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Physics > Chapter 5-7 > Flashcards

Chapter 5-7 Flashcards

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1
Q

What is friction?

A

A force

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2
Q

What are the two types of friction?

A

Static and Kinetic

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3
Q

Static Friction

A

The friction that arises to keep an object which is not sliding from starting to slide when another force on the object would otherwise cause it to slide. (varies from zero to a maximum value)

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4
Q

Kinetic friction

A

The friction between an object and another surface when they are sliding along each other.

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5
Q

Rolling friction

A

Occurs when one object rolls over another surface

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6
Q

Fluid friction

A

Occurs between a fluid and a solid surface moving past each other.

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7
Q

What is the kind of friction that allows a car to drive along the road?

A

Static friction between tires and road

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8
Q

What is the direction of the friction force?

A

Always opposite to the direction of sliding if it is kinetic friction, or opposite to the way it would slide without static friction to stop it from sliding.

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9
Q

What determines the amount of friction between an object and the surface it is on?

A

The amount of kinetic friction between two surfaces is determined by only two things:
1. The nature of the two surfaces - waxing floor changes friction, rubber on ice or pavement
2. The force pushing the surfaces together, i.e., the normal force - Fn is not always equal to Fg

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10
Q

Formula for kinetic friction:

A

Ff, kinetic = μk FN

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11
Q

μk

A

coefficient of kinetic friction for the two surfaces (just a number, no units)

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12
Q

Ff

A

force friction

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13
Q

Fn

A

Normal Force

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14
Q

Does friction force depend on the surface area of contact or speed?

A

No, it does not

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15
Q

What is the amount of static friction?

A

The amount of static friction between two surfaces is anywhere from zero to a maximum amount that depends on the same two factors as kinetic friction does, except the coefficient of static friction has a different value than the coefficient of kinetic friction for the same two surfaces.

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16
Q

μs

A

coefficient of static friction

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17
Q

Why do friction formulas, that have vector quantities on both sides, not include arrows over the letters?

A

Because they are only for the magnitude of forces since the directions do not match.

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18
Q

What is always the direction of Ff and Fn

A

Perpendicular to each other

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19
Q

When do we want a high coefficient of friction?

A

Tires and roads, sports shoes, skirt and chair, book and table.

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20
Q

When do we want a low coefficient of friction?

A

Skates, box and floor

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21
Q

Is the coefficient of static friction or kinetic friction generally higher for a given pair of surfaces?

A

Coefficient of static friction

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22
Q

Why is it normally harder to get an object moving than to keep it moving?

A

The maximum static friction holding back is greater than the kinetic friction that acts when the object is sliding.

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23
Q

What is the resultant?

A

Two vectors in two different directions (e.g. 2 velocities), they are equivalent to a single vector

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24
Q

How do we replace a single vector?

A

Add 2 perpendicular ones that add up to a single one, (these are called the components of the single vector)

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24
Q

If the angle is measured from the x-axis what is Voy equal to?

A

Voy = V (sin θ)

24
Q

What is the process of finding the x and y components that make up a vector called?

A

Resolving the vector into its scalar components.

25
Q

If the angle is measured from the x-axis what is Vox equal to?

A

Vox = V (cos θ)

26
Q

Why are no arrows included whenever a subscript x or y is used?

A

We are working with the scalar components of the vector.
In the calculations of the components, we are dealing with the magnitudes only, so arrows are not included at all.

27
Q

When describing the scalar components of a vector, what do we define each component as?

A

Positive or negative component. East or North pointing components are positive and West or South pointing components are negative.

28
Q

What trajectory do objects thrown horizontally follow? Why?

A

Parabolic, because it accelerated downwards as it fell.

29
Q

Since motion is downwards what do you set down as?

A

positive - so d = positive and a = g

30
Q

If a rock is being thrown sideways off the cliff instead of straight down, how do you find how long it would take to hit the ground?

A

Take the same time to hit the ground.

31
Q

Does the horizontal motion of a projectile affect vertical motion?

A

No and vertical motion does not affect horizontal motion down either.

32
Q

Why is time the same whether an object is dropped or thrown from a height?

A

Because it makes no difference whether an object falls straight down or as a projectile, downward acceleration is exactly the same whether the object is going sideways or not.

33
Q

Projectile definition

A

an object that has both vertical and horizontal motion

33
Q

What two parts do we analyze projectiles in?

A

Horizontal and vertical components.

34
Q

When are the two components of a projectile combined?

A

when you require the resultant velocity.

35
Q

Notes to find a projectile when the object is launched at an angle:

A
  1. Horizontal or x, component of velocity is constant
  2. The vertical, or y component of the velocity decreases to zero, (due to downwards acceleration), then increases until it is equal in magnitude to what it was the the start but in the opposite direction
  3. The horizontal distance traveled is called the range of the projectile.
36
Q

How to solve problems involving projectiles launched at an angle?

A

take the initial velocity and calculate the horizontal and vertical components.

37
Q

which direction is velocity and acceleration always in?

A

the direction of motion, downwards - or towards the center of the earth

38
Q

What rate does the surface of the earth curve at?

A

4.9m per 8km - so after a horizontal distance of 8km the surface has “dropped” 4.9m

39
Q

What would happen if a projectile were fired horizontally at 8km/s?

A

It would constantly “fall” downward, but never get any closer to the earth’s surface. - go in a circular orbit around the earth.

39
Q

If a projectile were fired horizontally at 8km/s would it be accelerating?

A

Yes! Because of gravity, at every moment it is accelerating towards the surface of the earth.

40
Q

Is the moon falling?

A

Yes, and every other satellite circling the earth is constantly falling towards the earth, they just never get any closer because of their tangential velocity.

41
Q

If a projectile were fired horizontally at 8km/s, how do its velocity and acceleration relate?

A

Always perpendicular to each other.
The acceleration is always towards the center of the earth, whereas velocity is always tangent to the circular path.

42
Q

What kinds of objects have gravity?

A

All objects!

42
Q

Centripetal acceleration

A

An object traveling in a circle at a constant speed experiences acceleration toward the center of the circle. The velocity of the object is always tangent to the circle.

43
Q

When Earth exerts a force of gravity on the moon what force does it exert back?

A

Equal force, but we can not use Fg=mg to find the value of force at that distance because acceleration is not 9.80m/s^2 because the value of gravity varies.

44
Q

The general formula that gives the force of gravity between any objects:

A

Newtons law of universal gravitation - F = (Gm1m2) / (r^2)

45
Q

What does the force of gravity between two objects depend on?

A

Fg = masses (both)- direct relationship
Fg = (1/r^2) (distance between the centers of the 2 objects) - inverse square relationship

46
Q

What keeps planets in motion around the sun(and the moon around the earth)?

A

The gravitational force on the planets provides the centripetal acceleration to keep them moving in a circle.

47
Q

What is the distance between objects in centripetal motion?

A

the radius of the orbit.

48
Q

If one mass is doubled what does it do to Fg?

A

Double

49
Q

If the distance is doubled what happens to Fg?

A

1/4 as much

50
Q

If both masses are doubled and the distance is halved what does it do to Fg?

A

16 x as much

51
Q

Who did a precise experiment to determine the universal gravitational constant 100 years after Newton, and what was it?

A

Henry Cavendish and found it was 6.67 x 10 ^-11 Nxm^2/kg^2

52
Q

When is your apparent weight zero?

A

When all objects are accelerating toward earth together and there are no contact forces exerted on you

53
Q

What is an object with mass surrounded by?

A

Gravitational field

54
Q

What is gravity? - from Einstein’s theory

A

not a force, but rather an effect of space as mass changes the space around it.

55
Q

What does light follow in space?

A

the curvature of space around the massive object and is deflected.

Physics (13 decks)

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