Getting an A in Science

1
Getting an A in Science

• Taking ownership or control of the information
  learned in class and taking responsibility for
  your work
• This means
• Practicing (reviewing, restating, preparing)
• Applying (creating, thinking, using)
• Planning (studying, completing projects and
  homework, being prepared-pencils, books)

2
Intro to Physics

• What is physics?
• https//www.youtube.com/watch?v2vYI2NcVsXY
• https//www.youtube.com/watch?vNd4PlvzMsaQ

3
Motion Forces

• Chapter 2

4
Motion

• What does motion mean?
• How can we tell if something is in motion?
• Is this marble in motion?
• How do you know?
• How can we provide evidence?

5
Motion

• Motion a change in position, measured by
  distance and time. 

6
Frame of Reference

• Determining if an object changes position
  requires a frame of reference.
• Frame of reference is the object or point from
  which movement is determined

What is the frame of reference for this video?
How do I know the skater is in motion?
7
Frame of Reference

• Movement is relative to an object that appears
  stationary
• Earth is the most common frame of reference

8
Motion of the Marble

• We can tell if the marble is in motion by
  measuring its change in position along the ruler
  (our frame of reference).
• Right now, the marble is NOT in motion. It is
  not changing position. How can we get it to
  change position?

9
How to make things move

• https//www.youtube.com/watch?vLEs9J2IQIZY

10
Force

• A force is a push or a pull
• Forces can affect motion in several ways.
• Forces can make objects
• Start moving
• Move faster
• Move slower
• Stop moving
• Change direction
• Change shape

11
What forces are at work

• During a rocket launch?
• Upwards force (thrust) from engines
• Downwards force from gravity
• Downwards force (drag) from air resistance

12
What forces are at work

• pushing a truck?
• Forwards force from pushing
• Backwards force from friction
• Downwards force from gravity
• Upwards force from ground.

13
What forces are at work

• Kicking a football?
• Upwards force from kick
• Downwards force from gravity
• Backwards force from air friction

Fair
Fkick
Fgrav
14
Friction

• Friction
• force that opposes motion between 2 surfaces
• It depends on the
• types of surfaces
• force between the surfaces

15
Friction

• Friction is greater...
• between rough surfaces
• when theres a greater force between the surfaces
  (e.g. more weight)

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How strong is the force?

• Forces are measured in Newtons

https//www.youtube.com/watch?vPCxP24qj2UQ
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Wrong kind of force
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Measuring Forces that cause motion

• Units are Newtons (N)
• Forces are measured using a spring scale or force
  meter.

https//www.youtube.com/watch?vAFu1advEnhw
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Forces occur in pairs

• The force pairs can be balanced or unbalanced

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Balanced Forces

• Balanced forces cause NO change in motion
• Balanced forces are equal in size and opposite in
  direction

21
Balance forces no motion
22
Unbalanced Forces

• Unbalanced forces always cause a change in
  motion, speed, or direction
• Which way will the boy in the middle be pulled?

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Unbalanced ForcesCauses Motion
Adding Forces
Subtracting Forces
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Unbalanced ForcesCauses motion
Adding Forces
Object
5 N
5 N
Notice that all the forces are pointed in the
same direction. Hence they add together.
25
Subtracting Forces
10 N
5 N
5 N

• Two forces can subtract to produce a net force in
  the direction of the larger force.
• Negative motion occurs (Slow Down)

26
Unbalanced ForcesCauses Motion
Subtracting Forces
Object
5 N
5 N
5 N
Notice that all the forces are unequal and
pointed in the opposite direction. Hence they
are unbalanced and in opposition to each other
or one partially cancels the other.
27
Which game will result in a winning team? How
do you know?
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Balanced Forces
Balanced Push i.e. Pushing a Car No motion
Balanced Pull i.e. Tug-of-war No motion
10N
10N
10N
10N
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Un-Balanced Forces
Additive Un-Balanced Same Direction i.e. sled Faster motion
Subtractive Un-Balanced Opposite Direction i.e. football tackle Slower motion
10N
10N
10N
10N
10N
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Why do I care?

• Understanding an objects motion can help you
  understand how to interact with the world around
  you.

31
Review of balanced and unbalanced forces
https//www.youtube.com/watch?vHEJOybRxclk
32
Stop here
33
Universal Law of Gravitation

• Mass and gravity have a directly proportional
  relationship.
• The distance between the centers of masses of the
  two objects and gravity have an inversely
  proportional relationship.

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Gravity

• Gravity
• force of attraction between any two objects in
  the universe
• increases as...
• mass increases
• distance decreases

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Gravity

• Who experiences more gravity - the astronaut or
  the politician?

• Which exerts more gravity -
• the Earth or the moon?

36
Gravity

• Would you weigh more on Earth or Jupiter?

• Jupiter because...

greater mass
greater gravity
greater weight
37
Balanced Forces (Balanced Forces No Motion)
Ground pushes up
Gravity pulls down
Gravity pulls down on you The ground pushes
back up THIS KEEPS YOU WHERE YOU ARE!
If these football players push on each other
equally as hard, will either one move?
38
Newtons First Law

• Newtons First Law of Motion
• An object at rest will remain at rest and an
  object in motion will continue moving at a
  constant velocity unless acted upon by a net or
  unbalanced force.

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Two Key Points
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Newtons First Law

• Newtons First Law of Motion
• Law of Inertia
• Inertia
• tendency of an object to resist any change in its
  motion
• increases as mass increases

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Can you explain how this trick works, using
newtons first law of motion?
42
(No Transcript)
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Newtons Second Law

• Newtons Second Law of Motion
• The acceleration of an object is directly
  proportional to the net force acting on it and
  inversely proportional to its mass.

F ma
44
Newtons Second Law
F
m
F ma
F force (N) m mass (kg) a accel (m/s2) 1 N
1 kg m/s2
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Calculations

• What force would be required to accelerate a 40
  kg mass by 4 m/s2?

GIVEN F ? m 40 kg a 4 m/s2
WORK F ma F (40 kg)(4 m/s2) F 160 N
46
Calculations

• A 4.0 kg shotput is thrown with 30 N of force.
  What is its acceleration?

GIVEN m 4.0 kg F 30 N a ?
WORK a F m a (30 N) (4.0 kg) a 7.5
m/s2
47
ConcepTest

• Is the following statement true or false?
• An astronaut has less mass on the moon since the
  moon exerts a weaker gravitational force.

• False! Mass does not depend on gravity, weight
  does. The astronaut has less weight on the moon.

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Newtons Third Law

• Newtons Third Law of Motion
• When one object exerts a force on a second
  object, the second object exerts an equal but
  opposite force on the first.

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Newtons Third Law

• Action-Reaction Pairs

• The hammer exerts a force on the nail to the
  right.
• The nail exerts an equal but opposite force on
  the hammer to the left.

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Newtons Third Law

• Action-Reaction Pairs

• The rocket exerts a downward force on the exhaust
  gases.
• The gases exert an equal but opposite upward
  force on the rocket.

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Newtons Third Law

• Action-Reaction Pairs

• Both objects accelerate.
• The amount of acceleration depends on the mass of
  the object and the force applied.
• Force mass x acceleration Fma

• Small mass ? more acceleration
• Large mass ? less acceleration