Mechanics 3 - Newton's Laws, Terminal Velocity, Momentum Flashcards

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1
Q

What is Newton’s 1st Law of Motion?

A

If no resultant force acts on a body, then it will either remain at rest, or continue moving with constant velocity (no acceleration)

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2
Q

What is Newton’s 2nd Law of Motion?

A

The rate of change of momentum (acceleration) of a body is directly proportional to the resultant force acting on it

Fres ∝ a

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3
Q

What is Newton’s 3rd Law of Motion?

A

When two objects interact, they exert an equal and opposite force on each other and the forces are of the same type

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4
Q

If the forces acting on an object are balanced what can you say about its motion?

A

There is no resultant force so it will continue moving at a constant velocity. It won’t accelerate.

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5
Q

What’s wrong with this?

A

In F=ma, F must be the resultant force!!!

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6
Q

How does an object reach terminal velocity?

A

As it speeds up, air resistance increases, decreasing the resultant force.

Eventually air resistance = driving force, Fres=0 so a=0.

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7
Q

What two things are the case for tension?

A
  1. Tension always acts away from the contact points
  2. Tension is constant throughout the rope/wire/ cable
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8
Q

Why are objects never truly in freefall?

A

There will always be air resistance opposing the weight

(Apart from when v=0)

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9
Q

What is the condition for terminal velocity?

A

The drag force = driving force (or weight) so Fres=0 and so a=0

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10
Q

What factors affect the drag force on an object?

A
  • Fluid density
  • Shape of object
  • Cross sectional area of object
  • Velocity of object
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11
Q

Why does air resistance increase with velocity?

A

The object is colliding with more air molecules per second

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12
Q

What does the velocity time graph of an object reaching terminal velocity look like?

A
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13
Q

What’s wrong with this?

A

The acceleration is not constant so you cannot use SUVATs

Instead use area under graph

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14
Q

How is momentum calculated?

A
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15
Q

What’s wrong with this?

A

Direction must be taken into account (as momentum is a vector)

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16
Q

What two things is impulse equal to?

A
  1. Rate of change of momentum
  2. Impact force x impact time
17
Q

What are the units of impulse and momentum?

A
18
Q

What does the area under a force-time graph represent?

A

The change of momentum or impulse

19
Q

What is the conservation of momentum?

A

For a system of interacting objects, the total momentum remains constant…

provided no external resultant force acts

20
Q

In any interaction, what is conserved?

A

Total momentum is always conserved

Total energy is always conserved

Kinetic energy is only conserved if collision is elastic

21
Q

What is an elastic collision?

A

A collision where kinetic energy is conserved

(as well as momentum)

22
Q

What is wrong here?

A

You have to calculate kinetic energies separately for each object

23
Q

In physics terms what is an explosion?

A

The total momentum = 0

24
Q

How do you answer flow rate questions? (momentum of a flowing liquid)

A

Consider the cylinder made by a liquid’s flow after 1 second

Where the length of the cylinder = velocity of the fluid

And use density equation to get volume of cylinder

25
Q

How do you work out the area of a curved graph?

A
  1. Split into boxes
  2. Count the boxes (pairing up incomplete boxes)
  3. Multiply number of boxes by area of each box