Transcript WORK DONE - whs10science

CHAPTER 6
On the Move
What you should have completed so far:
1.
Distance Vs. Displacement sheets
2.
Text questions 1-8 on 140-141 on speed and velocity.
3.
Acceleration questions.
4.
The practical on measuring and graphing speed.
5.
The 3 page worksheet on interpreting graphs.
6.
The practical on the Ticker timer with both tables of
results completed and the questions.
If NOT YOU HAVE TO SEE ME IN in my office AT 1pm on
Tuesday & Wed so I can help you finish the work.
Recap- Speed Vs Velocity
Speed is scalar. Scalars are quantities with only magnitude. The
direction does not matter. If you are on the highway whether
traveling 100 km/h south or 100 km/h north, your speed is still 100
km/h.
Distance traveled / Time taken (remember to write the units also and
make sure you are able to convert numbers also to get the correct
answer)
Ex 1: Ruby walks 2 km in 30 min on her way home from school.
What is her average speed for the trip in m/s.
Ex 2: You drive a car for 2.0 hr at 40 km/h, then for another 2.0 hr at
60 km/h.
a. What is your average speed?
b. Do you get the same answer if you drive 100 km at each of the two
speeds?
Ans
2 a. The total distance driven = [(2 h )(40
km /h) + (2 h)( 60 km/h)] = 200 km
The total time = 2 + 2 = 4 h
average speed = (200 km)/(4 h) = 50km/h

2b. total distance = 100 + 100 = 200 km
total time = [(100 km)/(40 km/h) + (100
km)/ 60 km/h)] = 4.17 h
average speed = (200 km)/(4.17 h) = 48
km/h

Velocity
Velocity is a vector. Both direction and
quantity must be stated. If one train has a
velocity of 100km/h north, and a second train
has a velocity of 100km/h south, the two
trains have different velocities, even though
their speed is the same.
Displacement (change in position) / Time taken.
(It is different to speed as there is a
directional component.)
Question: A hiker traveled 80.0 m [S] at
1.00 m/s, then 80.0 m [S] at 5.00 m/s.
What is the hiker's average velocity?
Question: What if Ruby’s journey home
involved her walking from school. 0.5 km
north to Sarah's place, to deliver Sarah's
homework and then walking 1.5 km due
south to her own home, in 2 hours.
What’s the displacement and velocity?
Answers
Answer: displacement = 160.0 m [S] time for the first
part is 80.0÷1.00= 80.0 s,
time for the second part is 80.0 m ÷ 5.00 m/s = 16.0 s.
Total time = 80.0+16.0 = 96.0 s Therefore, the
velocity is (160.0 m [S])÷96.0 s = 1.67 m/s (S)
Gravity's pull on objects is a constant here on Earth (an object will fall at a constant
acceleration of 9.8 or 10 m/s2) and it always pulls toward the center of the planet
(Note: Gravity decreases as you move far away from the surface of the planet.). We
can see how quickly an object gains speed as it falls. It travels at about 10 m/s after
one second, 20 m/s after two seconds, 30 m/s after three. This translates to speeds
of about 36, 72 and 108 km/h after just three seconds. Objects won’t really fall quite
this fast for the same reason that a feather and a rock won’t hit the ground at the
same time when dropped together. Air resistance will slow them to some extent.
Terminal velocity- These 2 sky divers will reach what is called Terminal Velocity which
is the maximum speed where the acceleration is zero. This happens when the
downward force due to gravity is equal to the upward force of air resistance. The
terminal velocity of the sky divers can increase if they change their body shape.
ACCELERATION
What’s acceleration: the rate of change in speed or velocity.
A= Change in Speed/ Change in Time
1. In the graph below (right) what is the acceleration of the sprinter in the first 30
sec?.
2. A car speeds up from 8 m/s to 18 m/s over a
period of 20 s. What is its acceleration in
m/s2?
1. What is deceleration?
1Choose words from the box to complete the sentences. (You may use the
same word more than once.)
distance direction time speed velocity rate acceleration
a How fast an event happens is its ____________________________
b How fast somethingΥsposition changes is its __________________
c Speed is calculated using __________ and ___________________
d Velocity is different from speed because it has ________________
e Speeding up and slowing down are both types of _______________
f Like velocity, acceleration has ____________________________ .
g An object speeds up when its acceleration and _________ have the
same direction.
Forces on a car
Engine pushes
forward
Resistance forces
that push against
the direction of
movement: Drag
etc. pulls back (air
resistance and
friction)
(Horizontal forces only shown)
Forces on a car
Road pushes up
Engine pushes
forward
Resistance forces
that push against
the direction of
movement: Drag
etc. pulls back (air
resistance and
friction)
Weight pulls
car down
NEWTON’S FIRST LAW OF
MOTION
An object will remain at rest or will not change its speed or
direction unless it is acted upon by an outside, unbalance force
 Also known as the Law of Inertia; inertia is the scientific principle
behind why we wear seat belts. Inertia makes things difficult to stop
as well as hard to get started.
 All objects possess inertia or a tendency to resist change. The
larger the mass, the greater the inertia. This is why it is much easier
to stop an empty runaway shopping trolley than a full one.
Coins on elbow trick! How does it work according to this law?
http://www.youtube.com/watch?v=NWE_aGqfUDs&feature=related
Lego one
http://www.youtube.com/watch?v=7_fbH-muvlw&feature=fvw

NEWTON’S FIRST LAW OF
MOTION is not always apparent.
Friction and air resistance are always
present
 You don’t always realise you’re moving!

◦ Is it your train moving forward, or the one
next to you going backwards?

You can get a false sense of security in a
car.
ACTIVITY
LOOKING AT INERTIA (Pg 148)
 FORCING THE ISSUE (Handout)
 EXPERIMENT: Crash test dummies
 EXPERIMENT: Inertia in Motion

CRASH TEST DUMMIES
WITHOUT
SEAT BELT
Shallow Slope
20cm to Brick
40cm to Brick
60cm to Brick
80cm to Brick
100cm to Brick
WITH SEAT
BELT
WITH CRUMPLE
ZONE
FORCING THE ISSUE HANDOUT
Situation 2:
Situation 1:
A.
A.
B.
A
B,
Describe what would happen in these situations
to the people in trolleys A and B.
FORCING THE ISSUE
HANDOUT
Situation 1:
Situation 2:
Describe why it is possible to
perform the following tricks
Crash testing
Ready
Set
Go!
Source: http://www.inrialpes.fr
Crash testing
Ready
Set
Go!
Source: http://www.inrialpes.fr
NEWTON’S SECOND LAW OF
MOTION
The mass of an object affects the way
that it moves when acted upon by
one or more forces
a= F/ m
Acceleration
(measured in
meters per
second
squared- m/s )
The total force of
the object
(measured in
Newtons- N)
The mass of the
object (measured in
kg)
NEWTON’S SECOND LAW OF
MOTION
Describes the fact that larger masses
accelerate less rapidly than smaller
massed acted on by the same total force
 Can also be expressed as F= ma
Force=Newtons Mass=Kg Acceleration=m/s/s
 If the mass of the brick and car is 2kg and
the force applied is 4 newtons, what is ‘a’?

NEWTON’S THIRD LAW OF
MOTION
For every action there is an equal
an opposite reaction
When an object applies a force to a
second object, the second object applies
an equal and opposite force back to the
first object
ACTIVITY

Think about three examples that demonstrate
an action – reaction forces
1)
Pushing off a starting block. Action- the push
and the Reaction- is the block pushing back
Pushing water while swimming- water pushes
back on your hands as you swim
http://www.wonderhowto.com/how-todemonstrate-newtons-third-law-motion223910/
2)
3)
WORK DONE



When ever you apply a force (a push or a pull. Measured in
Newtons) to an object it moves in direction of that force
Energy is the capacity of something to do work and there are many
types of energy (gravitational, electrical, heat, potential, kinetic).
Energy can never be created or destroyed (this is the Law of
Conservation of Energy), it just changes from one form to another.
Work is another physical term. Work is done on an object when a
force is applied over a distance. If an object does not move then
work done= 0. Work is measured in joules (like energy)
Work Done (newton*meter= joules)
= Force (N) X distance travelled in the
direction of the force ( M)
1. A man digging for an hour to create a hole for a well, and a women trying
unsuccessfully for 10 minutes to loosen a tough lid.
Who Has Done Work?
Ans: The woma n has done no work on the lid as there has been no moveme nt. The man
has done work on his surroundings. The women has done work within her own body and
she may be getting a sweat up.
2. If a woman pushes a car with force of 400 N for 200 M, how much work has
been done?
Ans:
W= 400 N * 200M
W= 8000 j
Think pg 155 - answers

A) Gardener pushes with a horizontal
force of 300 N, the lawnmower moves
5meters across the lawn.
Work done = F x Distance
= 300 x 5
= 1500 Nm
Think pg 155
B) Three people try to push a car out of a
mud bog. Each person pushes with a force
of 400 N , however the car does not
move.
Work Done = F x distance travelled
= 400 X 0
=0
Energy transformations
Energy can be transformed from one
form to another, for example:
◦ Fuel (chemical) energy to kinetic energy
◦ Gravitational potential energy to kinetic
energy
◦ Kinetic energy to heat
◦ Etc.
Conservation of energy
When energy is transformed from one
form to another, the total amount of
energy remains the same.
This is a very useful principle if you can
identify where all the energy has come
from and where it is going.