Transcript Document

Forces
SPECIFIC LEARNING OUTCOMES:
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By the end of this topic you should be able to:
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Give examples of everyday forces (push/pull/twist)
Label simple force diagrams (driving/thrust, reaction/support/buoyancy,
friction/drag, gravity)
Recognise the distance forces of gravity and magnetism and the contact force of
friction and be able to apply their knowledge to contexts as given
Use arrows as indicator of force size and direction
Label a simple net force diagram, and calculate net forces
demonstrate/describe the effect of net force (collisions)
describe the difference between weight (a force due to gravity) and mass (the
amount of matter in an object)
stopping distances / reaction times
understand how simple machines work (levers or pulleys or…)
Motion (optional)
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– calculate speed
– interpret speed-time graphs
– interpret distance-time graph
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A force is a push or a pull
For example the following forces are:
Gravity
A strong wind blowing
a car over.
Removing bluetack
from the wall with
your fingers.
Push / pull
(Circle correct)
Push / pull
Push / pull
Advanced examples for experts…
A piston in a car.
Air passing over an
Airplane wing.
Push / pull
Push / pull
Forces have direction.
We use arrows to show the direction that the force is acting. Example:
Direction of force is indicated by the arrow.
Forces Motion and Velocity
• In Science we like to use the word velocity instead of
the word speed. This is because velocity is a measure
of speed and direction. Speed does not normally
include the direction in which an object was traveling.
For example
• A train is traveling at a speed of 68 km hr-1
• A train is traveling north west at a velocity of 68 km hr-1
How to calculate velocity
• The formula for velocity is
Velocity (v) =
distance (d)
time (t)
d
v t
Distance Vs Time Graphs
• Distance time graphs clearly show the motion (movement) of an object.
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More slope (steeper) means the velocity of the object is ________.
Less slope (flatter) means the velocity of the object is ________.
If the line is completely flat then the object is ________.
Time
Distance Vs Time
Distance
On the graph above – label where:
The object is stationary
The object has the greatest velocity
The object is moving backwards -
Distance vs time Graphs
Distance
Distance Vs Time
Time
Distance
Distance Vs Time
Time
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Copy the graph above and label where:
The object is stationary
The object has the greatest velocity
The object is moving backwards -
The faster you go the bigger the mess
SPEED
THINKING DISTANCE BRAKING DISTANCE
OVERALL STOPPING
DISTANCE
32 km hr-1
20 ft. (6 m)
20 ft. (6 m)
40 ft. (12 m)
48 km hr-1
30 ft. (9 m)
45 ft. (14 m)
75 ft. (23 m)
64 km hr-1
40 ft. (12 m)
80 ft. (24 m)
120 ft. (36 m)
80 km hr-1
50 ft. (15 m)
125ft. (38 m)
175 ft. (53 m)
96 km hr-1
60 ft. (18 m)
180 ft. (55 m)
240 ft. (73 m)
112 km hr-1
70 ft. (21 m)
245 ft. (75 m)
315 ft. (96 m)
Mass and Weight
• It's important to understand the difference
between weight and mass.
• We often use the terms "mass" and "weight"
interchangeably in our daily speech, but in their
true meanings they are completely different
things.
• The mass of a object is how much matter
(atoms) it contains.
• Your weight is the force on the ground caused by
gravity.
• To measure your weight all you have to do is
stand on a scale! Scales measure the force of
attraction between you and the Earth. This force
of attraction between you and the Earth (or any
other planet) is called your weight.
• If you are in a spaceship far between the stars
and you put a scale underneath you, the scale
would read zero. Your weight is zero. You are
weightless. There is an anvil floating next to you.
It's also weightless. But are you or the anvil massless?.... No.