Physics Exam III Flashcards
Work
describes what happens when a force is exerted on an object as it moves; if a system/object has energy it may be able to have work
kinetic energy
energy that an object has due to its motion
potential energy
energy associated not with an object’s motion but with its shape and position
internal energy
energy stored within an object sometimes in a way that can’t be easily extractd
equation for work
w=fd
what is the unit for work
joule
equation for work done on an object by a constant force that points at an angle to the object’s displacement
W= (Fcosθ)d
If F is in the same direction as the motion:
θ = 0, cosθ = cos0 = 1
W = Fd
If F is perpendicular to the direction of the motion:
θ = 90, cosθ = cos 90 = 0
F does zero
If the angle is greater than 90
the value of cosθ is negative
- F does NEGATIVE work so W is less than 0
What if more than one force acts on an object as it moves?
Then the total work done on the object is the SUM of the work done on the object by each force individually
Net work?
Wnet = (Fnetcostheta)d
Work energy theorem (in words)
When an object undergoes a displacement, the work done on it by the net force equals the object’s kinetic energy at the end of the displacement minus the kinetic energy done at the beginning of the displacement
Work energy theorem equation
Wnet = Kf - Ki
Kinetic Energy
k = 0.5mv^2
= units: kg times m^2 all over s^2
If Wnet is positive?
What happens to kinetic energy and speed?
Kf-Ki is positive (increase kinetic energy)
speed increases
If Wnet is negative?
What happens to kinetic energy and speed?
Kf-Ki is negative (decrease kinetic energy)
speed decreases
If Wnet is zero?
What happens to kinetic energy and speed?
Kf-Ki = 0 (no change)
speed doesn’t change
Work done by a constant force
- area under the curve (a box)
- W = Fd
Work done by a varying force (SPRING)
- What is Hooke’s Law
Fx = -kx –> Hooke’s law
- this is the force exerted by an ideal spring
- the force that the spring exerts on you is directly proportional to the amount of stretch
spring constant
- why negative in front of k
- what are the units of K
k (measure of its stiffness)
spring trying to pull you in the opposite direction
units: N/m
Work done by a varying force equation
- what are x1 and x2
- what is area under curve
W = 1/2x2^2 - 1/2x1^2
X2 = final stretch of spring
X1 = initial stretch of spring
- it’s a triangle A =1/2bh
does a stationary object have the ability to do work?
example?
yes
example: barbell (lifted above head but has no kinetic energy but if dropped it would crash)
gravitational potential energy
- equation
- object of mass is at a vertical coordinate (y)
Ugrav = mgy
y = height of object
m = object
g = gravity
Change in kinetic energy if only the gravitational force does work
ΔK = -ΔUgrav
- the change in kinetic energy of object is equal to the negative of the change in the gravitational potential energy
- transfer energy into motion
If the object rises, the gravitational potential energy does what? the kinetic energy?
gravitational - increase
kinetic - decreases
If the object descends, the gravitational potential energy does what? the kinetic energy?
gravitational decreases
kinetic increases
Where should you set y for gravitational potential energy?
- what does the change in gravitational potential energy depend on?
doesn’t matter
-the change in gravitational potential energy depends only on the difference between the initial and final heights
Ugrav equation
Ugrav = Uf-Ui = mgyf-mgyi = mg(yf-yi)
Ideal spring potential energy?
what x value means a relaxed spring?
Uspring = 0.5kx^2
x=0
Conservative force
a force that can be associated with a potential energy like the gravitational force or the force exerted by an ideal spring
- path independent
nonconservative force
example is friction
- force which we cannot use the concept of potential energy
- friction force DOES depend on the path taken
Energy transformation example of heart
- what kind of energy do we lose?
heart contracts and pushes blood into arteries which stretch to accommodate increased volume
- stretched arterial walls behave like stretched spring and possess spring potential energy
- blood gains kinetic energy as arterial walls push on it between heart beats
- spring potential energy of arteries transformed to kinetic energy of blood
- we lose heat (friction)
