motion
DESCRIPTION
Motion. There are four sections to this…. 1. Speed and Acceleration. 2. Forces. 3. Work. 4. Power. Click on the section you want. 1. Speed and Acceleration. D. distance. Speed =. time. S. T. D. x. S =. T. Speed. Speed tells you how fast something is travelling. - PowerPoint PPT PresentationTRANSCRIPT
Motion
There are four sections to this….
1. Speed and Acceleration
2. Forces
3. Work
4. Power
Click on the section you want.
1. Speed and Acceleration
Speed
S
D
TxS =
Speed =distance
time
D
T
Speed tells you how fast something is travelling
A turtle travels 2 metres in 300 seconds. What speed
is it travelling?
S
D
Tx
S = D
T
S =2 m
300 s
S = 0.007 m/s
Distance
S
D
TxS =
Speed =distance
time
D
T
Distance = speed x time
D = S x T
A 747 travels from L.A. to Auckland at 700 km/hr. The trip takes 9 hours. How far
has the plane travelled?
D = S x T
S
D
TxD = 700 x 9
D = 6,300 km
Time
S
D
TxS =
Speed =distance
time
D
T
Distance = speed x time
D = S x T
Time = distance
speed
T =D
S
A train travels from Wellington to Auckland at an average speed
of 110 km/hr. The distance it covers is 550km. How long does
the trip take?
S
D
Tx
T =D
S
T =
550 km
110 km/hr
T = 5 hours
Units
The Standard units are :
• distance = metres = m
• time = seconds = s
• speed = metres per second = m/s
Be careful!
• Look at the units given to you in a question.
•Convert all parts of a question into the same units.
•Remember to include the units in your answer.
Distance – Time Graphs
Time (s)
Distance
(m)
A.
Time (s)
Distance
(m)
B.
Time (s)
Distance
(m)
C.
Constant speed Constant speed (but slower than A)
Stationary
The gradient of the line is the speed.
The steeper the line, the faster the speed!
Distance – Time Graphs
Time (s)
Distance
(m)
A. Constant speed
Time (s)
Distance
(m)
B.Acceleration
Time (s)
Distance
(m)
C.Deceleration
If the line on a distance – time graph is curved then the object
must be changing speed.
i.e. getting faster or slower.
Acceleration
Acceleration =change in speed (m/s)
time (s) a
s
tx
Units = m/s
s= metres per second squared.
= change
= m/s2 = ms-2
When the parachutist jumps from the plane his speed downwards goes from 0m/s to 20m/s in just 2 seconds.
20 m/sa =2s
= 10 m/s2
a
s
tx
His change in speed is from 0m/s to 20 m/s
So his acceleration is…….
(10 m/s2 is the acceleration due to gravity)
Speed – Time graphs
speed
m/s
Time (s)
Increasing speed = acceleration
Steady speed = no acceleration
Decreasing speed = deceleration
Acceleration can also be shown on distance – time graphs:
Distance (m)
Time (s)
Acceleration
Deceleration
2. Forces
A Force is a:
push
pull
twist
Forces may be:
Contact Non-contact (field)
friction upthrust
buoyancyair-resistance
magnetic electrical
gravitational
Forces always act in pairs.
ActionReaction
ThrustDrag
Gravity Buoyancy (Upthrust)
If the forces are balanced
the object will not
change its speed,
direction or shape
Unbalanced forces however….
Gravity buoyancy
… will make an object
change speed, shape or direction.
The Stages of a Parachute
Jump 2. Freefall
3. Deployment
4. Descent
There are different forces acting at each
stage. Try to work them out before going to the next
slide.
1. Exit
Exit
The force pulling the man down is his weight.
At first this is far more than the upthrust (air resistance) pushing him upwards
… so he accelerates downwards!
Freefall
As he accelerates down his weight stays the same
… but the upthrust (air resistance) increases rapidly.
Once they are the same he stops accelerating and falls at a steady speed (terminal
velocity).
Deployment
As he opens his parachute his weight
is still the same.
… but the upthrust due to air resistance is suddenly
huge!
So he decelerates
rapidly.
Descent
Once again, his weight hasn’t
changed
… but as he slows the upthrust
decreases until it equals his weight
again.
So, once again he descends at a steady (but slower) speed.
3. Work
Work done = Force x Distance Moved
W
F x D
•Work is actually measuring the amount of energy that has been converted from one form to another……
….. So its units is JOULES
1.5m
2kg2kg
How much work is done to lift the two bricks on to the table?
Work = Force x Distance moved
Force (weight) = mass x acceleration
= 4kg x 10 m/s2
= 40N
40N x 1.5m = 60J
1.5m
2kg2kg
How much work is done to lift the two bricks on to the table?
Work = Force x Distance moved 40N x 1.5m = 60J
It takes 1 joule of energy to do move a force of 1N for 1 metre
So in this example the work done is 60Joules
1.5m
How much work is done to lift the two bricks on to the table one at a time?
2kg2kg
2kg => 20N 2kg => 20N
20N x 1.5m = 30 J Two bricks needing
30J of work each = 60J
The work done is the same!
1.5m
2kg2kg
Lifting the bricks separately means less force each time, but twice as much distance moved overall.
2kg2kg
1.5m1.5m
Some machines work this way …..
….. Using a small force over a large distance to move a larger force over a smaller one.
60J 60J
1.5m
3m
The ramp is an inclined plane. It increases the distance moved so it decreases the force needed.
(small force , big movement big force, small movement)
20N10N
Levers Levers are usually used as force multipliers.
A small force moving a long distance
Lifts a large force a short distance
Here are some more force multipliers.(small force, big movement big force, small movement)
….. Alternatively levers can be distance
multipliers
Big force, small distance
Small force, big distance
For instance:
4. Power (Watts)Power measures how fast work is being done.
P
w
t
Power =work done
time taken
A weightlifter lifts 350kg up 2.5 metres in 2 seconds.
How much power does he use?
Force (weight) = 350 kg x 10 m/s2 = 3,500N
Force = mass x acceleration
Work done = force x distance
Work done = 3,500 N x 2.5 m = 8,750 J
Power `=work done
time taken
Power = 8,750 J
2 s= 4,375 W
END