9: Momentum, Force and Energy Flashcards

1
Q

What is a perfectly elastic collision?

A

One where momentum is conserved and kinetic energy is conserved

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2
Q

What is an inelastic collision ?

A

One where some of the kinetic energy is converted into other forms during the collision. But momentum is always conserved

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3
Q

What are the two types of friction? What are the differences?

A

Contact friction between solid surfaces
Fluid friction, drag or fluid resistance or air resistance

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4
Q

Three things you need to know about or frictional forces:
Their direction?
Their affect on the speed of the object?
Which types of energy do they convert?

A

They always act in the opposite direction to the motion of the object
They can never speed things up or start something moving
They convert kinetic energy into heat

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5
Q

You will reach your terminal velocity at some point, if you have…

A

A driving force that stays the same all the time e.g. weight
A fictional or drag force that increases with speed

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6
Q

When does something reach terminal velocity?

A

When the frictional force equals the driving force e.g. when weight = drag due to air resistance

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7
Q

Describe the graph of velocity against time for an object reaching terminal velocity

A

Curve, where the gradient decreases, eventually turning into a horizontal line. Graph starts at the origin and increases

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8
Q

Describe the graph of acceleration against time for an object reaching terminal velocity

A

Acceleration starts off high then decreases. The rate of decrease of acceleration starts slow then speeds up then slows again. The acceleration finally reaches zero. The graph looks like an unexaggerated S shape

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9
Q

Describe the graph of the velocity against time for the parachutist

A

First half of the graph looks like the normal velocity against time for an object reaching terminal velocity. Then the graph suddenly drops to a lower velocity and flattens out, where it reaches a new terminal velocity

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10
Q

When is work done?

A

Whenever energy is transferred

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11
Q

What is power?

A

The rate of doing work – it’s the amount of energy transferred from one form to another per second

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12
Q

How do you answer a question where somethings projected at an angle

A
  1. resolve initial velocity into horizontal and vertical components
  2. use vertical component to find how high it goes and and to work out how long it’s in the air for
  3. use horizontal component to find distance travelled horizontally while in air
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13
Q

How do you use vertical component to find highest point of projectile’s motion and how long it’s in the air for?

A

If the graph is symmetrical: at the halfway point, vertical velocity is 0 for an instantaneous time, so the value of vv must be multiplied by 2

To get the second time vv = 0 which is when it will hit the ground
so vv = 0, uv = resolve into vertical and horizontal to find out
a = g = -9.81 (upwards as neg)
t = ?
use v = u + at to get time
This is time taken to get to highest point; multiply by 2 to get the time to reach ground again

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14
Q

How do you use horizontal component to find distance travelled after you have found time before it hits the ground again using vertical component

A

There is no acceleration horizontally so
a = g = 0
this means that uH = vH (speed)
time for it to hit ground found from vertical component = t
use this to find s, distance travelled
you can use speed = distance / time
distance = speed * time

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15
Q

how to answer question where projectile is thrown horizontally, you want how long it takes to hit ground and how far it travelled

A

break it into vertical and horizontal

vertical: thrown horizontally with speed Vh so
for vertical u = o
a = g = (-) 9.81
if done above ground level s = xm
so use s = ut + 1/2 at^2
usually just goes to s = 1/2 at^2
use this to find time

horizontal: Vh isnt affected by gravity so Uv = Vv and a = N/A
use time t from vertical component
usually can use speed = distance / time
distance = speed * time
s = Vv (Uv) * t

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16
Q

What are the forces acting on a person falling at constant speed

A

if falling at constant speed, forces are equal, drag = mg

17
Q

Key points to remember for work done

A

Work done is not necessarily the total energy. If you move an object higher up you have increased its potential energy, but it already had some potential energy to start with

Force, F, will usually be fixed.
Equation assumes force is in same direction as direction of movement

18
Q

Equation for work done at an angle

A

Work done = force * distance * cos theta

19
Q

Equation linking power, force and velocity
Equation linking power, force at an angle and velocity

A
Power = Force \* velocity 
P = Fv 
Power = Force cos theta \* velocity 
P = F cos theta \* velocity
20
Q

Equation linking power, force and velocity
Equation linking power, force at an angle and velocity

A
Power = Force \* velocity 
P = Fv 
Power = Force cos theta \* velocity 
P = F cos theta \* velocity