Download ConcepTest 4.1a Newton`s First Law I 1) there is a net force but the

QotD
• Make a list of what types of forces we have on Earth?
4-1 Force
A force is a push or
pull. An object at rest
needs a force to get it
moving; a moving
object needs a force to
change its velocity.
4-2 Newton’s First Law of Motion
Newton’s first law is often called the law of
inertia.
An object in motion stays in motion and an
object at rest stays at rest, unless a force acts
upon it.
4-3 Mass
Mass is the measure of inertia of an object. In
the SI system, mass is measured in
kilograms.
Mass is not weight:
Mass is a property of an object. Weight is the
force exerted on that object by gravity.
If you go to the moon, whose gravitational
acceleration is about 1/6 g, you will weigh
much less. Your mass, however, will be the
same.
4-4 Newton’s Second Law of Motion
Newton’s second law is the relation between
acceleration and force. Acceleration is
proportional to force and inversely
proportional to mass.
(4-1)
4-4 Newton’s Second Law of Motion
Force is a vector, so
each coordinate axis.
is true along
The unit of force in the SI system is the
newton (N).
Your turn:
What average force is required to stop an 1100-kg car in 8.0 s
if the car is traveling at 95 km/hr?
ConcepTest 4.1a Newton’s First Law I
A book is lying at
rest on a table.
The book will
remain there at
rest because:
1) there is a net force but the book has too
much inertia
2) there are no forces acting on it at all
3) it does move, but too slowly to be seen
4) there is no net force on the book
5) there is a net force, but the book is too
heavy to move
ConcepTest 4.1a Newton’s First Law I
A book is lying at
rest on a table. The
book will remain
there at rest
because:
1) there is a net force but the book has too
much inertia
2) there are no forces acting on it at all
3) it does move, but too slowly to be seen
4) there is no net force on the book
5) there is a net force, but the book is too
heavy to move
There are forces acting on the book, but the only
forces acting are in the y-direction. Gravity acts
downward, but the table exerts an upward force
that is equally strong, so the two forces cancel,
leaving no net force.
ConcepTest 4.1b Newton’s First Law II
A hockey puck
A hockey puck
slides on ice at
slides on ice at
constant velocity.
constant velocity.
What is the net
What is the net
force acting on the
force acting on the
puck?
puck?
1) more than its weight
2) equal to its weight
3) less than its weight but more than zero
4) depends on the speed of the puck
5) zero
ConcepTest 4.1b Newton’s First Law II
A hockey puck
A hockey puck
slides on ice at
slides on ice at
constant velocity.
constant velocity.
What is the net
What is the net
force acting on the
force acting on the
puck?
puck?
1) more than its weight
2) equal to its weight
3) less than its weight but more than zero
4) depends on the speed of the puck
5) zero
The puck is moving at a constant velocity, and
therefore it is not accelerating. Thus, there must
be no net force acting on the puck.
Follow-up: Are there any forces acting on the puck? What are they?
ConcepTest 4.1c Newton’s First Law III
You put your book on
the bus seat next to
you. When the bus
stops suddenly, the
book slides forward
off the seat. Why?
1) a net force acted on it
2) no net force acted on it
3) it remained at rest
4) it did not move, but only seemed to
5) gravity briefly stopped acting on it
ConcepTest 4.1c Newton’s First Law III
You put your book on
the bus seat next to
you. When the bus
stops suddenly, the
book slides forward
off the seat. Why?
1) a net force acted on it
2) no net force acted on it
3) it remained at rest
4) it did not move, but only seemed to
5) gravity briefly stopped acting on it
The book was initially moving forward (since it was
on a moving bus). When the bus stopped, the book
continued moving forward, which was its initial state
of motion, and therefore it slid forward off the seat.
Follow-up: What is the force that usually keeps the book on the seat?
ConcepTest 4.1d Newton’s First Law IV
You kick a smooth flat
stone out on a frozen
pond. The stone slides,
slows down and
eventually stops. You
conclude that:
1) the force pushing the stone forward
finally stopped pushing on it
2) no net force acted on the stone
3) a net force acted on it all along
4) the stone simply “ran out of steam”
5) the stone has a natural tendency to be
at rest
ConcepTest 4.1d Newton’s First Law IV
You kick a smooth flat
stone out on a frozen
pond. The stone slides,
slows down and
eventually stops. You
conclude that:
1) the force pushing the stone forward
finally stopped pushing on it
2) no net force acted on the stone
3) a net force acted on it all along
4) the stone simply “ran out of steam”
5) the stone has a natural tendency to be
at rest
After the stone was kicked, no force was pushing
it along! However, there must have been some
force acting on the stone to slow it down and stop
it. This would be friction!!
Follow-up: What would you have to do to keep the stone moving?
4-5 Newton’s Third Law of Motion
Newton’s third law:
Whenever one object exerts a force on a second
object, the second exerts an equal force in the
opposite direction on the first.
(Every action has an equal and opposite
reaction)
4-5 Newton’s Third Law of Motion
Helpful notation: the first subscript is the object
that the force is being exerted on; the second is
the source.
This need not be
done indefinitely, but
is a good idea until
you get used to
dealing with these
forces.
(4-2)
4-6 Weight – the Force of Gravity;
and the Normal Force
Weight is the force exerted on an
object by gravity. Close to the
surface of the Earth, where the
gravitational force is nearly
constant, the weight is:
4-6 Weight – the Force of Gravity;
and the Normal Force
An object at rest must have no net force on it. If
it is sitting on a table, the force of gravity is still
there; what other force is there?
The force exerted perpendicular to a surface is
called the normal force.
Your turn!
•A 20.0-kg box rests on a table.
•(a) What is the weight of the box and the
normal force acting on it?
•(b) A 10.0-kg box is placed on top of the 20.0kg box. Determine the normal force that the
table exerts on the 20.0-kg box and the normal
force that the 20.0-kg box exerts on the 10.0kg box.
4-7 Solving Problems with Newton’s Laws –
Free-Body Diagrams
When a cord or rope pulls on
an object, it is said to be under
tension, and the force it exerts
is called a tension force.
QotD:
Without looking at your notes, what are Newton’s 3 Laws of
Motion?
4-8 Applications Involving Friction, Inclines
Static friction is the frictional force between
two surfaces that are not moving along each
other. Static friction keeps objects on
inclines from sliding, and keeps objects from
moving when a force is first applied.
4-8 Applications Involving Friction, Inclines
For kinetic – sliding –
friction, we write:
is the
coefficient of kinetic
friction, and is
different for every
pair of surfaces.
4-8 Applications Involving Friction, Inclines
4-8 Applications Involving Friction, Inclines
The static frictional force increases as the
applied force increases, until it reaches its
maximum. Then the object starts to move, and
the kinetic frictional force takes over.
4-8 Applications Involving Friction, Inclines
An object sliding down an incline has three forces acting
on it: the normal force, gravity, and the frictional force.
• The normal force is always perpendicular to the surface.
• The friction force is parallel to it.
• The gravitational force points down.
If the object is at rest,
the forces are the same
except that we use the
static frictional force,
and the sum of the
forces is zero.
Your turn!
•A box slides down a 25° ramp with an
acceleration of 1.08 m/s2. Determine the
coefficient of kinetic friction between the box
and the ramp.
QotD
A 2-kilogram block slides down a 30 incline as
shown below with an acceleration of 2 meters
per second squared.
Draw the FBD and find the frictional force along
the plane.
Coefficient of Friction Lab
Question: Which material will have the
highest coefficient of friction?
Hypothesis:
Data Table:
Procedure:
1 2 3
1. Find the mass of the
angle (wood)
block
angle (carpet)
2. Set the block on the
ramp
3. Slowly raise the ramp
until the block slides
4. Measure the angle of
the ramp
5. Repeat for other side
Analysis:
1. Using sum of forces,
calculate FN and Fg
2. Calculate the coefficient
of static friction