12.3.A.1Multiple Choice
A book rests on a table, pressed down by gravity and held up by the normal force. Which pair of forces is the genuine Newton's third-law pair in this situation?
Select A, B, C, or D to check your answer.
Directions: Work each problem on your own paper first. For multiple choice, select A, B, C, or D. You will see right away whether you are correct. If you miss, you will see the most likely reason, and you can try again. For short response, type your answer, then compare it with the model solution. Questions mix the essential knowledge threads of Topic 2.3 — identifying genuine Newton's third-law pairs (always on two different objects), why internal forces never move a system's center of mass, tension as the macroscopic result of segments pulling on each other, the ideal-string shortcuts (negligible mass, no stretching, uniform tension), how tension varies along a nonideal (heavy) string, and the ideal-pulley assumptions that let tension pass through unchanged.
A book rests on a table, pressed down by gravity and held up by the normal force. Which pair of forces is the genuine Newton's third-law pair in this situation?
Select A, B, C, or D to check your answer.
A person pushes against a wall but the wall doesn't move. Using Newton's third law, explain what force the wall exerts on the person, and explain why the wall doesn't noticeably move even though it's exerting just as much force as the person.
A rocket in deep space expels exhaust gas backward to accelerate forward. Which statement correctly describes the Newton's third-law pair at work?
Select A, B, C, or D to check your answer.
Two carts connected by a spring are released from rest on a frictionless track. Using Newton's third law, explain why the spring's force on cart 1 and the spring's force on cart 2 do not change the overall motion of the two-cart system's center of mass.
Why can't a person lift themselves off the ground by pulling upward on their own hair?
Select A, B, C, or D to check your answer.
A firework at rest in the air explodes into two fragments that fly off in different directions. Using Newton's third law and the idea of internal forces, explain what happens to the system's center of mass immediately after the explosion (ignoring air resistance and assuming it was in free fall before exploding).
A system consists of two blocks connected by a spring, both initially at rest, with no friction and no net external horizontal force. According to EK 2.3.A.2, what happens to the center of mass of this two-block system as the spring pushes the blocks apart?
Select A, B, C, or D to check your answer.
Explain, in your own words, what tension "really is" at the microscopic level, in terms of the forces internal to a string.
Why can a rope pull an object but not push it (for example, you can pull a cart with a rope, but you cannot push a cart by pushing on a slack rope)?
Select A, B, C, or D to check your answer.
A mass hangs at rest from a string attached to the ceiling. Explain, in terms of "infinitesimal segments pulling on their neighbors," why the string must be under tension throughout its entire length, from the mass up to the ceiling.
Two blocks are connected by an ideal string and pulled across a frictionless floor by a force applied to the front block. What can you conclude about the tension at the front end of the string versus the tension at the back end?
Select A, B, C, or D to check your answer.
Prove, using Newton's second law applied to a tiny massless segment of string, why the tension in an ideal string must be the same at every point along its length.
An ideal string does not stretch when placed under tension. Why is this assumption useful when solving a problem involving two connected blocks?
Select A, B, C, or D to check your answer.
Explain why both "negligible mass" and "does not stretch" are necessary parts of the definition of an ideal string — that is, explain what could go wrong in a problem if only one of those two properties were true.
A heavy rope with significant mass hangs vertically from the ceiling, with nothing attached to its bottom end. Compare the tension at the very bottom of the rope to the tension at the very top (where it attaches to the ceiling).
Select A, B, C, or D to check your answer.
A heavy chain of mass M and length L hangs from a hook, supporting no additional load. Find the tension in the chain at a point located a distance x below the hook (measured along the chain, with x = 0 at the hook and x = L at the free bottom end), in terms of M, L, x, and g.
Why does accelerating a rope with significant mass (rather than just letting it hang at rest) make its tension vary along its length even more than when it's at rest?
Select A, B, C, or D to check your answer.
State the two defining properties of an ideal pulley, and explain what role each one plays in keeping the tension on both sides of the pulley equal.
A string passes over a pulley that has significant mass and noticeable friction at its axle. How does the tension on the two sides of this pulley compare, and why?
Select A, B, C, or D to check your answer.
A string passes over two separate ideal pulleys in sequence before connecting a hanging mass to a block on a table. Explain why the tension throughout the entire string is still the same single value, even though it passes over two pulleys.