Download Chapter 4 - Forces and Newton`s Laws of Motion w./ QuickCheck

Chapter 4 - Forces and Newton’s
Laws of Motion
w./ QuickCheck Questions
© 2015 Pearson Education, Inc.
Anastasia Ierides
Department of Physics and Astronomy
University of New Mexico
September 8, 2015
Review of Last Time
Looking at 1-D (x or y) and 2-D (x and y) motion
diagrams, position, velocity, & acceleration graphs
Equations of motion in 1-D and 2-D, Projectile
Motion
Vectors, Free Fall, Coordinate System, & Motion
on a ramp
Relative Velocity, Circular Motion
QuickCheck Question 3.15
A factory conveyor belt rolls
at 3 m/s. A mouse sees a
piece of cheese directly
across the belt and heads
straight for the cheese at 4 m/
s. What is the mouse’s speed
relative to the factory floor?
A.
B.
C.
D.
E.
1 m/s
2 m/s
3 m/s
4 m/s
5 m/s
QuickCheck Question 3.15
A factory conveyor belt rolls
at 3 m/s. A mouse sees a
piece of cheese directly
across the belt and heads
straight for the cheese at 4 m/
s. What is the mouse’s speed
relative to the factory floor?
A.
B.
C.
D.
E.
1 m/s
2 m/s
3 m/s
4 m/s
5 m/s
Break the
problem up
into the
“relative
parts”!
QuickCheck Question 3.16
A heavy red ball is released from rest 2.0 m above a flat, horizontal surface. At exactly the same instant, a yellow ball with the same mass is fired horizontally at 3.0 m/s. Which ball hits the ground first?
A. The red ball hits first.
B. The yellow ball hits first.
C. They hit at the same time.
v = 3.0 m/s
2.0 m
QuickCheck Question 3.16
A heavy red ball is released from rest 2.0 m above a flat, horizontal surface. At exactly the same instant, a yellow ball with the same mass is fired horizontally at 3.0 m/s. Which ball hits the ground first?
A. The red ball hits first.
B. The yellow ball hits first.
C. They hit at the same time.
v = 3.0 m/s
2.0 m
Projectile Acceleration
Ignoring air
resistance, vertical
acceleration, ay, is the
same at all points
Why? - Free Fall
ay = afree fall = -g = -9.8 m/s2
Horizontal (x-)
component, ax = 0 m/s2
QuickCheck Question 3.17
A 100-g ball rolls off a table and lands 2.0 m from the base of
the table. A 200-g ball rolls off the same table with the same
speed. It lands at distance
A.
B.
C.
D.
E.
1.0 m
Between 1 m and 2 m
2.0 m
Between 2 m and 4 m
4.0 m
QuickCheck Question 3.17
A 100-g ball rolls off a table and lands 2.0 m from the base of
the table. A 200-g ball rolls off the same table with the same
speed. It lands at distance
A.
B.
C.
D.
E.
1.0 m
Between 1 m and 2 m
2.0 m
Between 2 m and 4 m
4.0 m
Remember no
acceleration in the
x direction only in
the y.
Analyzing Projectile Motion
The ball finishes its
motion moving
downward at the same
speed as it started moving
upward
Two independent motions:
uniform motion at
constant velocity in the xdirection and free-fall
motion in the y-direction
QuickCheck Question 3.18
Projectiles 1 and 2 are launched over level ground with the
same speed but at different angles. Which hits the ground
first? Ignore air resistance.
A.
B.
C.
D.
Projectile 1 hits first.
Projectile 2 hits first.
They hit at the same time.
There’s not enough information to tell.
QuickCheck Question 3.18
Projectiles 1 and 2 are launched over level ground with the
same speed but at different angles. Which hits the ground
first? Ignore air resistance.
Remember: The time in
the air is determined by
the vertical component of
the velocity
A.
B.
C.
D.
Projectile 1 hits first.
Projectile 2 hits first.
They hit at the same time.
There’s not enough information to tell.
What have we learned?
Acceleration is a vector
that points in the direction
of change in velocity; if
there is a change in
velocity, there is an
acceleration
What is the Cause of Motion?
Acceleration is a vector
that points in the direction
of change in velocity; if
there is a change in
velocity, there is an
acceleration
What causes this change in
velocity or acceleration?
What Causes Motion?
What Causes Motion?
When you (agent) push
on an object to cause it
to move from rest
(accelerate) what are
you doing?
What Causes Motion?
When you (agent) push
on an object to cause it
to move from rest
(accelerate) what are
you doing?
You are applying a
FORCE!
What Causes Motion?
When you (agent) push
on an object to cause it
to move from rest
(accelerate) what are
you doing?
You are applying a
FORCE!
A push or pull, an
action on an object,
is a FORCE
Newton’s First Law
A push or pull, an action on an object, is a
FORCE
Newton’s First Law
A push or pull, an action on an object, is a
FORCE
A force acts on an object
Newton’s First Law
A push or pull, an action on an object, is a
FORCE
A force acts on an object
An agent imparts that force on the object
What is FORCE?
Forces cause acceleration
What is FORCE?
Forces cause acceleration
A force is a vector, denoted by the symbol F, whose
magnitude (size) is F.
What is FORCE?
Forces cause acceleration
A force is a vector, denoted by the symbol F, whose
magnitude (size) is F.
Contact forces are forces that act on an object by
touching it at a point of contact
What is FORCE?
Forces cause acceleration
A force is a vector, denoted by the symbol F, whose
magnitude (size) is F.
Contact forces are forces that act on an object by
touching it at a point of contact
Long-range forces are forces that act on an object
without physical contact
Contact Forces
Contact Forces
Contact Forces
Contact Forces
Long Range Forces
Long Range Forces
Force Vectors
More than One Force?
Many objects in motion
experience more than one
force acting on them
More than One Force?
Many objects in motion
experience more than one
force acting on them
The sum of all the forces
acting on an object is the
resultant or net force
More than One Force?
Many objects in motion
experience more than one
force acting on them
The sum of all the forces
acting on an object is the
resultant or net force
FNET = ∑ F
= F1 + F2 + F3 + …
QuickCheck Question 4.1
The net force on an object points to the left.
Two of three forces are shown. Which is the
missing third force?
QuickCheck Question 4.1
The net force on an object points to the left.
Two of three forces are shown. Which is the
missing third force?
We know that FNET points to the left
QuickCheck Question 4.1
The net force on an object points to the left.
Two of three forces are shown. Which is the
missing third force? We also know that the sum
of F1 and F2 points to the top
right
FNET
Fr
QuickCheck Question 4.1
The net force on an object points to the left.
Two of three forces are shown. Which is the
missing third force?
Subtracting Fr from FNET will
give us the third force
FNET
Fr
QuickCheck Question 4.1
The net force on an object points to the left.
Two of three forces are shown. Which is the
missing third force?
Adding -Fr and FNET together
we see that F3 has to be …
FNET
-Fr
Fr
QuickCheck Question 4.1
The net force on an object points to the left.
Two of three forces are shown. Which is the
missing third force?
Adding -Fr and FNET together
we see that F3 has to be …
FNET
-Fr
Fr
Where Are Forces Found?
There is a plethora of forces out there
Where Are Forces Found?
There is a plethora of forces out there
You just have to know where to look!
Where Are Forces Found?
There is a plethora of forces out there
You just have to know where to look!
Think of your weight or falling down
Where Are Forces Found?
There is a plethora of forces out there
You just have to know where to look!
Think of your weight or falling down
Rolling or skiing down a hill
Where Are Forces Found?
There is a plethora of forces out there
You just have to know where to look!
Think of your weight or falling down
Rolling or skiing down a hill
Jumping on a trampoline or your car’s shocks
Types of Forces
Types of Forces - Weight
The gravitational pull
of the earth on an
object on or near the
surface of the earth
Types of Forces - Weight
The gravitational pull
of the earth on an
object on or near the
surface of the earth
What is the agent?
Types of Forces - Weight
The gravitational pull
of the earth on an
object on or near the
surface of the earth
What is the agent?
- Earth (pulling down)
Types of Forces - Weight
The gravitational pull
of the earth on an
object on or near the
surface of the earth
What is the agent?
- Earth (pulling down)
An object’s weight vector always points vertically downward
Types of Forces - Spring
Types of Forces - Spring
These come in many forms (springs,
trampoline, etc.)
Types of Forces - Spring
These come in many forms (springs,
trampoline, etc.)
They push or pull when pushed or pulled
Types of Forces - Tension
Types of Forces - Tension
Contact force exerted when pulling a string,
rope, or wire
Types of Forces - Tension
Contact force exerted when pulling a string,
rope, or wire
The direction of this force is in the direction the
string, rope, or wire is pulled
Types of Forces - Normal Force
Types of Forces - Normal Force
The force exerted by a
surface (the agent)
against an object that
is pressing against it
Types of Forces - Normal Force
The force exerted by a
surface (the agent)
against an object that
is pressing against it
This forces is always
perpendicular to the
surface
Types of Forces - Normal Force
The force exerted by a
surface (the agent)
against an object that
is pressing against it
This forces is always
perpendicular to the
surface
Even on inclined plane
Types of Forces - Normal Force
Responsible for
solidness of solids
Types of Forces - Normal Force
Responsible for
solidness of solids
Denoted by the
symbol n
n
Types of Forces - Friction
Types of Forces - Friction
Like Normal Force, exerted by a surface
Types of Forces - Friction
Like Normal Force, exerted by a surface
Unlike Normal Force, parallel to the surface
Types of Forces - Friction
Static Friction, fs, keeps an object from moving
Types of Forces - Friction
Static Friction, fs, keeps an object from moving
Kinetic Friction, fk, acts as an object moves
along the surface
Types of Forces - Drag
The force of a fluid (like
air or water) on a
moving object
Types of Forces - Drag
The force of a fluid (like
air or water) on a
moving object
Points opposite the direction of motion
Types of Forces - Drag
The force of a fluid (like
air or water) on a
moving object
Points opposite the direction of motion
Neglect air resistance
in all problems except
when told to include it
Types of Forces - Thrust
Types of Forces - Thrust
Occurs when a jet or
rocket engine expels
gas molecules at
high speed
Types of Forces - Thrust
Occurs when a jet or
rocket engine expels
gas molecules at
high speed
Directed opposite
the direction of the
exhaust gas
QuickCheck Question 4.2
A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp?
A. The weight of the ball acting vertically down.
B. A horizontal force that maintains the motion.
C. A force whose direction changes as the direction of motion
changes.
D. The weight of the ball and a horizontal force.
E. The weight of the ball and a force in the direction of motion.
QuickCheck Question 4.2
A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp?
A. The weight of the ball acting vertically down.
B. A horizontal force that maintains the motion.
C. A force whose direction changes as the direction of motion
changes.
D. The weight of the ball and a horizontal force.
E. The weight of the ball and a force in the direction of motion.
QuickCheck Question 4.3
A steel beam hangs from a cable as a crane lifts the beam.
What forces act on the beam?
A.
B.
C.
D.
Gravity
Gravity and tension in the cable
Gravity and a force of motion
Gravity and tension and a force of motion
QuickCheck Question 4.3
A steel beam hangs from a cable as a crane lifts the beam.
What forces act on the beam?
A.
B.
C.
D.
Gravity
Gravity and tension in the cable
Gravity and a force of motion
Gravity and tension and a force of motion
QuickCheck Question 4.4
A bobsledder pushes her sled across horizontal snow to get it
going, then jumps in. After she jumps in, the sled gradually
slows to a halt. What forces act on the sled just after she’s
jumped in?
A.
B.
C.
D.
E.
Gravity and kinetic friction
Gravity and a normal force
Gravity and the force of the push
Gravity, a normal force, and kinetic friction
Gravity, a normal force, kinetic friction, and the force of the
push
QuickCheck Question 4.4
A bobsledder pushes her sled across horizontal snow to get it
going, then jumps in. After she jumps in, the sled gradually
slows to a halt. What forces act on the sled just after she’s
jumped in?
A.
B.
C.
D.
E.
Gravity and kinetic friction
Gravity and a normal force
Gravity and the force of the push
Gravity, a normal force, and kinetic friction
Gravity, a normal force, kinetic friction, and the force of the
push
Identifying Forces
What Forces Are There?
Identifying Forces
Forces and Motion
What do forces do?
Forces and Motion
What do forces do?
- They cause an object to move, accelerate
Forces and Motion
What do forces do?
- They cause an object to move, accelerate
An object pulled with a constant force experiences a
constant acceleration (both pointing in the same
direction)
Forces and Motion
What do forces do?
- They cause an object to move, accelerate
An object pulled with a constant force experiences a
constant acceleration (both pointing in the same
direction)
Acceleration is directly proportional to the force (F ∝a)
Forces and Motion
What do forces do?
- They cause an object to move, accelerate
An object pulled with a constant force experiences a
constant acceleration (both pointing in the same
direction)
Acceleration is directly proportional to the force (F ∝a)
Acceleration is inversely proportional to an object’s mass
(a ∝ 1/m)
Forces and Motion
Forces and Motion
As the block starts to move, in order to keep the
pulling force constant you must move your hand in
just the right way to keep the length of the rubber
band—and thus the force—constant.
QuickCheck Question 4.5
A cart is pulled to the right
with a constant, steady force.
How will its acceleration
graph look?
QuickCheck Question 4.5
A cart is pulled to the right
with a constant, steady force.
How will its acceleration
graph look?
Remember: A constant force
means a constant acceleration!
QuickCheck Question 4.5
A cart is pulled to the right
with a constant, steady force.
How will its acceleration
graph look?
Remember: A constant force
means a constant acceleration!
Example 4.4 - Finding the mass of
an unknown block
When a rubber band is stretched to pull on a 1.0 kg block with
a constant force, the acceleration of the block is measured to
be 3.0 m/s2. When a block with an unknown mass is pulled
with the same rubber band, using the same force, its
acceleration is 5.0 m/s2. What is the mass of the unknown
block?
Example 4.4 - Finding the mass of
an unknown block
When a rubber band is stretched to pull on a 1.0 kg block with
a constant force, the acceleration of the block is measured to
be 3.0 m/s2. When a block with an unknown mass is pulled
with the same rubber band, using the same force, its
acceleration is 5.0 m/s2. What is the mass of the unknown
block?
PREPARE Each block’s acceleration is inversely proportional
to its mass.
Meaning a ∝ 1/m
Example 4.4 - Finding the mass of
an unknown block
SOLVE We
can use the result of the Inversely Proportional
Relationships box to write
Example 4.4 - Finding the mass of
an unknown block
SOLVE We
can use the result of the Inversely Proportional
Relationships box to write
Example 4.4 - Finding the mass of
an unknown block
SOLVE We
can use the result of the Inversely Proportional
Relationships box to write
ASSESS With
the same force applied, the unknown block had a
larger acceleration than the 1.0 kg block. It makes sense, then,
that its mass—its resistance to acceleration—is less than 1.0
kg.
Newton’s Second Law
This is the same as writing
Fnet = m a
QuickCheck Question 4.6
A constant force causes an object to accelerate at 4 m/s2. What
is the acceleration of an object with twice the mass that
experiences the same force?
A.
B.
C.
D.
E.
1 m/s2
2 m/s2
4 m/s2
8 m/s2
16 m/s2
QuickCheck Question 4.6
A constant force causes an object to accelerate at 4 m/s2. What
is the acceleration of an object with twice the mass that
experiences the same force?
A.
B.
C.
D.
E.
1 m/s2
2 m/s2
4 m/s2
8 m/s2
16 m/s2
Remember: Acceleration is
inversely proportional to mass
a ∝ 1/m
QuickCheck Question 4.6
A constant force causes an object to accelerate at 4 m/s2. What
is the acceleration of an object with twice the mass that
experiences the same force?
A.
B.
C.
D.
E.
1 m/s2
2 m/s2
4 m/s2
8 m/s2
16 m/s2
Remember: Acceleration is
inversely proportional to mass
a ∝ 1/m
If the new mass is twice as big
as the old one, mnew = 2 mold
QuickCheck Question 4.6
A constant force causes an object to accelerate at 4 m/s2. What
is the acceleration of an object with twice the mass that
experiences the same force?
A.
B.
C.
D.
E.
1 m/s2
2 m/s2
4 m/s2
8 m/s2
16 m/s2
Remember: Acceleration is
inversely proportional to mass
a ∝ 1/m
If the new mass is twice as big
as the old one, mnew = 2 mold
anew = (mold/mnew) aold
= 1/2 aold
QuickCheck Question 4.6
A constant force causes an object to accelerate at 4 m/s2. What
is the acceleration of an object with twice the mass that
experiences the same force?
A.
B.
C.
D.
E.
1 m/s2
2 m/s2
4 m/s2
8 m/s2
16 m/s2
Remember: Acceleration is
inversely proportional to mass
a ∝ 1/m
If the new mass is twice as big
as the old one, mnew = 2 mold
anew = (mold/mnew) aold
= 1/2 aold
QuickCheck Question 4.7
An object, when pushed with a net force F, has an acceleration
of 2 m/s2. Now twice the force is applied to an object that has
four times the mass. Its acceleration will be
A.
B.
C.
D.
½ m/s2
1 m/s2
2 m/s2
4 m/s2
QuickCheck Question 4.7
An object, when pushed with a net force F, has an acceleration
of 2 m/s2. Now twice the force is applied to an object that has
four times the mass. Its acceleration will be
A.
B.
C.
D.
½ m/s2
1 m/s2
2 m/s2
4 m/s2
Remember: Acceleration is directly proportional
to force and inversely proportional to mass
a = F/m
QuickCheck Question 4.7
An object, when pushed with a net force F, has an acceleration
of 2 m/s2. Now twice the force is applied to an object that has
four times the mass. Its acceleration will be
A.
B.
C.
D.
½ m/s2
1 m/s2
2 m/s2
4 m/s2
Remember: Acceleration is directly proportional
to force and inversely proportional to mass
a = F/m
If the new force is twice as big as the old one,
Fnew = 2 Fold and the new mass is 4 times as big as
the old one, mnew= 4 mold, then
QuickCheck Question 4.7
An object, when pushed with a net force F, has an acceleration
of 2 m/s2. Now twice the force is applied to an object that has
four times the mass. Its acceleration will be
A.
B.
C.
D.
½ m/s2
1 m/s2
2 m/s2
4 m/s2
Remember: Acceleration is directly proportional
to force and inversely proportional to mass
a = F/m
If the new force is twice as big as the old one,
Fnew = 2 Fold and the new mass is 4 times as big as
the old one, mnew= 4 mold, then
anew = Fnew/mnew
= (2 Fold)/(4 mold) = 2/4 Fold/mold
= 2/4 aold = (1/2)(2 m/s2) = 1 m/s2
QuickCheck Question 4.7
An object, when pushed with a net force F, has an acceleration
of 2 m/s2. Now twice the force is applied to an object that has
four times the mass. Its acceleration will be
A.
B.
C.
D.
½ m/s2
1 m/s2
2 m/s2
4 m/s2
Remember: Acceleration is directly proportional
to force and inversely proportional to mass
a = F/m
If the new force is twice as big as the old one,
Fnew = 2 Fold and the new mass is 4 times as big as
the old one, mnew= 4 mold, then
anew = Fnew/mnew
= (2 Fold)/(4 mold) = 2/4 Fold/mold
= 2/4 aold = (1/2)(2 m/s2) = 1 m/s2
QuickCheck Question 4.8
A 40-car train travels along a straight track at 40 mph. A skier
speeds up as she skis downhill. On which is the net force
greater?
A.
B.
C.
D.
The train
The skier
The net force is the same on both.
There’s not enough information to tell.
QuickCheck Question 4.8
A 40-car train travels along a straight track at 40 mph. A skier
speeds up as she skis downhill. On which is the net force
greater?
A.
B.
C.
D.
The train
The skier
The net force is the same on both.
There’s not enough information to tell.
QuickCheck Question 4.9
An object on a rope is lowered at constant speed. Which is true?
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.9
An object on a rope is lowered at constant speed. Which is true?
Remember: Constant
velocity means zero
acceleration
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.9
An object on a rope is lowered at constant speed. Which is true?
Remember: Constant
velocity means zero
acceleration
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.9
An object on a rope is lowered at constant speed. Which is true?
Remember: Constant
velocity means zero
acceleration
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.10
An object on a rope is lowered at a steadily decreasing
speed. Which is true?
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.10
An object on a rope is lowered at a steadily decreasing
speed. Which is true?
Remember: Decreasing downward velocity
means acceleration vector points up
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.10
An object on a rope is lowered at a steadily decreasing
speed. Which is true?
Remember: Decreasing downward velocity
means acceleration vector points up
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.10
An object on a rope is lowered at a steadily decreasing
speed. Which is true?
Remember: Decreasing downward velocity
means acceleration vector points up
so the net force is also pointing up
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
QuickCheck Question 4.10
An object on a rope is lowered at a steadily decreasing
speed. Which is true?
Remember: Decreasing downward velocity
means acceleration vector points up
so the net force is also pointing up
A. The rope tension is greater than the object’s
weight.
B. The rope tension equals the object’s weight.
C. The rope tension is less than the object’s weight.
D. The rope tension can’t be compared to the object’s
weight.
Units of Force
The basic unit of force, in SI, is called a Newton (N)
1 N = 1 kg m/s2
One newton causes 1 kg mass to accelerate at 1 m/
2
s
The unit for force in the English system is a pound
(lb)
1 pound = 1 lb = 4.45 N
Summary
Summary
Summary
Summary
Things that are due
Homework #3
Due September 14, 2015 by 11:59 pm
Reading Quiz #4
Due September 15, 2015 by 4:59 pm
EXAM #1
Covers Chapters 1-3 & Lectures 2-6
Thursday, September 10, 2015
Bring a calculator and cheat sheet (turn in with
exam, one side of 8.5”x11” piece of paper)
Practice exam is available on website along
with solutions
QUESTIONS?