Physics Exam III

Question 1

Work

Answer 1

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

Question 2

kinetic energy

Answer 2

energy that an object has due to its motion

Question 3

potential energy

Answer 3

energy associated not with an object’s motion but with its shape and position

Question 4

internal energy

Answer 4

energy stored within an object sometimes in a way that can’t be easily extractd

Question 5

equation for work

Answer 5

w=fd

Question 6

what is the unit for work

Answer 6

joule

Question 7

equation for work done on an object by a constant force that points at an angle to the object’s displacement

Answer 7

W= (Fcosθ)d

Question 8

If F is in the same direction as the motion:

Answer 8

θ = 0, cosθ = cos0 = 1
W = Fd

Question 9

If F is perpendicular to the direction of the motion:

Answer 9

θ = 90, cosθ = cos 90 = 0
F does zero

Question 10

If the angle is greater than 90

Answer 10

the value of cosθ is negative
- F does NEGATIVE work so W is less than 0

Question 11

What if more than one force acts on an object as it moves?

Answer 11

Then the total work done on the object is the SUM of the work done on the object by each force individually

Question 12

Net work?

Answer 12

Wnet = (Fnetcostheta)d

Question 13

Work energy theorem (in words)

Answer 13

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

Question 14

Work energy theorem equation

Answer 14

Wnet = Kf - Ki

Question 15

Kinetic Energy

Answer 15

k = 0.5mv^2
= units: kg times m^2 all over s^2

Question 16

If Wnet is positive?
What happens to kinetic energy and speed?

Answer 16

Kf-Ki is positive (increase kinetic energy)
speed increases

Question 17

If Wnet is negative?
What happens to kinetic energy and speed?

Answer 17

Kf-Ki is negative (decrease kinetic energy)
speed decreases

Question 18

If Wnet is zero?
What happens to kinetic energy and speed?

Answer 18

Kf-Ki = 0 (no change)
speed doesn’t change

Question 19

Work done by a constant force

Answer 19

• area under the curve (a box)
• W = Fd

Question 20

Work done by a varying force (SPRING)
- What is Hooke’s Law

Answer 20

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

Question 21

spring constant
- why negative in front of k
- what are the units of K

Answer 21

k (measure of its stiffness)
spring trying to pull you in the opposite direction
units: N/m

Question 22

Work done by a varying force equation
- what are x1 and x2
- what is area under curve

Answer 22

W = 1/2x2^2 - 1/2x1^2
X2 = final stretch of spring
X1 = initial stretch of spring
- it’s a triangle A =1/2bh

Question 23

does a stationary object have the ability to do work?
example?

Answer 23

yes
example: barbell (lifted above head but has no kinetic energy but if dropped it would crash)

Question 24

gravitational potential energy
- equation

Answer 24

• object of mass is at a vertical coordinate (y)
  Ugrav = mgy
  y = height of object
  m = object
  g = gravity

Question 25

Change in kinetic energy if only the gravitational force does work

Answer 25

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

Question 26

If the object rises, the gravitational potential energy does what? the kinetic energy?

Answer 26

gravitational - increase
kinetic - decreases

Question 27

If the object descends, the gravitational potential energy does what? the kinetic energy?

Answer 27

gravitational decreases
kinetic increases

Question 28

Where should you set y for gravitational potential energy?
- what does the change in gravitational potential energy depend on?

Answer 28

doesn’t matter
-the change in gravitational potential energy depends only on the difference between the initial and final heights

Question 29

Ugrav equation

Answer 29

Ugrav = Uf-Ui = mgyf-mgyi = mg(yf-yi)

Question 30

Ideal spring potential energy?
what x value means a relaxed spring?

Answer 30

Uspring = 0.5kx^2
x=0

Question 31

Conservative force

Answer 31

a force that can be associated with a potential energy like the gravitational force or the force exerted by an ideal spring
- path independent

Question 32

nonconservative force

Answer 32

example is friction
- force which we cannot use the concept of potential energy
- friction force DOES depend on the path taken

Question 33

Energy transformation example of heart
- what kind of energy do we lose?

Answer 33

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)