PHYS-121 Ch 6 Questions with complete
solution 2025
definition of work, and is it a vector or scalar? - correct answer ✔an
expression of energy used to move an object
its a scalar
in order for work to be done, what has to happen? - correct answer ✔there
has to be a displacement, simply holding an object in place does not require
work.
description of work - correct answer ✔work describes what is accomplished
when a force acts on an object and the object moves a certain distance.
work done by a constant force: definition - correct answer ✔the displacement
moved multiplied by the component of the force in the direction of the
displacement
work done by a constant force: equation and important notes - correct answer
✔FΔXcos(θ) or Fdcos(θ)
F and d are magnitudes only
cosine selects the parallel force component
theta is the angle between the force and the displacement
Example: Holding + walking with a bag of groceries - correct answer
✔Holding: when you hold the bag in place, no work is done. this is because
the displacement is zero, even though you exert a force on the bag ---> you
need both a force AND a displacement to do work
,Walking: you can also do no work if you carry the bag while walking at a
constant velocity. No horizontal force is required to move the bag horizontally
at constant velocity. However... when you start and stop walking there is a
horizontal acceleration and therefore a horizontal force, so work is done.
if we are exerting a force on the bag, then why aren't we doing any work? -
correct answer ✔the person exerts an upward force on the bag because he's
holding it above ground and gravity acts on it downwards. BUT... the upward
force is perpendicular to the displacement.
when a force is perpendicular to the displacement, what does this mean? -
correct answer ✔that force does no work! the angle between the force and
displacement is 90 degrees and cos(90) is 0, so work is 0.
discuss specificity - correct answer ✔1. its important to specify whether you
are talking about work done BY a specific force, our work done ON a specific
object
2. its also important to specify whether the work done is due to one particular
force (and which one) OR the total (net) work done by the net force on an
object.
how can we calculate Wnet? - correct answer ✔1. Fnet(Δx)cos(θ)
2. sum of the work done by each force
what is negative work? - correct answer ✔work done by forces that oppose
the direction of motion, such as friction
how can work be negative when it is a scalar? - correct answer ✔the angle
between the frictional force and the displacement is 180 degrees and cosine
of 180 is -1.
, what do + and - work mean in terms of energy and angles? - correct answer
✔positive work: energy into the system, angles between 0 and 90
negative work: energy out of the system, angles between 90 and 180
how are pos and neg work related to KE and speed? - correct answer ✔pos
work: KE increases and object speeds up
neg work: KE decreases and object slows down
A box is pushed with a downward angle and pulled with an upward angle,
which requires less force? - correct answer ✔the pull because it reduces FN,
which reduces Ffr, requiring less force to move at a constant velocity. less
force = less work
does the earth do work on the moon? - correct answer ✔NO! the forces that
cause centripetal motion do zero work because they are perpendicular to the
direction of motion
work done by a varying force - correct answer ✔not all forces are constant
and they can vary in magnitude and direction during a process
examples of varying forces? - correct answer ✔Gravity: as we move farther
and farther away from earth, the force of gravity becomes smaller and smaller
Springs: the longer the spring is stretched, the more force is exerted by the
spring
limitations of the work equation? - correct answer ✔only one number can be
plugged in for each variable. so it can't calculate work for forces where the
variables are constantly changing (variable forces)
solution 2025
definition of work, and is it a vector or scalar? - correct answer ✔an
expression of energy used to move an object
its a scalar
in order for work to be done, what has to happen? - correct answer ✔there
has to be a displacement, simply holding an object in place does not require
work.
description of work - correct answer ✔work describes what is accomplished
when a force acts on an object and the object moves a certain distance.
work done by a constant force: definition - correct answer ✔the displacement
moved multiplied by the component of the force in the direction of the
displacement
work done by a constant force: equation and important notes - correct answer
✔FΔXcos(θ) or Fdcos(θ)
F and d are magnitudes only
cosine selects the parallel force component
theta is the angle between the force and the displacement
Example: Holding + walking with a bag of groceries - correct answer
✔Holding: when you hold the bag in place, no work is done. this is because
the displacement is zero, even though you exert a force on the bag ---> you
need both a force AND a displacement to do work
,Walking: you can also do no work if you carry the bag while walking at a
constant velocity. No horizontal force is required to move the bag horizontally
at constant velocity. However... when you start and stop walking there is a
horizontal acceleration and therefore a horizontal force, so work is done.
if we are exerting a force on the bag, then why aren't we doing any work? -
correct answer ✔the person exerts an upward force on the bag because he's
holding it above ground and gravity acts on it downwards. BUT... the upward
force is perpendicular to the displacement.
when a force is perpendicular to the displacement, what does this mean? -
correct answer ✔that force does no work! the angle between the force and
displacement is 90 degrees and cos(90) is 0, so work is 0.
discuss specificity - correct answer ✔1. its important to specify whether you
are talking about work done BY a specific force, our work done ON a specific
object
2. its also important to specify whether the work done is due to one particular
force (and which one) OR the total (net) work done by the net force on an
object.
how can we calculate Wnet? - correct answer ✔1. Fnet(Δx)cos(θ)
2. sum of the work done by each force
what is negative work? - correct answer ✔work done by forces that oppose
the direction of motion, such as friction
how can work be negative when it is a scalar? - correct answer ✔the angle
between the frictional force and the displacement is 180 degrees and cosine
of 180 is -1.
, what do + and - work mean in terms of energy and angles? - correct answer
✔positive work: energy into the system, angles between 0 and 90
negative work: energy out of the system, angles between 90 and 180
how are pos and neg work related to KE and speed? - correct answer ✔pos
work: KE increases and object speeds up
neg work: KE decreases and object slows down
A box is pushed with a downward angle and pulled with an upward angle,
which requires less force? - correct answer ✔the pull because it reduces FN,
which reduces Ffr, requiring less force to move at a constant velocity. less
force = less work
does the earth do work on the moon? - correct answer ✔NO! the forces that
cause centripetal motion do zero work because they are perpendicular to the
direction of motion
work done by a varying force - correct answer ✔not all forces are constant
and they can vary in magnitude and direction during a process
examples of varying forces? - correct answer ✔Gravity: as we move farther
and farther away from earth, the force of gravity becomes smaller and smaller
Springs: the longer the spring is stretched, the more force is exerted by the
spring
limitations of the work equation? - correct answer ✔only one number can be
plugged in for each variable. so it can't calculate work for forces where the
variables are constantly changing (variable forces)
