Download Forces

Review of Newton’s Laws

Newton’s First Law
 Objects at rest tend to stay at rest and
objects in motion tend to stay in motion at
the same velocity unless acted upon by an
outside force.
Inertia
Newton’s 1st Law is also called the law
of inertia.
 Inertia

 An object’s resistance to change in motion.
 Where does inertia come from?
○ Mass
Which of the objects in each pair
has more inertia?
Feather
or
bird
Which of the objects in each pair
has more inertia?
cat
or
bird
Which of the objects in each pair
has more inertia?
Cat
or
dog
Which of the objects in each pair
has more inertia?
Dog
or
bear
Which of the objects in each pair
has more inertia?
Bear
or
Bill Nye

If Bill Nye was being chased by a bear in
the woods, which path should Bill
choose? Why?
Newton’s 3rd Law (we’ll get back
to the 2nd Law later)
For every action force, there is an equal
and opposite reaction force.
 Forces come in
action-reaction pairs.

Name the force pair:
Person walking off of a boat. What is the
action force? Reaction force?
Name the force pair:
Air coming out of a balloon. What is the
action force? The reaction force?
If the action force is 3 Newtons, how big is the reaction force?
Newton’s Second Law
Relates force to mass and acceleration.
 Commonly written as F=ma
 Force equals mass times acceleration

 F=force in Newtons
 m=mass in kilograms
 a=acceleration in m/s/s

Newton is the SI unit of force and is
shortened to just “N”.
Before we do any math…
F=ma
 If you push your pencil across the desk
with more force, what happens to mass?
what happens to acceleration?
 If you sharpen your pencil and then want
it to have the same acceleration as
before, do you have to use more or less
force than before?

Rearranging the equation:
F=ma
•
force equals mass times
acceleration
m=F/a
•
mass equals force divided by
acceleration
a=F/m
•
acceleration equals force
divided by mass
Number your notes from 1 to 5. Leave space
for each problem.
Problem 1:
An object with a mass of 10 kg is
accelerated across the table at 2 m/s/s.
What force is required?
 Looking for: Force
 equation: F=ma
 m=10 kg and a=2 m/s/s
 F= 10kg*2m/s/s
 F=20 N

Problem 2:
A force of 20 N acts upon a 5 kg block.
What is the acceleration of the object?
 Looking for: Acceleration
 Equation: A=F/m
 F=20 N and m=5 kg
 A=20 N/5 kg
 A=4 m/s/s

Problem 3
A net force of 15 N is exerted on a book
to cause it to accelerate at a rate of 5
m/s/s. Determine the mass of the book.
 Looking for: Mass
 Equation: M=F/a
 F=15 N and a=5 m/s/s
 M= 15 N/5 m/s/s
 M= 3 kg

Problem 4
A 10 kg bowling ball would require what
force to accelerate it down an alley at a
rate of 3 m/s/s?
 Looking for: Force
 Equation: F=ma
 m=10 kg and a=3 m/s/s
 F=10 kg* 3 m/s/s
 F=30 N

Problem 5
What is the mass of a truck if it produces
a force of 14,000 N while accelerating at
a rate of 5 m/s/s?
 Looking for: mass
 Equation: m=F/a
 F=14,000 N and a=5 m/s/s
 m=14,000 N/5 m/s/s
 m=2,800 kg

Exit Slip: Turn in when finished!
Answer the following questions on
your own.
1. The SI unit of force is __________.
 2. The equation used to solve for force is
F=_______.
 3. If I increase the mass of an object and
want to keep acceleration the same, force
must ____________. (increase or decrease)
 4. A boy pushes a box with a mass of 30 kg
and it moves at an acceleration of 5 m/s/s.
With what force does the boy push on the
box?

Force Equation Sheet
Solving
For…
Force
Mass
Acceleration
Givens…
Equation
Unit for
Answer…
Quick Review on Forces:
A force is a push, pull or any action that
can change motion.
 Force is measured in the unit of

 Newtons.

We will look at types of forces later.
Mass
Every object is made up of ________.
 The ______ (more/less) matter an
object has, the ______ (bigger/smaller)
the object is, and the _______
(more/less) mass it has.

Mass
Every object is made up of matter.
 The more matter an object has, the
bigger the object is, and the more mass
it has.
 Mass is measured in kilograms (kg) or
grams (g).


Mass is how heavy something is without
gravity.
 this means that an object has the same
mass here on earth or in space.
Weight
Weight is a force caused by gravity.
 Also measured in Newtons. (because
it’s a force)
 Example:

You can solve for weight (w) by
multiplying mass (m) and acceleration
due to gravity (g).
 W=mg


but what is acceleration due to
gravity???
Acceleration due to gravity
The acceleration due to gravity on earth
is 9.8 m/s/s.
 For this class we will round.
 We will write g=______ m/s/s

Acceleration due to gravity
The acceleration due to gravity on earth
is 9.8 m/s/s.
 For this class we will round.
 We will write g=10 m/s/s

Example Problem
If you have a mass of 60 kg, how much
do you weigh on earth?
 Looking for: Weight
 Equation: w=mg
 Given: m= 60 kg and g=10 m/s/s
 W=60 kg*10 m/s/s
 W=600 N

More about Weight

Weight problems are almost identical to
force problems.
 (Weight is a force, so it makes sense)

The only difference is that acceleration
for weight problems will always be 10
m/s/s.
Types of Forces
Forces occur from interactions.
 Contact forces are from physical
interactions.
 Examples include:

 Frictional Force
 Normal force
 Applied force
 Air resistance

Frictional Force: A force that opposes
motion
More on Friction
Without frictional forces, a moving object
may continue moving for a longer
period.
 Frictional forces are usually greater on
rough surfaces than on smooth
surfaces.

Normal Force: Support force exerted
upon an object in contact with another
stable object.
 Always at a 90° angle
from the surface.

Applied Force: force exerted on an
object by another person or object.
 Example: Pushing


Air Resistance: a force that acts against
the force of gravity on an object in free
fall.
Action-at-a-distance forces
AKA non-contact forces
 Result from non-physical interactions.
 Examples:

 Electrical forces
 Magnetic forces
 gravity
Gravity
Gravity is an attractive force between
two objects.
 Anything with mass has gravity.

 More massive objects
have more gravity.
Electrical Force

Attractive or repulsive force between two
charged objects.
Magnetic Force
Attractive or repulsive force based on a
magnetic pole or field.
 All magnets have a north and south
pole.

What types of forces are
working?

For a book on a table?
What types of forces are
working?

On a parachute?
What types of forces are
working?

When you push the grocery cart?
Foldable
Gravity
Contact Forces
Normal
Electrical
Force
Applied
Air resistance
Magnetic
Force
Non-contact forces
Frictional
Force Diagrams
Used to show force on an object.
 An arrow, with a name, length and
direction is used to represent a force.
 Example:

In a force diagram, the bigger the arrow,
the bigger the force.
 Notice how the 5 N force arrow is larger
than the 2 N force arrow.

How to draw force diagrams
The object is drawn as a box.
 The arrow for each force is drawn from
the box outward.
 All forces for a situation must be drawn

Steps for Force Diagrams
Read the situation and figure out which
forces are present (friction, gravity,
normal, etc.)
2. Determine the direction each force acts in.
(For example, gravity is always down)
3. Draw the box (for the object) and the
arrows.
4. label each arrow with the type of force,
and if available, the strength of the force.
1.
Example Problems

Sketch a force diagram for a book being
held up by a person.
Example 2

Sketch a force diagram of a laptop
sitting on a table.
Example 3

You are trying to push a heavy box
across the floor. It is not very easy
because the floor is not very smooth.
Sketch a force diagram of the box.
Example 4

A sled is falling down a sloped surface.
Sketch a force diagram of the sled.
Balanced Forces
Forces that are acting in the opposite
direction and are the same magnitude
(meaning they’re the same size)
 Anytime there is a balanced force on an
object, the object stays still or continues
moving at the same speed and in the
same direction.

Examples of Balanced Forces
Unbalanced Forces
When two forces are not equal in size.
 Unbalanced forces cause acceleration,
which is change in velocity.


Unbalanced forces causes can cause:
 a still object to move
speed
 moving object to __________
up or
________
slow down
 moving object to stop
 moving object to change direction
Examples of Unbalanced forces
Balanced or Unbalanced?

Riding a bike at a constant speed.
Balanced
Balanced or Unbalanced?

Lifting weights off the ground
Unbalanced
Balanced or Unbalanced?

A skydiver in the air
Unbalanced
Balanced or Unbalanced?

Person (or cat) Sleeping
Balanced
Adding Forces

What is net force?
 Sum of all forces acting on an object.
What is the net force on this object?
3 N to the right.
Finding net force

If no force is acting on an object, then
the net force is zero.
 Although this happens in physics problems,
it is unlikely to happen in real life.
Force Diagram:
Finding net force

If there is just one force acting on an
object, then that force is the net force.
 In free fall, the net force acting on an object
is its weight (mass times gravity (10 m/s/s)
What is the
net force on
this object?
5N
Finding Net Force

If 2 forces push or pull in opposite
directions, and they cancel each other
exactly, the net force is zero.
5N
5N
Finding Net Force

If 2 forces push or pull in opposite
directions, and they don’t cancel each
other exactly, the net force is what is left
over (the difference).
3N
5N
What is the
net force?
Finding Net Force

If 2 or more forces are acting in the
same direction, the net force is the sum
of the forces.
5N
5N
What is the
net force?
Net Force
Any time a stationary object stays still,
its resultant force (or net force) is zero.
 As soon as force is applied, acceleration
begins. The rate of acceleration will
force applied and
depend on the ________
mass of the object.
the _______

Examples:
Practice Problem 1
During 2nd period, Ladoris was a beast.
She resisted the forces applied by 5
people in her class all at once. They
applied a total force of 95 N to the right,
and Ladoris still did not move.
 1. Draw a force diagram
 2. What was the net force?

95 N
95 N
If Ladoris didn’t move, how should we
finish the force diagram?
What is the net force?
Practice Problem 2



During 7th period, we put Vincent in a box
because he was talking too much. We still
heard his voice through the box, so we
decided to push him outside. The force of
friction of the ground on the box was 68 N to
the left. Mrs. Nipper can apply a force of 25 N
to the right and every other 9th grade student
can apply a force of 6 N to the right.
Draw a force diagram.
How many students would Mrs. Nipper need to
make the box start moving and go outside?
Applied by
Mrs. Nipper=
25 N
Friction = 68 N
Applied by Student = 6 N
How many students do we need to
make the box move? (remember to
include Mrs. Nipper in the calculation!)