# Newtons First Law - PowerPoint

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```					         Newton’s First Law
• An object at rest will remain at rest and an
object in motion will remain in motion
unless acted on by an outside net force.
Newton’s Second Law
• The net force acting on an object equals the
object’s mass multiplied by its acceleration.

• F = ma

• a.k.a. an object’s acceleration is inversely
proportional to it’s mass and directly
proportional to the force applied
What Changes?
• Almost nothing changes

• We now have a few more types of
problems.
Constant Velocity/Rest
• Some objects are still at rest or moving at a
constant velocity.
• The acceleration of these objects is zero,
thus ma = 0

• Therefore the net force in both directions is
zero.
Find Acceleration
• You are given all of the forces acting on an
object and you have to find the acceleration
of the object.

• Find the net force in both directions and set
that equal to ma. Find a.
Red Wagon
• You are pulling a small child down the
street in a wagon. The wagon and the child
have a combined mass of 48 kg. You are
pulling to the right with a force of 120 N.
There is a frictional force of 74 N opposing
the motion. Find the resulting acceleration
of the wagon.
Could You Get That?
• You’re going to pick a 3.5 kg bag up off of
the floor. You are pulling up with a force of
37 N. What is the acceleration of the bag
after it has left the ground?
Atwood Machine
• You have two masses suspended over a
pulley as shown. Assuming the pulley to
massless and frictionless find the
acceleration of the two blocks.

4 kg   3 kg
Moving In
• Two people are pulling a 32 kg couch up to
their second floor apartment. One person
pulls with a force of 177 N at an angle of
55o above the horizontal while the other
person pull with a force of 203 N at an
angle of 60o with the opposite horizontal.
Find the acceleration of the couch after it
leaves the ground.
Find Some Force
• You are given the acceleration of an object
and now you have to find what forces are
causing this.
• You often have to find one particular force
that is acting on the object.
Sledding Season
• You are sledding down a hill that is inclined
at 18o. You and your sled have a combined
mass of 75 kg and you travel down the hill
with an acceleration of 2.3 m/s2. What is the
force of friction slowing down your trip?
Rolling On The Table
• A cart is on a table as shown. Find the
acceleration of the car assuming no friction.

875 g

85 g
Big Boat
• A supertanker (mass = 1.70 x 108 kg) is
moving with a constant velocity. Its engines
generate a forward thrust of 7.4 x 105 N.
Determine the magnitude of the resistive
force exerted on the tanker by the water and
the magnitude of the upward buoyant force
exerted on the tanker by the water.
Ramps and Blocks
• A frictionless car is sitting on the ramp as shown.
What is the acceleration of the system?

0.6 kg

32o
Remember Kinematics
• You are usually looking for either a force or
a kinematics quantity.
• You are given enough information to find
acceleration by using F = ma or using
kinematics.
• Find a and use it to find what you are
looking for.
I’m Not A Good Skier
• An 83 kg skier starts from rest and skis
down a hill with an angle of 30o. The
kinetic friction between himself and the
ground is 75 N. Given that the hill is 100 m
long find his final velocity and time when
he reaches the bottom of the hill.

• T = 7.07 seconds
• Vf = 28.3 m/s
Hallway Bowling
Turk is bowling a patient down a hallway of Sacred Heart Hospital.
What force did Turk have to exert on the 78 kg wheel chair in order to
get it going from 2.8 m/s to 3.5 m/s in 0.5 seconds. After Turk let go of
the wheel chair it slowed down due to friction (in the wheels) to a speed
of 3.0 m/s over the 15 m long hallway to the “pins”. Find what average
force the “pins” had to exert on the wheel chair to get it to stop in 1.2
seconds? (Remember that friction is constant until the chair stops.)

Fpush = 117.65 N
FF = -8.45 N
Fpins = -186.55 N
It’s A Bird…
• The Superman roller coaster uses powerful
magnets to accelerate a car and its riders
from rest to 45 m/s (about 100 mph) in a
time of 7.0 sec. The mass of the car and the
riders is 5.5 x 103 kg. Find the average net
force exerted on the car and the riders.

• F = 35357 N
Throw Up
• A boy throws a steel ball straight up. Consider the motion of the ball
only after it has left the boy’s hand but before it touches the ground,
and assume that the forces exerted by the air are negligible. For these
conditions, the force(s) acting on the ball is (are):
A. A downward force of gravity along with a steadily decreasing
upward force.
B. A steadily decreasing upward force from the moment it leaves the
boy’s hand until it reaches its highest point; on the way down there
is a steadily increasing downward force of gravity as the object gets
closer to the earth.
C. An almost constant downward force of gravity along with an
upward force that steadily decreases until the ball reaches its
highest point; on the way down there is only a constant downward
force of gravity
D. A constant downward force of gravity only.
E. None of the above. The ball falls back to the ground because of its
natural tendency to rest on the surface of the earth.
Bang
• A 15 g bullet is fired from a rifle. It takes
2.50 x 10-3 s for the bullet to travel the
length of the barrel, and it exits the barrel
with a speed of 715 m/s. Assuming that the
acceleration of the bullet is constant, find
the average net force exerted on the bullet.

• F = 4290 N
Could MJ Be More Useless?
Spider-Man has to save the day again, but can he? How strong must
his web be? At one point the Green Goblin hits Spider-Man making
him lose his grip on the cable. We can assume that the cable and car
with people has a weight of around ?????? N. Now if Spider-Man
lost his grip on the car for ?#? seconds what force must Spider-Man
and his web hold to return the car to rest in ?#? seconds?
Don’t Forget
• The following mass is hung via ropes. Find
the tensions in each of the ropes

35o

16 kg
Friction
• Friction (FF) is the force that opposes
motion between 2 surfaces in contact.

• There are two types of friction.
Static Friction
• The friction force that exists between two
objects that are NOT in motion relative to
one another.

• Example: Try pushing a school bus that’s in
park. Guess why it isn’t moving.
Kinetic Friction
• A.K.A. Sliding Friction

• The friction force between two objects that
ARE in motion relative to one another.

• Example: Slide your desk along the floor.
The resistance is kinetic friction.
Finding Friction
• Friction is found by multiplying the
coefficient of friction by the normal force

• FF = mN

• Or FF = mFN
What Is m?
• The coefficient of friction (m) is a constant
value based on the surfaces in contact.

• It is a pure number (no unit)
Will It Move?
• A block whose weight is 45.0 N rests on a
horizontal table. A horizontal force of 36.0
N is applied to the block. The coefficients
of static and kinetic friction are 0.650 and
0.420, respectively. Will the block move
under the influence of the force, and, if so,
what will be the block’s acceleration?
Slide It Again
• If you use a horizontal force of 30.0 N to
slide a 12.0 kg wooden crate across a floor
at a constant velocity, what is the
coefficient of sliding friction between the
crate and the floor?
• A rock is thrown straight up from the earth's
surface. Which one of the following statements
concerning the net force acting on the rock at the
top of its path is true?

a)   The net force is equal to the weight of the rock.
b)   The net force is instantaneously equal to zero newtons.
c)   The net force’s direction changes from up to down.
d)   The net force is greater than the weight of the rock.
e)   The net force is less than the weight of the rock, but
greater than zero newtons.
I Love This Movie
An EMP took out the power of the ship Serenity. The pilot must coast
the ship to a landing. If the runway that they landed on was 500 m
long and they were coming in at 42 m/s what was the coefficient of
friction between Serenity and the ground? (Assume Earth gravity)
Ramps and Friction
• A block is at rest before sliding down the ramp as
shown. Find how far the block will slide down the
ramp after 4 seconds if m = 0.28.

32o
Ramps and Friction II
• Assuming each surface has a coefficient of friction
with the blocks of 0.36 find how far the system will
be displaced after 5.0 seconds.

3 kg

23o                                  34o
Box Glide
• A woman exerts a constant force on a large box. As a
result, the box moves across a horizontal floor at a
constant speed “v”. The constant horizontal force
applied by the woman:
A. Has the same magnitude as the weight of the box.
B. Is greater than the weight of the box.
C. Has the same magnitude as the total force which resists
the motion of the box
D. Is greater than the total force which resists the motion
of the box.
E. Is greater than either the weight of the box or the total
force which resists its motion
Box Glide II
• If the woman in the previous question doubles the constant
horizontal force that she exerts on the box to push it on the
same horizontal floor, the box then moves:
A. With a constant speed that is double the speed “v” in the
previous question.
B. With a constant speed that is greater than the speed “v” in
the previous question, but not necessarily twice as great.
C. For a while with a speed that is constant and greater than
the speed “v” in the previous question, then with a speed
that increases thereafter.
D. For a while with an increasing speed, then with a constant
speed thereafter.
E. With a continuously increasing speed
Box Glide III
• If the woman in the previous questions suddenly
stops applying a horizontal force to the block, then
the block will:
A. Immediately come to a stop.
B. Continue moving at a constant speed for a while
and then slow to a stop.
C. Continue at a constant speed.
D. Immediately start slowing to a stop.
E. Increase its speed for a while and then start
slowing to a stop.
I Saw This Movie 11 Times In The
Theater
Neo kicks Agent Smith in the chest and sends him flying back
through the hallway. If Agent Smith hits the ground with a velocity
of ??? m/s away from Neo how far should he have slid after he made
contact with the ground assuming the standard coefficient of friction
between clothing and a wooden floor of 0.3?
• The wheels of an automobile are locked as it
slides to a stop from an initial speed of 30.0 m/s.
If the coefficient of kinetic friction is 0.600 and
the road is horizontal, approximately how long
does it take the car to stop?
A. 4.22 s
B. 8.75 s
C. 15.3 s
D. 5.10 s
E. 10.4 s
Newton’s Third Law
• For every action there is an equal and
opposite reaction. Equal in magnitude and
opposite in direction.

• Forces occur in pairs.
You Can Always Say…

• If you push on the wall with 10 N of force,
• Then the wall pushes on you with 10 N of
force.
with 7 N of force,

with 7 N of force.
• If Object A pushes on Object B,

• Then Object B pushes back equally and in
the opposite direction on Object A
Newton’s Apple
• Isaac Newton once saw on apple fall from a
tree. He realized that the force of gravity is
what pulls the apple to the Earth. Now that
you know Newton’s 3rd Law find the force
that the apple exerts on the Earth given that
the apple has a mass of 0.3 kg.
Tug of War
• Two students A & B are going to pull on one another via a
rope in a game of tug of war. Each student is sitting on a
cart with frictionless wheels. Student A has a mass of 100
kg and student B has a mass of 50 kg. Student A pulls with
a force of 40 N and student B pulls with a force of 10 N.
Which of the following is true:
A.   Student A’s acceleration is 2 times greater than student B.
B.   Student A’s acceleration is 4 times greater than student A.
C.   Student B’s acceleration is 2 times greater than student A.
D.   Student B’s acceleration is 4 times greater than student A.
E.   Student A and student B have the same acceleration.
•Two identical blocks are pulled along a rough surface as
suggested in the figure. Which one of the following
statements is false?

a)   The coefficient of kinetic friction is the same in each case.
b)   A force of the same magnitude is needed to keep each block
moving.
c)   A force of the same magnitude was required to start each block
moving.
d)   The normal force exerted on the blocks by the surface is the same
for both blocks.
e)   The magnitude of the force of kinetic friction is greater for the
block on the right.
• A large truck collides head on with a small compact
car. During the collision:
A. The truck exerts a greater amount of force on the car than
the car exerts on the truck.
B. The car exerts a greater amount of force on the truck than
the truck exerts on the car.
C. Neither exerts a force on the other, the car gets smashed
simply because it gets in the way of the truck.
D. The truck exerts a force on the car but the car does not
exert a force on the truck.
E. The truck exerts the same amount of force on the car as the
car exerts on the truck.

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