Forces
Chapter 2.06 – 2.12
Do Now:
• Create a mind map (spider diagram) with
everything you believe you know about forces.
– What is a force?
– Types of forces?
– Where do you find forces? Etc.
Force
• Force – a push or a pull on an object
– Measured in Newtons (N)
– 1 Newton is the force required to give a mass of 1
kilogram an acceleration of 1 m/s2
– 1N = 1kg·m/s2
Types of Forces:
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Gravity
Friction
Air Resistance
What others can you think of?
Force of Gravity
• What is gravity?
– Think about our free-fall lab
– Where else can you find gravity on Earth?
Weight
• Weight – Force of gravity that pulls you
towards the Earth
• Mass – how much space an object occupies
• How does your weight change on the moon?
• How does your change on the moon?
What about other planets?
• How would your weight change? Mass?
Balanced Forces
• Balanced Forces – Forces that cancel each
other out
• Gravity pulls you towards the Earth
– You do not fall into the centre of the Earth. Why?
What if the forces do not balance?
• Resultant force – Total force acting on an
object
What is the force?
Free-Fall Forces
• What is the acceleration of a free-falling
object?
• What forces are acting on that object?
Terminal Velocity
• Terminal Velocity – maximum velocity of an
object in free-fall due to air resistance
– No longer accelerating, air resistance and gravity
will even out causing the object to fall at a
constant velocity
Homework:
• Page 37 #1-2
Do Now:
• Find the result force for each of the following:
3N
7N
5N
2N
9N
2N
3N
Page 37
• Lets look at question #3
Newton’s 1st Law of Motion
• If there is no resultant force acting on an
object, it will either stay at rest, or move with
constant velocity
• Inertia – Resistance to change in velocity
• What happens to you when you slam on the
breaks in your car?
• https://www.youtube.com/watch?v=6gzCeXD
hUAA
Newton’s 2nd Law of Motion
• What happens if forces acting on an object are
not balanced?
• F = ma
• Force = mass x acceleration
Balanced Forces
Practice
• A net force of 15 N is exerted on an
encyclopedia to cause it to accelerate at a rate
of 5 m/s2. Determine the mass of the
encyclopedia.
One More…
• Engineers at the Johnson Space Center must
determine the net force needed for a rocket to
achieve an acceleration of 70 m/s2. If the
mass of the rocket is 45,000 kg, how much net
force is needed?
Newton’s 3rd Law
• For every action there is an equal, but
opposite reaction.
• If a body A exerts a force on body B, body B
will exert an equal but opposite force on body
A.
Activity:
• Practice using Newton’s 2nd Law to solve for:
– Force
– Mass
– Acceleration
Do Now:
• Page 39 # 1-2
Weight
• What force holds each of us to the Earth?
• Why does it change on other planets?
True or false?
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If the earth stopped spinning we would float into space.
There is no gravity on the moon.
NASA have an anti-gravity room where gravity can be
turned off and people can float.
It is possible to shield the force of gravity by using lead or
other materials.
Isaac Newton discovered gravity.
Ms. Petit is the greatest science teacher you’ve ever had.
You have no mass in space.
There is no gravity in space because you are far from the
earth.
Weight is measured in kilograms.
There is no gravity in space because space is a vacuum.
If we could suck all the air out of the classroom, we would
float around.
What happens if there is no gravity?
• Gravitational Force – downward pull from the
Earth
• Weightlessness – No gravitational pull
Freefall dog!
Water on the ISS
Using Gravity to Calculate Weight
• Recall: Newton’s 2nd Law of Motion
– F = ma
• What was the acceleration due to gravity on a
free-falling object?
W=mg
Lets Practice:
• A ball as a mass of 10 g. What is the ball’s
weight in Newtons?
One More:
• A block has a weight of 150 N. Determine the
mass of the block.
Activity:
• Complete the worksheet
Hooke’s Law
• Please copy the table into your notes:
Activity:
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You are going to investigate how much a spring stretches when a force is applied to
it.
The amount a spring stretches is called its extension. This is the difference in
length between the stretched spring and the length of the spring when it was
unstretched.
(Remember we are looking at the force on the spring. A mass of 100g will have a
weight (force of gravity pulling it down) of 1 Newton.
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Add masses to your spring and carefully measure its extension. You can do this
until the spring breaks! (but you must wear goggles and be careful during this
investigation).
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Your experimental report will include the following
A table of results
A graph of your results
A conclusion describing what you have discovered (think about this!)
Graph:
Set-Up
How to Read Your Graph:
F = kx
• k is called the spring constant and is a
measure of the stiffness of the spring or
material
• It has units of N/m (newtons per metre)
• The higher the k the stiffer the spring
• Materials with a high k need a large force to
for a given extension
Practice Problem:
• A force is applied to a spring of 5 N. The
original length of the spring is 5 cm, the force
cause the spring to extend to a length of 8 cm.
What is the spring constant of the spring?