Forces and Motion
Vocabulary
• Force - A push or pull on an object.
• Net Force – vector sum of all forces
• Equilibrium:
– the state in which the net force on an
object is zero or balanced
– no change in velocity
Sir Isaac Newton
•
1643 - 1727
1st Law of Motion
(Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.
1st Law of Motion
(Law of Inertia)
Translation: Objects keep doing
what they are doing, in a
straight line.
Newton’s 1st Law –
Inertia
• Inertia is a measurement of
RESISTANCE TO CHANGE
______________
mass The
• It is measured in terms of ______.
mass the greater the
greater the _____,
inertia.
What is Mass?
• Mass is the amount of material a
body/object contains
• You are what you eat
• Bigger does not equal more mass
2nd Law
The net force of an object is
equal to the product of its
mass and acceleration, or
F=ma.
Newton’s 2nd Law – Simplified
• So, a little easier to read would be:
a = ΣF
OR
ΣF = ma
m
• So… what are the units for force
• Forces are measured in newtons
• N = (kg)(m/s2).
What does F = ma mean?
• Force is directly proportional to
mass and acceleration.
Imagine a ball of a certain mass
moving at a certain
acceleration. This
ball has a certain
force.
What does F = ma mean?
Now imagine we make the ball twice as big
(double the mass) but keep the
acceleration constant. F = ma says that
this new ball has twice the force of the old
ball.
What does F = ma mean?
Now imagine the original ball moving at
twice the original acceleration. F = ma
says that the ball will again have twice the
force of the ball at the original
acceleration.
More about F = ma
• If you double the mass, you double the force. If you
double the acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration quadruples the
force.
So . . . what if you decrease the mass by half? How
much force would the object have now?
What does F = ma say?
• F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still
have great force.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still
have a great force. Something very
small changing speed very slowly will
have a very weak force.
Gravity causes weight
• The weight of an object is caused by the force
of attraction between the earth and objects on
the surface of the earth.
Gravitation force is equal to weight, therefore…
How much does a person with
a mass of 70.0 kg weigh on
the earth?
Weight = mass X 9.8 m/s2
Weight = 70.0 kg X 9.8 m/s2
Weight = 686 N
Remember that 1 N = (1 kg) (1m/s2)
Mass vs. Weight
Your MASS is the amount of material in
your body.
Your MASS doesn’t depend on where you
are.
• Your WEIGHT is how much your body
pushes down on a scale.
• Your WEIGHT depends on how much
MASS you have and where you are.
• Weight is caused by gravity.
Marty
m = 68 kg
What does
Marty weigh?
•
g(sun)= 275 m/s2
18,700 N
g(pluto)= 0.654 m/s2
44 N
Gravity causes weight
What is the accelration due to gravity if Mr.
Batchelder has a force of
•
•
•
500N on Earth
80N on the moon
1000N on Saturn
Newton’s 2nd Law –
Let’s practice
1. You push your 100kg friend in a rolling
chair down the hall. They accelerate at
a rate of 5 m/s2. How much force did
you apply?
2. You are texting while driving. You and
your car have a mass of 500kg. You
accidently hit someone while
accelerating at 8 m/s2. What force did
you hit them with?
Newton’s 2nd Law –
Let’s practice
1. Determine the accelerations that result
when a 12-N net force is applied to a 3kg object.
2. A net force of 15 N is exerted on an
encyclopedia to cause it to accelerate
at a rate of 5 m/s2
Newton’s 2nd Law –
Let’s practice
1. You have run out of gas and have to
push your car. Your car weighs 560 N
and you push with a force of 395 N, and
your friend helps with 275 N. What is
the acceleration of the car?
2. You push a rolling chair (m=20kg) with
a force of 17 N. A friend pushes with
15 N at 30o. What is the net force?
What is the acceleration of the chair?
• For every action,
there
is
an
3rd Law
equal and opposite reaction.
What does this mean?
• For every force acting on an object, there is
an equal force acting in the opposite
direction.
• Right now, gravity is pulling you down in your
seat, but Newton’s Third Law says your seat
is pushing up against you with equal force.
This is why you are not moving.
• There is a balanced force acting on you–
gravity pulling down, your seat pushing up.
• Put really simply: F = F
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).
Newton’s Third Law
• It states:
Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first object
• It requires two forces:
Action
– _____________
Force
Reaction
– ______________
Force
Newton’s Third Law
• Does it matter which force we call the
action and which is the _________?
reaction
________
pairs
• Forces always occur in ________.
doesn’t matter which is
Therefore, it ________
the action and the reaction, as long as
you can identify both.
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.
Free Body Diagrams
• FBD’s are a neat way to show the
forces acting on an object.
• The object is drawn as a dot or a box,
and then vectors are drawn to show the
forces acting on it.
• Ex:
10 N
5N
6N
FBD’s
• Can you draw the FBD for you sitting on
your chair?
• Draw the FBD for your friend pushing
you in a rolling chair.
FBD’s
• Solve the net force for the following
FBD.
10 N
5N
6N
FBD
• The picture at the bottom shows a
snowman pulling a sled. Draw a freebody diagram for this sled. The
magnitudes of the forces acting on the
sled are 60N by the string, 130N from
gravity, and 90N upward by the ground.
Quick Quiz
Consider the following free-body
diagram. Ignoring air resistance and
friction, can this object be moving in a
horizontal direction?
YES!!!! IT CAN!!!
Quick Quiz
Think about this situation.
What are the forces on the
boy on the sled between
points B and C?
I hope you got
something that
looked a lot like
this.
Quick Quiz
This means a force is
not required to keep
a moving object in
motion.
Forces
• Types of forces
– Contact
– Non-Contact
Forces
• Contact
– Normal
– Frictional
– Tension
– Air Resistance
– Spring
Forces
• Non-Contact
– Electrical
– Magnetic
– Nuclear
The Normal Force
• The normal force (FN)
is one component of
the force that a surface
exerts on an object
with which it is in
contact
• Simply, the component
Perpendicular
that is ____________
to the surface.
The Normal Force
• How do I find the normal force?
• Easy, it is equal and opposite to the
weight of an object if the object is on a
horizontal surface.
• The Fn is not always horizontal.
• The Fn can be found by
• Fn=mg cosΘ (where Θ is the angle between the
contact surface and horizontal)
The Normal Force
• Where is the normal
force?
What is Friction?
• is a force resisting the relative motion of two
surfaces in contact with each other.
• is the parallel component of the force on an
object that is in contact with a surface.
• is a force that prevents motion from
occurring.
• Types
– Static
– Kinetic
– Fluid
What causes friction?
• There are two factors which affect
friction between two surfaces:
– Kind of surfaces in contact (rough or
smooth)
– Amount of force pressing the surfaces
together.
– The rougher the surface and the stronger
the force between the surfaces, the greater
the amount of friction.
From Glencoe Physical Science
Static and Kinetic Friction
• Static Friction- Friction force that acts on
objects that are not moving
• Kinetic Friction - Friction force that opposes
the direction of motion of an object as it slides
over a surface.
I better be
safe Ump!!
Static Friction
• Static Friction is the force
that prevents the start of
motion.
• With no movement,
Fs = Fapplied
• Has a range of 0 – Fs,max
Kinetic Friction
• Once motion has begun, the static friction force is no
longer a concern.
• Instead, kinetic friction is important.
• Kinetic friction is resistance to movement of an
already moving object.
Fluid Friction
Friction force that opposes the motion of
an object through a fluid.
Frictional Forces
• The frictional force will vary depending
on the surface interactions.
– Ex. Carpet vs tile
• Friction can be calculated by using the
coefficient of friction.
• Use the basic formula of:
– Ff = μFn
• It can be rearranged to:
– μk = (Fk/Fn) and μs = (Fs/Fn)
Frictional Forces
• Use the formula of:
– Ff = μFn
– Fk = μkFn
and Fs = μsFn
• A 24kg crate initially at rest on a horizontal
floor requires a 75N horizontal force to set
it in motion. Find the coefficient of static
friction between the crate and the floor.
Force of Tension
• Tension commonly
means the tendency of
a rope to be pulled
apart.
• Tension is typically the
same everywhere in a
rope because the rope
is treated as massless.
If it isn’t (and is vertical)
you must account for
the mass of the rope.
Finally, AIR RESISTANCE!!!
Terminal Velocity
The velocity at which a free falling object ceases to accelerate.
Force of Gravity
Air Resistance
T.V. is the constant velocity of a falling object when the
force of gravity equals the force of air resistance
Spring Force
• We will dive deep into
spring force when we
get to elastic energy.
• Until then, play safely!!
- forces that act between charged objects or
particles such as protons and electrons
Electric Forces
Magnetic Forces
-Forces that act on certain metals, poles of magnets,
and on moving charges
Nuclear Forces
- Nuclear forces act
over very short
distances and are
what hold the
particles of an atomic
nucleus together.
Frictional Forces
• Use the formula of:
– Ff = μFn
– Fk = μkFn
and Fs = μsFn
• A 24kg crate initially at rest on a horizontal
floor requires a 75N horizontal force to set
it in motion. Find the coefficient of static
friction between the crate and the floor.
Quick Review (Net Force)
• Derek leaves his physics book on top of a
drafting table that is inclined at a 35°
angle. There is a frictional force of 11N, a
normal force of 18N and gravitational force
of 22N. What is the net force?
On your own!! (Net Force)
• A student is designing a support to keep a tree
upright. Two wires have been attached to the
tree and placed at right angles to each other.
One wire exerts a force of 30N on the tree; the
other wire exerts a 40N force. Determine where
the third wire should be placed and how much
force is should have so that the net force acting
on the tree is zero.
• A stationary, flying kite is acted on by a force of
9.8N downward. The wind exerts a force of 45N
at an angle of 50° above the horizontal. Find
the force that the string exerts on the kite.
Putting it all together!!
• A student attaches a rope to a 20kg box
of books. He pulls with a force of 90N at
an angle of 30° with the horizontal. The
coefficient of kinetic friction between the
box and the sidewalk is 0.50. Find the
acceleration of the box.
• Slow down, take a deep breath and lets
take this one step at a time.
Putting it all together!!
• Now lets try pg 141 Practice F, # 1-4
• Don’t get bogged down with #2
What is momentum?
• The momentum of an object is equal to
the product of its mass and its velocity.
The unit for momentum is kg m/s
How some things react
without forces…