Exam 1: Conservation Laws Constrain Interactions

The emergency blankets in some first-aid kits consist of very thin Mylar that has been coated with aluminum to make it shiny. How can such thin plastic be a blanket? - Which mechanism of a person's heat loss is most effectively suppressed by the blanket?

A deep space probe moving forward in space needs to make a course correction and so fires a thruster on the side of the probe $90^{\circ}$ away from its direction of motion, as shown below. This thruster, while on, exerts a constant force on the probe perpendicular to its original direction of motion. While the thruster is operating, which trajectory best describes the probe's motion? (Hint: Note that the thrust force delivers an upward impulse to the probe during every successive time interval. Consider what happens to the ball in figure C2.4 as it receives analogous continual downward impulses.)

Which of the following statements involve notation, unit, or conceptual errors? For each statement, answer $A$ if the statement is acceptable, or $E$ if it is erroneous. - If $\vec{v}=5 \mathrm{~m} / \mathrm{s}$ and $m=2.0 \mathrm{~kg}$ , then $\vec{p}=10 \mathrm{~kg} \cdot \mathrm{m} / \mathrm{s}$ .

In the hypothetical atomic interaction shown in figure C9.13, suppose the atoms initially are separated by a very large distance but are approaching each other with kinetic energy $\frac{1}{2} \mu|\vec{v}|^{2}=5 \times 10^{-21} \mathrm{~J}$ . Roughly how close will the atoms get to each other eventually?

A reference frame attached to a train It is a good inertial frame if it is moving at a fixed speed in a straight line

Suppose a star has twice the surface temperature of the sun and also twice the radius. How many times larger is the rate at which this star emits energy compared to the rate at which the sun emits energy?

A reference frame attached to a train It is a good inertial frame if it is moving at a fixed speed around a curve

An astronaut floating in the International Space Station catches a Frisbee thrown directly toward his or her center of mass. Does the astronaut + Frisbee's change in thermal energy $\Delta U^{\text {th }}$ depend on whether the Frisbee is spinning or not, and if so, which case makes it larger?

The $x, y$ , and $z$ directions of a reference frame point up, west, and north, respectively. Such a coordinate system is right-handed.

An object moving with speed $\left|\vec{v}_{0}\right|$ collides head-on with an object at rest that is very much more massive. If the collision is elastic, how does the lighter object's speed $|\vec{v}|$ after collision compare to $|\vec{v}|$ ?

A human being's weight could be $150 \mathrm{~kg}$

A solid ball and a hollow ball are released from rest at the top of an incline and roll without slipping to the bottom. Which reaches the bottom first?

You and a classmate are discussing how much work one must do to lift a box of mass $m$ a vertical distance $h$ . Your classmate argues, "You have to exert an upward force on the box at least equal in magnitude to the box's weight $m|\vec{g}|$ , and you lift the box upward a distance $h$ , so you must do work equal to $m|\vec{g}|_{t}$ . But in addition," your classmate continues, "you must increase the box's gravitational potential energy by $m|\vec{g}| h$ . Therefore, you must actually do work equal to or greater than $2 m|\vec{g}| h$ to lift the box." What amount of work do you think you need to do on the box to lift it a height $h$ ? Be prepared to defend your response. (Hint: What is the system here?)

A deep space probe moving forward in space needs to make a course correction and so fires a thruster on the side of the probe $90^{\circ}$ away from its direction of motion, as shown below. After the thruster is turned off, which trajectory best describes the probe's subsequent motion? Possible final trajectories (thruster off):

When I was a teenager, I saw a Candid Camera sequence in which they filmed a person struggling with a suitcase that (unknown to that person) contained a large and rapidly spinning gyroscope. Imagine you are carrying such a suitcase where the gyroscope's angular velocity is oriented horizontally to your right as you carry it in your right hand. Imagine you try to make a left turn, thus turning the suitcase to the left around a vertical axis. What does the suitcase do?

Various objects are released from rest at the same time and vertical position and roll without slipping along an incline. -Which will reach the bottom last?

Consider a rock interacting gravitationally with the earth, and choose the interaction's potential energy to be zero if the rock is at the ground level. A person standing at the bottom of a well throws the rock vertically upward from $20 \mathrm{~m}$ below ground level. The rock makes it all the way up to $1 \mathrm{~m}$ below ground level before falling back into the well. The total energy of the rock-earth system is

A basketball player lands on the ground after dunking the ball. Is the work done on the player by the contact interaction between the player and ground positive, negative, or zero?

Suppose you are standing due east of a car traveling southwest. What is the direction of the car's angular momentum around you? Choose from the possible directions below (as seen by a person hovering over you), where A is north, C is east, and so on. If you are using the letters on the book's back cover, you can indicate a double letter by pointing to the letter with two fingers.

The $x$ and $y$ axes of a reference frame point forward and to your right, respectively. The frame is righthanded if the $z$ axis points up