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Physics > Newton's Laws > Flashcards

Newton's Laws Flashcards

1
Q

The First Law

A

Newton’s first law says that an object will continue in its state of motion unless compelled to change by a force impressed upon it

  • An object at rest will stay at rest, an object in motion will stay in motion (at a constant speed in a straight line)
  • No force= Δv
  • Inertia: Property of objects, their natural resistance to changes in the state of motion
    • First Law also called law of inertia
    • Mass is a measure of inertia; the more mass an object has the more the object resists changing its velocity
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2
Q

The Second Law

A

Acceleration, a, is directly proportional to the magnitude of the total, net force, (Fnet) and inversely proportional to the object’s mass

  • Fnet= ma
  • This law defines force
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3
Q

Force

A
  • The larger the force the larger the acceleration
  • Forces are represented by vectors; they have magnitude and direction
    • If several different forces act on an object simultaneously, then the net force is the vector sum of all these forces (resultant force= net force)
      • There can be forces acting on an object without causing a net acceleration; this happens when the forces cancel each other out Fnet= 0
  • An object does not have to move in the direction of the net force in the samne way an object doesn’t have to move in the direction of acceleration
    • The relationshipo between the direction of net force and velocity is the same as the relationship between acceleration and velocity
      • Foward forces speed up objects, backward forces slow down objects, and forces perpendicular to the velocity are responsible for turning
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4
Q

Newton

A

A force of 1 kg•m/s2 is renamed 1 newton (N)

A medium sized apple weighs about 1 N

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5
Q

Mass

A

The mass of an object is directly related to its weight

  • The heavier an object is, the more mass it has
  • Mass is measured in kilograms (kg)
  • Mass measures an object’s inertia- resistance to acceleration
  • Mass and weight are not the same thing
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6
Q

The Third Law

A

If object 1 exerts a force on object 2, then object 2 exerts a force back on object 1, equal in strength but in the opposite direction

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7
Q

Newton’s Law of Gravitation

A

Any two objects in the universe exert an attractive force on each other- the gravitational force- whose strength is proportional to the product of the objects’ masses and inversely proportional to the square of the distance between them

  • G is the universal gravitational constant, 6.67 x 10-11 N•m2/kg2
  • F= G(m1m2)/r2
  • Forces form along the line that joins the bodies and forms an action/reaction pair
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8
Q

The Gravitational Attraction Due to An Extended Body

A

Newton’s law of gravitation is a statement about the force between two point particles

  • Newton proved that a uniform sphere attracts another body as if all of the sphere’s mass were concentrated at its center
    • Newton’s law of gravitation can be applied to objects that are not small, relative to the distance between them
  • r is the distance between the centers of mass of the two objects; for objects with uniform density this is the distance from center to center
  • The force of gravity is very small unless at least one of the objects is large such as a planet or moon
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9
Q

Point Particles

A

Objects that are very small in comparison with the distance between them

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10
Q

Centripetal and velocity of satellite

A

Centripetal Force: mv2/R

Velocity of Satellite: v= squareroot(GM/R)

The Satellite’s speed doesn’t depend on its mass; even if it were a baseball, if its orbit radius were R then its orbit speed would still be given by the formula above

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11
Q

Weight

A

Gravitational force exerted on it by earth (or whichever planet it is on)

  • Since weight is a force, it can be computed by Fnet= ma, a=g
    • Fw= mg
    • Mass and weight are proportional but not identical
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12
Q

Mass Vs Weight

A
  • The weight of an object is the gravitational force exerted on it by earth (or planet is on)
  • Mass is an intristic property of an object that measures interia
  • An object’s mass does not change with location
    • An object’s weight on the moon will be less than on earth but its mass would stay the same
  • Mass and weight are proportional but not identical Fw= mg
  • Mass is measured in kilograms, while weight is mesaured in newtons
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13
Q

G vs g

A
  • G is a universal constant equal to 6.67 x 10-11 N•m2/kg2
  • g depends on what planet or moon an object is on
  • Near the surface:
    • mg= GMm/R2
      • M and R are the mass and radius of the planet or moon
      • g= GM/R2 (cancelling m)
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14
Q

Normal

A

Perpendicular

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15
Q

Contact Force

A

When an object is in contact with a surface, the surface exerts a contact force on the object

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16
Q

The Normal Force

A

The component of the contact force that’s perpendicular to the surface is called the normal force of the object

  • Normal force prevents objects from falling through table tops or from falling through the floor
  • Denoted by FN or N( not to be confused with N for newton)
  • Normal is not always equal to mg
17
Q

Friction

A

The component of the contact force that’s parallel to the surface is called the friction force on the object

  • Friction, like normal force, arises from electrical interactions between atoms of the object and surface
  • Two main categories:
    • Static friction
    • Kinetic (Sliding) friction
  • Strength of friction force dependent on
    • Nature of the surfaces
      • Represented by the coefficient of friction, μ; the greater this number the stronger the friction force will be
    • Strength of the normal force
18
Q

Static Friction

A

Occurs when there is no relative motion between the object and the surface (no sliding)

Fstatic friction, max= μsFN

  • Equation for the magnitude of static friction force is for the maximum value
    • Static friction can vary, counteracting weaker forces that are less than the mimium force required to move an object
    • Static friction can take on all values, up to a certain maximum, and the maxmium static friction force must be overcome for the object to slide
      • Example, If maximum static force exerted is 50N and object is pushed with 20N, then the static friction force will be 20N in the opposite direction not 50N; the object won’t move: the net force on a stationary object must be zero
  • The direction of Fstatic friction= Ff(static) is opposite to that of the intended motion
19
Q

Kinetic Friction

A

Occurs when there is relative motion (when there’s sliding)

Fkinetic friction= μkFN

The direction of Fkinetic friction= Ff(kinetic) is opposite to that of motion (sliding)

20
Q

Static Friction Vs. Kinetic Friction

A
  • Kinetic friction is generally weaker than static friction (it’s easier to keep an object sliding once it’s sliding than it is to start the object sliding in the first place)
  • Two coffecients of friction
    • μs static friction
    • μk kinetic friction
  • For a given pair of surfaces μk < μs is virtually always true
21
Q

Person walking static or kinetic friction

A

For a person walking, the friction between the person’s shoes and the floor is static (no sliding) and is directed foward (in the direction the person is walking) The person pushes on the floor in the backward direction, static friction prevents it from moving backward and so therefore must me forward

Objects that are rolling without slipping, rolling normally, not skidding, roll because of static friction

22
Q

Pulleys

A

Devices that change the direction of the tension force in the cords that slide over them

  • In the case of two single masses m1 and m2 that are attatched to a pulley and cord, the downward forces are due to the weight (mass and gravity exerted on it) of the masses
    • The upward forces are due to the tension (T) in the cord
23
Q

Inclined Plane

A

Ramp

  • If object of mass, m, is on the ramp, then the force of gravity on the object, Fw= mg has two components
    • One that is parallel to the ramp (mgsinθ)
    • One that is normal to the ramp (mgcosθ)
    • θ is the incline angle
  • The force driving the block down the inclined plane is the component of the block’s weight that’s parallel to the ramp: mgsinθ
24
Q

Incline plane Mneomic

A

mgsinθ is the component of gravity down the inclined plane by thinking (sine) equals (sliding)

25
Q

Short hand Pulley

m= 2kg

M= 10kg

coefficient of kinetic friction between the small block and tabletop is 0.5

what is the acceleration of the blocks

A

Mg- Mkmg= (M + m)a

Mg= forward force

Mkmg= backward force

(M + m)= total mass

Physics (15 decks)

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