Motion and Energy (Notes)

8/18/2019 Motion and Energy (Notes)

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Motion and Energy


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Motion

•

Motion – an object’s changein position relative to areference point.

• The Earth’s surface isused as a commonreference point

• A moving object can beused as a reference

point as well


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Speed

•

Speed is the distance traveled divided bythe time interval during which the motionoccurred.

• Normally, objects do not travel at aconstant speed.

• Average peed !total distance "m# total time "s#


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Velocity

•

Velocity is the speed of an object in aparticular direction.• $magine two birds leave the same tree at the

same time.

• They both fly at %&'m(hr for ) minutes.

*hy don’t they end up atthe same place+


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Velocity• Velocity appears to be very similar to

speed, however, when describing thevelocity of an object you need toprovide a magnitude and a direction.

• Magnitude – the speed of the object.• Direction – the direction in which the

object is moving.• elocity ! total displacement "m#

total time "s#


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Speed vs. Velocity

peed is simply how fast you are travelling-

,elocity is .speed in a given direction/-

This car is travellingat a speed of 0&m(s

This car is travelling at avelocity of 0&m(s east


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Acceleration

• Acceleration is the rate at which velocitychanges over time.

• Average acceleration (m/s2) =final velocity(m/s) – starting velocity(m/s)time taen to c!ange velocity (s)

• As velocity increases, so does acceleration•

As velocity decreases, so does acceleration• *hen direction changes, so does

acceleration•

*hen there is a constant velocity, there is


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'rap!s and "#uations• E2uations are great for describing

ideali3ed situations, but they don4t alwayscut it.• ometimes you need a picture to show

what4s going on a mathematical picturecalled a graph of several sentences.

• 6raphs are often the best way to conveydescriptions of real world events in a

compact form.• 6raphs of motion come in several typesdepending on which of the 'inematic2uantities "time, displacement, velocity,

acceleration# are assigned to which a7is.


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Distance ime 'rap!s(nderstanding and interpreting)

•

8lotting distance against time can tell you a lot about a journey. 9et4s loo' atthe a7is:

• Time "s# always runs hori3ontally "the 7;a7is#.

•


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Distance ime 'rap!s *ot moving+ ,o- -ill it loo lie+++• $f something is not moving, a hori3ontal line is

drawn on a distance;time graph "dt;graph#.

• Time is increasing to the right, but itsdistance does not change. $t is stationary.

• =ere both velocity and acceleration is 3ero.


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Distance ime 'rap!s Moving+ ,o- -ill it loo lie+++

• $f something is moving at a steady velocity, itmeans we e7pect the same increase indistance in a given time:

• Time is increasing to the right, and distanceis increasing steadily with time.• $t moves at a steady velocity ">ero

acceleration.


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Distance ime 'rap!s !at is t!e effect of line Steepness0

or slope0 or gradient0+++• ?oth the lines below show that each object moved

the same distance, but the steeper yellow line gotthere before the other one:

• lope of distance;time graph represents velocity.• A steeper gradient indicates a more distance moved

in a given time. $n other words, higher velocity.• ?oth lines are of constant gradient, so both speeds

are constant but of different value.


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Distance ime 'rap!s Moving+ !at if it is not steady+++• The line below is curving upwards. Thisshows an increase in velocity, since the

gradient is getting steeper:

• $n other words, in a given time, thedistance the object moves is larger. $t is

accelerating.


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Distance ime 'rap!s Moving+ !at if it is not steady+++• The line below is curving upwards. Thisshows an decrease in velocity, since the

gradient is getting lesser:

• $n other words, in a given time, thedistance the object moves is lesser. $t is

decelerating.


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Distance ime 'rap!s (Summary)

ero velocity


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Velocity ime 'rap!s(nderstanding and interpreting)

•

8lotting velocity against time can tell youa lot about a journey. 9et4s loo' at thea7is:

• Time"s# always runs hori3ontally "the 7;a7is#.• elocity"m(s# runs vertically "the y;

a7is#.


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Velocity ime 'rap!s *ot moving+ ,o- -ill it loo lie+++• $f something is not moving, a hori3ontalline is drawn on a velocity;time graph "vt;

graph#.

• Time is increasing to the right, but itsvelocity remains 3ero.

• =ere also velocity is constant but isalwa s 3ero.


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Velocity ime 'rap!s

Moving+ ,o- -ill it loo lie+++• $f something is moving at a steady

velocity, a hori3ontal line is drawn on avelocity;time graph "vt;graph#.

• Time is increasing to the right, but itsvelocity does not change.

V l i i ' !


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Velocity ime 'rap!s Moving+ ,o- -ill it loo lie+++•

$f something is moving with a steadyacceleration(deceleration, it means we e7pectthe same increase(decrease in velocity in agiven time

• Time is increasing to the right, and velocity issteadily increasing(decreasing with time.

•

@oves at a uniform acceleration(deceleration.

niformAcceleration

niform


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Velocity ime 'rap!s !at is t!e effect of line Steepness0

or slope0 or gradient0+++• ?oth the lines below show that each object moved atthe same velocity, but the steeper yellow line gotthere before the other one:

• lope of such graph represents acceleration.• A steeper gradient indicates a more velocity is

gained "more acceleration# in a given time.• ?oth lines are of constant gradient, so both

accelerations are constant but different.


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Velocity ime 'rap!s Moving+ !at if it is not steady+++

• The line below is curving upwards(downwards.This shows an increase(decrease in velocity,since the gradient is getting steeper:

• $n other words, in a given time, the velocitywith which the object moves is increasing ordecreasing. $t is non;uniform acceleration or

deceleration.

NonUniformacceleration

NonUniformdeceleration


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Area nderneat! v$t 'rap!

• Area underneath a v;t graph would

get the distance covered.• $f you calculate the area underneath a

v;t graph, you would multiply height B

width.• ?ecause height is actually velocity and

width is actually time, areaunderneath the graph is e2ual to» elocity B time or ,;t

"


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"nergy• Energy can be defined as the ability to do wor'

"measured in Coules C/ #.• 1inetic "nergy (1.". = 3Mass3velocity4) –The energy due to motion is called 'inetic

energy. –Dinetic energy is proportional to mass and

velocity. –E7ample ; moving a s'ateboard, blowing wind,

motion of pendulum.• 5otential "nergy (5." = -eig!t3!eig!t = mg!) – 8otential Energy is stored energy.

! t0 i t t 6 t


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!at0s so important a6out5" and 1"+

• *e call the sum of 8E and DE mechanicalenergy.

@.E ! D.E 1 8.E

• @echanical energy is important because it isconserved "as long as there are no forces, li'efriction#

• Therefore, if one goes down, the other goesup by the same amount.• 9oss of 8.E ! 6ain of D.E and vice;versa.


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"nergy ransfers


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8 d i d f t


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8ar design and safety• To be a safe driver you need to understand

the factors that affect a car4s stoppingdistance.• 8rimary safety factors li'e car’s road holding,

bra'es, steering, handling and above all the

driver help to prevent accidents.• There are secondary safety devices which aid

survival in the event of an accident. These areas follows ;

• rumple >ones – 8resent in front and at the bac'. Absorbs 'inetic energy. E7tends collision time Feduces decelerating force and potential injury.

8 d i d f t


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8ar design and safety• E7tensible eat ?elts –

E7erts a bac'ward force "of %&&&& N# over &.) m. $n a car moving at %) m(s, the effect of notwearing belt is same as that produced by jumpingoff from %0m high.

• Air ?ags – $t inflates and protects from injury by the

steering wheel.• =ead restraints – Ensures that if the car is hit from behind, the

head goes forward with the body and notbac'wards over the seat.

8revents damage to the top of the spine.

9 lli 6 di


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9alling 6odies• All bodies falling freely under the force of

gravity do so with uniform acceleration ifthe air resistance is negligible "all bodies fallat the same rate in vacuum#.

• This acceleration, also called as acceleration

of free fall, is denoted as Gg’.• $ts appro7imate value is H.I m(sJ but we will

ta'e it as %& m(sJ.

• $f two bodies with different mass arethrown from the same height in vacuum theywill reach the ground at same time.


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Motion and Energy (Notes)