object, it must accelerate. Does this mean that when two or more forces are applied to an object simultaneously, it must accelerate?
No, because two or more forces can cancel each other, leading to a net force of zero
All of the following, except one, cause the acceleration of an object to double. Which one is the exception?
A) all forces acting on the object double
B) the net force acting in the object doubles
C) both the net force acting on the object and the mass of the object double
D) the net force acting on the object remains the same, while the mass of the object is reduced by a factor of two
C
A father and his seven-year-old daughter are facing each other on ice skates. With their hands, they push off against one another. Which one or more of the following statements is/are true?
A) each experiences an acceleration that has a different magnitude
B) each experiences an acceleration of the same magnitude
C) each experiences a pushing force that has a different magnitude
D) each experiences a pushing force that has the same magnitude
A and D
When a body is moved from sea level to the top of a mountain, what changes?
A) the body’s mass
B) its weight
C) both its mass and weight
B
Object A weighs twice as much as object B at the same spot on earth. Would the same be true at a given spot on mars?
Yes, because the ratio of the two weights depends only on the mass of the objects, which are the same on the earth and on mars
Three particles have identical masses. Each particle experiences only the gravitational forces due to the other two particles. How should the particles be arranged so each one experiences a net gravitational force that has the same magnitude?
A) on the corners of an equilateral triangle
B) on three of the four corners of a square
C) on the corners of a right triangle
A
A stack of books whose true weight is 165N is placed on a scale in an elevator. The scale reads 165N. From this information alone, can you tell whether the elevator is moving with a constant velocity of 2m/s upward, is moving with a constant velocity of 2m/s downward, or is it at rest?
No, you can’t tell
A 10kg suitcase is placed on a scale that is in an elevator. In which direction is the elevator accelerating when the scale reads 75N and when it reads 120N?
Downward when it reads 75N upward and upward when it reads 120N
Suppose that the coefficients of static and kinetic friction have values such that us = 1.4 uk for a crate in contact with a cement floor. Which one of the following statements is true?
A) the magnitude of the static frictional force is always 1.4 times the magnitude of the kinetic frictional force
B) the magnitude of the kinetic frictional force is always 1.4 times the magnitude of the static frictional force
C) the magnitude of the maximum static frictional force is 1.4 times the magnitude of the kinetic frictional force
C
A person has a choice of either pushing or pulling a sled at a constant velocity. Friction is present. If the angle is the same in both cases, does it require less force to push or pull the sled?
To pull, because the upward component of the pulling force reduced the normal force and, therefore, also reduces the force of kinetic friction acting on the sled
A box has a weight of 150N and is being pulled across a horizontal floor by a force that has a magnitude of 110N. The pulling force can point horizontally, or it can point above the horizontal at an angle. When the pulling force points, horizontally, the kinetic frictional force acting on the box is twice as large as when the pulling force points at the angle. Find the angle.
43 degrees
A box rests on the floor of an elevator. Because of static friction, a force is required to start the box sliding across the floor when the elevator A) is stationary, B) accelerating upward, and C) accelerating downward. Rank the forces required in these three situations in ascending order- that is, smallest first.
C, A, B
A rope is used in a tug-of-war between two teams of five people each. Both teams are equally strong, so neither team wins. An identical rope is tied to a tree, and the same ten people pull just as hard in the loose end as they did in the contest. In both cases, the people pull steadily with no jerking. Which rope sustains the greater tension?
A) rope tied to the tree
B) rope in the tug-of-war
C) they sustain the same tension
A
In which of the following situations could an object possibly be in equilibrium?
A) three forces act on the object; the forces all point along the same line but may have different directions
B) two perpendicular forces act on the object
C) a single force acts on the object
D) in none of the situations described could the object possibly be in equilibrium
A
A stone is thrown from the top of a cliff. Air resistance is negligible. As the stone falls, is it a) in equilibrium or b) not in equilibrium?
B
During the final stages of descent, a sky diver with an open parachute approaches the ground with a constant velocity. There is no wind to blow him from side to side. Which one of the following statements is true?
A) the sky diver is not in equilibrium
B) the force of gravity of the only force acting on the sky diver, so he is not in equilibrium
C) the sky diver is in equilibrium because no forces are acting on him
D) the sky diver is in equilibrium because two forces act on him, the downward-acting force of gravity and the upward acting force of the parachute
D
A crate hangs from a ring at the middle of a rope, as the drawing illustrates. A person is pulling on the right end of the rope to keep the crate in equilibrium. Can the rope ever be made to be perfectly horizontal?
No, because there must always be a vertical component of the tension force in the rope to balance the weight of the crate
A circus performer hangs stationary from a rope. She then begins to climb upward by pulling herself up, hand over hand. When she starts climbing, is the tension in the rope a) less than, b) equal to, or c) greater than it is when she hangs stationary?
C
A freight train is accelerating on a level track. Other things being equal, would the tension in the coupling between the engine and the first car change if some of the cargo in the last car were transferred to any one of the other cars?
No, because the transfer described does not change the total mass being pulled by the engine
