Exam 1: Conservation Laws Constrain Interactions

The electric field inside a conductor must be zero in all possible circumstances.

In the hypothetical atomic interaction shown in figure C9.13, if the system had the appropriate total energy and initial position, could the atoms' separation remain stable and constant?

An isolated star collapses so that its radius is half its original radius. Both its angular momentum and its rotational energy must be conserved

Objects $A$ and $B$ are made of the same substance, but object $A$ is twice as massive as object $B$ . Object $A$ originally has a temperature of $100^{\circ} \mathrm{C}$ and $B$ has a temperature of $0^{\circ} \mathrm{C}$ . Suppose these objects are put inside a well-insulated cup and are allowed to come to a common temperature $T_{f}$ . What is that temperature? Select the closest answer.

Kepler's 3rd law applies to hyperbolic orbits.

Consider the displacement vectors shown below. -Which is not equal to $\vec{A}$ but has the same magnitude?

A person who exerts a 100-N force horizontally in the $+x$ direction on a wall for 10 minutes does positive work on the wall.

The following formulas are supposed to describe the speed $V$ of a sphere sinking in a thick fluid. $C$ is a unitless constant, $\rho$ is the fluid's density in $\mathrm{kg} / \mathrm{m}^{3}, A$ is the sphere's cross-sectional area, $m$ is its mass, and $g$ is the gravitational field strength in $\mathrm{N} / \mathrm{kg}$ . Which could be right?

Two hockey pucks are initially at rest on a horizontal plane of frictionless ice. Puck $A$ has twice the mass of puck $B$ . Suppose we exert a constant force on puck $A$ until it has travelled $2 \mathrm{~m}$ from its starting position. We exert a force of the same magnitude on puck $B$ until it has travelled $1 \mathrm{~m}$ from its starting position. How do the pucks' kinetic energies compare when we stop pushing them?

According to the definition of "science" given in this chapter, which of the following do you think are sciences? Choose the letter of the first discipline on the list that you think is not a science. (The answer is open to debate!)

Consider a collision where an object with a known mass and known speed collides with another object at rest. The objects are free to move in three dimensions. -c) If the collision is completely inelastic, how many quantities suffice to determine the system's final state?

An object is moving at a constant velocity. This means that an interaction must be delivering momentum to the object at a constant nonzero rate.

In a reference frame floating in space, which do you think follows the straighter path?

A person squeezes a balloon horizontally with both hands while holding it at rest, decreasing its width by a certain amount. Is the work done on the balloon by the person positive, negative, or zero?

When is friction a potential problem for applying conservation of momentum to a system?

An object of mass $m$ traveling at speed $\vec{v}_{0}$ in the $+x$ direction hits an identical object at rest. Characterize each of the collision outcomes shown below as being (A) absurd or (C) credible. (The collisions are not necessarily elastic, but they are not super-elastic.) -

Suppose we take a disk of radius $R$ and remove the material from $r=\frac{1}{2} R$ outward. Note that this decreases both the disk's mass and its radius. Will the value of $\alpha$ for the new object be larger, smaller, or the same as that for the original object?

In a situation where an object can move only along the $x$ axis, you see this statement in a book: "The object's velocity is $\mathrm{v}=\left(x-x_{0}\right) / t$ , where $x$ is the object's position at time $t$ to the left or right of its position $x_{0}$ at time $t=0$ ." Is $v$ being used here as a shorthand for $|\vec{v}|(\mathrm{A})$ or $v_{x}(\mathrm{C})$ ? $\bullet$ A $\bullet$ C

(Challenging!) Experienced players can throw a Frisbee so it skips off the ground (the Frisbee's front edge actually tips upward after the Frisbee hits, enabling it to climb back into the air). If a right-handed player throws a Frisbee in the usual way, it rotates counterclockwise when viewed from above. To skip the Frisbee, which edge has to hit the ground (as seen by the thrower)? (Hint: The Frisbee's angular momentum vector, and thus its rotation axis, changes in the direction of the external torque that the strike applies to the Frisbee.)

The components of a displacement vector $\Delta \vec{r}$ between two position vectors $\overrightarrow{r_{1}}$ and $\overrightarrow{r_{2}}$ depend on one's choice of reference frame origin.