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Forces
Forces
• Force: push or pull
• Any unbalanced force acting on a object will
cause that object to accelerate.
▫ This is Newton’s 2nd Law of Motion.
Fnet=ma
• Units:
▫ Force (F) – Newtons (N)
▫ Mass (m) ≈ amount of particles that are in an object
– kilograms (kg)
▫ Acceleration (a) – m/s2
Net Force
10 N
• What is the net force?
• What direction will it accelerate?
4 Newton
1 Newton
30 N
30 N
2N
80 N
4N
80 N
Fnet = ma Example Problems
(Not in
Packet)
1. If a 350-kg object accelerates at a rate of
30 m/s2, what is the net force acting on the object?
2. Find the mass of an object that accelerates at
5 m/s2 if you push it with a force of 40 N.
3. What is the acceleration of a 12-kg box that is being
pulled with a 10 N force to the left and a
15 N force to the right?
Force of Gravity
• Force of Gravity:
▫ All objects that have mass attract each other.
▫ Weak, long-distance force
• Objects that are close together experience a
stronger gravitational attractive force than
objects that are farther apart.
Free Fall
• Free Fall – the only force acting on an object
is gravity.
• Demo: Which will hit the ground first?:
▫ 1 penny?
▫ 4 pennies taped together?
• ALL objects accelerate towards the ground at
a rate of 9.8 m/s2!
Acceleration due to Gravity
• Acceleration due to gravity (g) will be different:
1. On different planets (on the moon, g = 1.6 m/s2)
2. At high vs. low elevations on Earth
F = ma
Fg = mg
 Applied to Gravity!
• Units:
▫ Force of Gravity (Fg) or Weight – Newtons (N)
▫ Mass (m) - kilograms (kg)
▫ Acceleration due to gravity (g) on Earth:
g = 9.8 m/s2
How do you measure
the force of gravity? –
Spring Scale!
Example Problems
• If your mass is 68.2 kg,
what is your weight?
▫ A) On Earth?
▫ B) On the moon?
MASS DOES NOT CHANGE!
Rock Falling:
Photo taken every
second.
Acceleration Due to Gravity
a=(vf-vi)/Δt
1) A rock is dropped from rest
off of a very high cliff.
What is its speed
after 3 seconds?
2) A rock is thrown downward
with an initial velocity of 2 m/s.
What is its speed
after 2 seconds?
Friction Forces
• Friction – A force that occurs between two
surfaces in contact & opposes motion.
• What causes Friction?
Static Friction
• Static Friction – the friction between surfaces
that are stationary (not moving).
• Ex. When a car is parked on a hill,
the static friction between the tires
and the road keeps the car from
sliding down the hill.
Kinetic Friction
• Kinetic Friction – The friction between moving
surfaces.
• 3 Types:
1) Sliding
2) Rolling
3) Fluid – liquid and gas (ex. air resistance)
Worksheet
Which is harder to overcome:
static or kinetic friction?
• Does it take more force to get the sofa started
moving (overcome static friction) or
to keep the sofa moving (overcome kinetic
friction)?
• Static Friction is always
greater than
Kinetic Friction.
How can friction
be harmful?
• Example: Your car engine heats up because of friction
between the moving parts in the motor. Some of the
energy that we get from gasoline changed into heat and
cannot be used to power the car (move the wheels).
• Machines cannot be 100% efficient because of
friction!
How can we reduce friction?
1) Make surfaces smoother.
2) Add a lubricant – ex. water, oil/grease,
air (like an air hockey table).
How can friction be helpful?
• Walking – What happens when you try to walk on a wet
floor? On ice?
• Writing with a pencil
• Washing Dishes – Use an abrasive (rough) sponge!
• Prevent Hydroplaning
How can friction be increased?
1. Make surfaces rougher.
2. Put sand between surfaces.
Friction Lab
Lab – Part 1
• Which side has greater static friction?
• Procedure: Place the block on the top surface of the dry
erase board. Lift one end of the dry erase board just
until the block begins to slide. Repeat with the bottom
surface of the board.
• Compare the height that you had to lift the board to get
the block started sliding for each side.
• The higher that you have to lift it, the greater the force of
static friction that you are overcoming.
Lab – Part 2
• Which is greater: static or kinetic friction?
• Procedure: Place block at one end of dry erase board
(on the rougher side). Place mass on top of block. Attach
spring scale to hook. Pull horizontally across board at
constant velocity. Watch closely and record the force readings
from the spring scale. Repeat several times to be sure that
you are getting accurate data.
• Record:
▫ Static Friction – Force necessary to get it started moving.
▫ Kinetic Friction – Force necessary to keep it moving after it has
already started.
Lab
• ANSWER LAB QUESTIONS IN
COMPLETE SENTENCES, PLEASE!!!!
Newton’s 3 Laws of Motion
Free Body Diagrams
• What are all the forces acting on the object?
man
car
Balanced Forces (Net Force = 0)
Newton’s 1st Law
(No Forces)
1st Law: When balanced forces are acting on an
object (net force = 0):
a) An object at rest will stay at rest.
b) An object in motion will
continue moving at constant velocity
(constant speed & straight line).
Newton’s 1st Law (cont’d)
• Also called the Law of Inertia.
• Inertia – resistance to a change in motion.
▫ Objects with more mass have more inertia.
▫ Ex. A car that is at rest
wants to stay at rest.
It will resist, if you try to
push it. 
Newton’s 1st Law
• Examples:
1. A book sitting on a table
2. Ice Skating, Air Hockey, Air Track (constant speed)
3. Paper Plate minus a piece of pie
(constant direction)
4. Turning a corner in a car
- Your body keeps going straight!
5. Why seatbelts and airbags are
important…
Why are
seatbelts
and airbags
important?
Your body keeps
going straight if
you don’t have a
seatbelt to stop
you!
Air Resistance &
Terminal Velocity
•As a person free-falls, the person
increases in speed.
accelerating
F air
•As the person increases in speed,
resistance the force of air resistance
increases.
accelerating
•When the Force of Air Resistance
(up) cancels out the Force of Gravity
(down), the person reaches
Constant
Terminal Velocity.
speed
•Once the forces balance out… the
person continues to move at a
constant speed.
Terminal Velocity
• When the force of air resistance (up) = the force
of gravity (down), an object falls at constant
speed.
Thought Question
• Who will have the higher terminal velocity:
heavier or lighter person? Why?
• The heavier person
 The heavier person must free fall for more
time (increase to a faster speed) before the air
resistance will cancel out the larger weight.
Unbalanced forces
Newton’s 2nd Law
2nd Law: When unbalanced forces
are acting on an object (net force is NOT zero),
the object will accelerate.
▫ Fnet = ma
▫ The more mass an object has,
the greater the force required
to accelerate it.
Thought Question
• Which would accelerate faster if
you kicked it with a 20 N force? Why?
▫ Soccer Ball?
▫ Bowling Ball?
F B on A
Newton’s 3rd Law
F A on B
• 3rd Law: For every action,
there is an equal and opposite reaction.
▫ All forces act in pairs. Force pairs do not act on
the same object and therefore do not cancel out.
#1 Reason why you should
not get in a fight…
Action: You punch someone in
the face.
Reaction: The face exerts an
equal force back on your hand.
3rd Law Examples
Action: The books exert a
downward force on the table.
Reaction: The table pushes
with an equal force up on the
books.
Action: The girl exerts a force
on the wall.
Reaction: The wall pushes
back at the girl with an equal
force.
3rd Law: Rockets
• Action Force:
Rocket engines push gas particles down and
out the bottom of the rocket.
• Reaction Force:
The gas particles also push back
(up) on the rocket. The rocket will be
propelled upward.
• Because of the First Law,
spaceships do not need much
fuel -- once they are moving,
they will stay in motion.
(In space: fewer particles
& therefore less friction.)
DEMO
Your Turn... 3rd Law
• Explain how paddling a canoe demonstrates
Newton’s 3rd Law of Motion.
Paddle exerts a
force on the water
Water exerts equal
force back on paddle
Newton’s 3rd Law
On frictionless surfaces…
Newton’s 3 Laws of Motion (recap)
1st: What happens when you have balanced forces?
1st a) Objects at rest stay at rest.
b) Objects in motion stay in motion at constant
velocity.
2nd: What happens when you have unbalanced forces?
2nd F = ma (Acceleration!)
3rd: All forces act in pairs.
3rd For every action,
reaction.
there is an equal but opposite
Which Law? WS
Frictionless Surfaces WS
Double Trouble – Intro
Use your understanding of this
equation to answer the following
questions:
•F = ma
Examples –
How would acceleration be affected?
• If you tripled the force that you apply to the
object?
• If you decreased the force to 1/3 its original
value?
• If you doubled the mass of your object (keeping
force constant)?
• If you decreased your mass to 1/2 its original
value?
Newton’s Laws Flashcards
Newton’s Laws Project
Newton’s 1st Law
Newton’s 2nd Law
{Magazine photo}
{Magazine photo}
Summary of Law:
Summary of Law:
How does your photo
demonstrate the law?:
How does your photo
demonstrate the law?:
Newton’s 3rd Law
{Magazine photo}
Label force pairs!
Summary of Law:
How does your photo
demonstrate the law?:
List the action/
reaction forces!
Motion Matching
• Centripetal Force: force that keeps objects
moving in a circle.
• Momentum: depends on the mass and velocity
of an object
▫ A large truck is going to have more momentum
(be more difficult to stop) than a small truck.
▫ Newton’s Cradle
REVIEW FOR FORCES TEST
Balanced & Unbalanced Forces:
Which smilies will accelerate?
A
D
G
B
E
C
F
If the forces are balanced, the
object will not accelerate.
If the forces are unbalanced, the
object WILL accelerate.
Which law is also called the
law of inertia?
Which has more inertia:
the toy car or the real car?
Terminal Velocity occurs
when the force of _______
cancels out the force of
_________ and the falling
object continues to move at
constant _______.
A 50 kg object is transferred
from the Earth to the Moon.
A) Does its mass change?
B) Does its weight change?
A) Kinetic friction occurs between
objects that are _________.
B) Static Friction occurs between
objects that are _______.
A) How could you reduce
friction?
B) How could you increase
friction?
A) How is friction harmful?
B) How is friction helpful?
The tendency of matter to
resist a change in motion is
called…
Newton’s Laws
Examples:
Which Law?
Which Law is This?
If the net force on an object
is NOT zero, the object will
accelerate.
Which Law is This?
An object at rest will remain at rest.
An object in motion will remain in
motion at constant speed in a
straight line.
Which Law is This?
All forces act in pairs.
These forces act on different objects.
The forces are equal in size
but in the opposite direction.
Which law explains what
happens to an object
when all forces are
balanced?
This diagram represents which law?
FA on B = - FB on A
F B on A
F A on B
You are driving your car and
you turn left.
Your body leans towards the
right side of the car.
You drop a pencil and it falls
to the ground.
You are heading to the lake
with a canoe on top of your
car. You make a sudden
stop and the canoe slides
forwards.
You hit a tennis ball with a
racket. The racket bounces
backwards.
You and your little cousin are
playing basketball. Your
cousin throws as hard as he
can, but he cannot make the
basketball reach the basket.
Which law explains how you
can move through the water
while swimming?
On a cold wintery day, you
step on some ice and go
sliding.
Which law explains why a
rocket goes up?
Use these equations for the
next problems:
F = ma
Fg = mg
a = ( vf – vi )/ t
a) What does “g” stand for?
b) What is “g” on Earth?
What is the weight of
a 30 kg object?
A force of 20 N causes an
object to accelerate at
4 m/s2.
What is the object’s mass?
If an object has a force of
gravity of 100 N acting on it,
what is the object’s mass?
A force of 3 N acts on a
6 kg object.
What is the object’s
acceleration?
An object is dropped from rest
and falls for 2.5 seconds.
What is its final velocity?