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Physics > Exam 2--141 > Flashcards

Exam 2--141 Flashcards

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1
Q

What force is work concerned with?

A

Force parallel

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2
Q

Work

A

the amount of energy a force can pump into (or take out) of a system

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3
Q

Kinetic energy

A

the energy of motion

generally, Wnet = ∆KE

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4
Q

Potential energy

A

stored energy / energy associated with position

shortcut to figuring out the amount of energy a force can pump into (or take out) of a system

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5
Q

How can we say that a collision is an isolated system and apply conservation of momentum?

A

1) the collision forces between the two objects we make internal forces by lumping the two objects together in a two object system
2) the friction force we artificially cancel out by making the ∆t so small that friction does not have time to act

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6
Q

How can we say that Bing on the skateboard with the fire extinguisher is isolated?

A

Friction is not that significant and normal force cancels out gravity

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7
Q

Conservation of momentum and Bing on the skateboard

A

some mass of air gets a big velocity in the negative direction from the fire extinguisher

so, Bing has to have a velocity forward to maintain the same momentum

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8
Q

What types of forces can do work?

A

Only forces parallel to the objects motion can do work

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9
Q

What types of forces can create torque?

A

Forces perpendicular to the object can twist the object

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10
Q

Is energy a vector?

A

No

Energy does not encode direction

Can simply add more energy or take away energy

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11
Q

What types of forces have potential energy?

A

conservative forces

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12
Q

Conservative forces

A

only the initial and final positions matter, not the path taken

no energy is lost by taking a longer path and correcting it (unlike something like friction)

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13
Q

Spring force

A

a contact force

linear force (force gets bigger as spring is stretched)

conservative force

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14
Q

What does the spring constant correspond to?

A

K corresponds to the stiffness of the spring

large K means a more stiff spring

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15
Q

Pendulum and energy

A

at bottom, all PE goes to KE energy

at top, all PE gravitational energy

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16
Q

Going from rest to rest? Like raising a book

A

Kinetic energy is 0

Allows Work to correspond to the change in potential energy

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17
Q

Power

A

the rate at which work is done

Unit: J/s (Watt)

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18
Q

Different between impulse and work

A

Work is force acting through a distance which produces a change in kinetic energy

Impulse is force acting through time which produces a change in momentum

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19
Q

Similarity between impulse and work

A

Both are ways of saying that forces change velocities

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20
Q

Why is bouncy ball better at knocking over a block than deadball?

A

Bouncy ball has velocity before and after the impact which creates a greater change in momentum (due to direction as well)

Since momentum is related to force through impulse, this means that the bouncy ball creates a larger force on the block than the deadball

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21
Q

When does conservation of momentum hold true?

A

an isolated system

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22
Q

Elastic collisions

A

total initial KE equals total final KE

ex: Newton’s cradle, billard balls

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23
Q

Are there ever any truly elastic collisions?

A

No

some energy is always lost

24
Q

Can elastic collisions have 0 final KE?

A

No

Final KE equals initial KE and we need some initial KE to make the collision happen

25
Q

Inelastic collisions

A

Some kinetic energy is lost

Initial KE is greater than final KE

losses energy to sound, heat, and especially permanent deformation

26
Q

Permanent deformation

A

a dent in an object

collision forces doing work

changes KEf

27
Q

Perfectly inelastic collisions

A

max amount of KE is lost

two objects have fused into one

don’t lose all KE energy because need some final velocity to balance momentum

28
Q

Can you have KE=0 in perfectly inelastic collisions?

A

Yes

Let’s say two clay balls are thrown at each other and then they collide and stop

29
Q

What does tangential velocity refer to?

A

the change in arc length over time

30
Q

Two dots are positioned on the outer edge of a circle and the inner edge of a circle. Which moves faster?

A

In tangential terms, the one one the outer edge moves faster

In angular terms, both dots move at the same speed (both cover the same amount of angle)

31
Q

Centripetal acceleration

A

points towards the center of a circle

due to velocity changing direction not magnitude

caused by familiar forces

always have centripetal acceleration when moving in a circle

32
Q

Different between centripetal and angular acceleration

A

angular acceleration is an angular quantity whereas centripetal acceleration is a linear quantity (m/s2)

angular acceleration just refers to if angular velocity is slowing down or speeding up

can be rotating without an angular acceleration (magnitude not changing)

33
Q

Orbit

A

a gravitational pull is the force that causes a centripetal acceleration

34
Q

Universal gravitation

A

any mass in the universe attracts any other mass

35
Q

r in universal gravitation

A

center to center distance between two objects

36
Q

Kepler’s 3rd Law

A

square of orbital period is proportional to the cube of the orbital radius

37
Q

Period

A

time for one complete revolution

38
Q

Kepler’s 1st law

A

orbits are ellipses with the sun at a focus point

39
Q

Keplers 2nd law

A

equal areas are swept out by the planet in equal times

this fits conservation of energy

40
Q

Top of loop-da-loop

A

normal force and gravity act downwards

set normal force equal to zero to solve for the minimum speed needed to make it around the loop

41
Q

What is force equivalent to in rotational motion?

A

torque

42
Q

Torque

A

a twist that causes an angular acceleration

torques come from known forces

43
Q

Why do you get more torque further from the hinge?

A

bigger distance that the force can act through

44
Q

What is mass equivalent to in rotational motion?

A

Inertia

45
Q

Why does a hoop roll slower than a disk?

A

Hoop has all the mass in a large radius to get around

this leads to greater resistance / rotational inertia

46
Q

Arc length velocity compared to translational velocity

A

both are measured in m/s

both are the same magnitude

arc length velocity is being laid down along the bath (one revolution lays down one circumference of arc length)

therefore, translational velocity equals arc length velocity

47
Q

Why is KE not zero at the apex of projectile motion?

A

there is still kinetic energy in the x direction

48
Q

Is momentum conserved in inelastic collisions? What about perfectly elastic?

A

Yes

Momentum is conserved in both

49
Q

What do you have to account for when using gravitational potential energy?

A

Gravity works on the object’s center of mass

only very specific cases, like a rod standing up, do we take this into consideration

50
Q

A ball rolls down a ramp. What happens to kinetic and potential energy?

A

Kinetic energy varies with velocity and velocity increases constantly as the ball accelerates. Therefore, the square of the increasing velocity gives the kinetic energy graph a concave up parabola shape

Potential energy varies with height and gets steeper and steeper as the ball loses height more quickly. Therefore, potential energy graph has a concave down curve

51
Q

What happens to total energy (KE+PE) if there is friction?

A

Some of the total energy is lost to the outside system

Mechanical energy is not conserved

52
Q

Does center of gravity mean that masses are equal on both sides?

A

No

Center of gravity accounts for both mass and radius

So, something with a larger radius would have less mass on a given side

53
Q

If something moves at a constant speed, what is the net work?

A

The net work is zero

There was no change in velocity which means there was no acceleration/net force, and W=F*x, so this would mean that net work is zero

Can still be doing work though when the potential energy changes (like moving books off ground)

54
Q

Why does conservation of momentum not apply to free fall?

A

the net force on the system does not equal 0

gravity is unbalanced in free fall and therefore, this is not an isolated system

55
Q

If you double angular acceleration do you double your final angular speed?

A

Yes

Physics (7 decks)

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