Phys207:chapter 4 check your understanding questions

during the final stages of a descent, a sky driver 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 following statements is true? (a) the sky driver is not in equilibrium (b) the force of gravity is the only force acting on the sky driver, so that he is 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) 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

a box has a weight of 150 N 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 θ (theta) when pulling force points horizontally, the kinetic frictional force acting on the box is twice as large as when the pulling force points of the angle θ. find θ

43 degrees

two boxes have mass m1 and m2 is greater than m1. the boxes are being pushed across a frictionless horizontal surface. As the drawing shows, there are two possible arrangements and the pushing force is same in each. in which arrangement (a) or (b) does the force that the left box applies to the right box have a greater magnitude or (c) is the magnitude the box same in both cases?

(a)

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) Each experiences an acceleration that has a different magnitude. (d) Each experiences a pushing force that has the same magnitude.

a 10-kg 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? (a) downwards when it reads 75N and upward when it reads 120N (b) upward when it reads 75N and downward when it reads 120N (c) downward in both cases (d) upward in both cases

(a) downwards when it reads 75N and upward when it reads 120N

three particles have identical masses. Each particle experiences only 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 the right triangle

(a) on the corners of an equilateral triangle

a rope is used in 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 on 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) the rope to the tree (b) the rope in tug-a-war (c) neither ropes sustain the same tension

(a) the rope to the tree

in which one 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) none of the situations described in (a,b,c) could the 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

you are standing on a scale in an elevator that is moving upward with a constant velocity. the scale reads 600N. the following table shows 5 options for what the scale reads when the elevator slows down as it comes to a stop, when it is stopped, and when it picks up speed on its way back down. Which one of the five options correctly describes the scale reading? (< = less than & > = greater than)

(b) <600 N, 600N, <600N

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.

(b) both A and B

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 its weight

(b) its weight

a stone is thrown from the top of a cliff. air resistance is negligible. as the stone falls, is it (a) in equilibrium (b) not in equilibrium

(b) not in equilibrium

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 on 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) Both the net force acting on the object and the mass of the object double.

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) The object can be either stationary or traveling with a constant velocity.

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 ________ it is when she hangs stationary? (a) less than, (b) equal to, (c) greater than

(c) greater than

suppose that the coefficients of static and kinetic friction have values such that μs= 1.4 μk for crate in contact with a cement floor. which 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 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) the magnitude of the maximum static frictional force is 1.4 times the magnitude of the kinetic frictional force

two objects with masses m1 and m2 are separated by a distance 2d. Mass m2 is greater than mass m1. A third object has a mass of m3. All three objects are located on the same straight line. the net gravitational force acting on the third object is zero. which one of the drawings correctly represents the locations of the objects?

(d)

In Case A an object is moving straight downward with a constant speed of 9.80 m/s, while in Case B an object is moving straight downward with a constant acceleration of magnitude 9.80 m/s2. Which one of the following is true? (a) A nonzero net external force acts on 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) A nonzero net external force acts on the object in Case B only.

a stack of books whose true weight is 165 N is placed on a scale in an elevator. the scale reads 165 N. 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 at rest?

no

a freight train is accelerating on a level track. other things being equal, would the tension in the coupling between the engine and the car 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

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.

a crate hangs from a ring at the middle of a rope as shown in the picture. a person is pulling on the right end of the rope to keep the crate in equilibrium. can rope ever be made to be perfectly horizontal?

no, because there must be a vertical (upward) component of the tension force in the rope to balance the weight of the crate

A person has a choice of either pushing or pulling a sled at a constant velocity, as the drawing illustrates. Friction is present. If the angle θ is the same in both cases, does it require less force to push or to pull the sled?

to pull,

Object A weighs twice as much as object B at the same spot on the earth. Would the same be true at a given spot on Mars

yes because the ratio of the two weights depends only on the masses of the objects, which are the same on the earth and on mars