chapter 6: work and energy Flashcards
If you push twice as hard against a stationary brick wall, the amount of work you do _______
remains zero
Is it possible for a system to have a negative value of potential energy?
Yes, since the value of potential energy at any given point is arbitrarily defined.
The total mechanical energy of a system is _______
the sum of the total kinetic energy and total potential energy
In a baseball game, the catcher stops a 90-mph pitch. What can you say
about the work done by the catcher on the ball?
the catcher has done negative work
A 2.0 kg rock falls 5.0 m. What is the work done by gravity
98 J
A 2.0 kg rock slides down a 5.0 m slope making an angle of 25°
with the horizontal. (a) What is the work done by gravity?
(b) What is the work done by the normal force?
Only the component of force that is parallel to
the displacement of the object does work.
true
I push a 20.0 kg crate up an incline, as shown in the figure. What is the work done on the crate by the normal force?
0 J
Work done by the normal force is
zero since it is perpendicular to the
displacement of the crate is always
perpendicular to the normal force.
A puck attached to a string follows a circular path at constant
speed on a horizontal frictionless surface. The work done on the
puck as it travels halfway around the circle is ________
zero
You throw a 2.0 kg rock straight up at the edge of a 12-m-tall cliff with an initial speed of 10 m/s. It rises and falls, eventually landing at the base of the cliff. (a) What is the total work done by gravity on the rock? (b) At any point in the rock’s flight is the work done by gravity equal to 0 J? (c) Where in the rock’s flight is the work done by gravity the most negative, and what
is its value at this point?
You throw a 2.0 kg rock straight up at the edge of a 12-m-tall cliff with an initial speed of 10 m/s. Now, answer the following questions about the previous example question: (a) At what point is the rock’s speed equal to its starting speed? (b) Where is its speed the greatest? (c) Where is its speed the smallest? (d) Is there any connection between the work done on the rock and its change in
speed?
Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one?
twice as much
At room temperature, an oxygen molecule, with mass of 5.31 × 10^−26 kg, typically has a kinetic energy of about 6.21 × 10^−21 J. How fast is it moving?
How much work must be done to stop a 1300-kg car traveling at 95 km/h?
A car starts from rest and accelerates to 30 mph. Later, it gets on a
highway and accelerates to 60 mph. Which acceleration requires more work, the 0 → 30 mph, or the 30 → 60 mph?
30 to 60 mph
The bigger energy change occurs in the second case, so more work was required
A 1.60-m tall person lifts a 1.95-kg book off the ground so it is 2.20 m above the ground. What is the potential energy of the book relative to (a) the ground, and (b) the top of the person’s head?
(c) How is the work done by the person related to the answers in parts (a) and (b)?
If it requires 5.0 J of work to stretch a particular spring by 2.0 cm from its equilibrium length, how much more work will be required to stretch it an additional 4.0 cm?
Two identical stones, A and B, are shot from a cliff from the same height and with identical initial speeds. Stone A is shot vertically up, and stone B is shot vertically down (see diagram).
Which one of the following
statements best describes which stone has a larger speed right before it hits the ground?
The stones have the same speed
A sled is initially given a shove up a frictionless 23.0° incline. It
reaches a maximum vertical height of 1.12 m higher than where it
started. What was its initial speed?
A 16.0-kg child descends a slide 2.20 m high and reaches the bottom with a
speed of 1.25 m/s. How much thermal energy due to friction was generated in
this process?
