Transcript forces

Page 11
S8P3a.
Motion
8/28/12
What are the similarities and differences between speed and
velocity?
Speed is the rate that an object moves. (How
fast something moves)
Speed is measured in meters per second (m/s),
kilometers per hour (km/hr) and miles per hour
(mph)
Speed is useful for knowing how fast vehicles
move, how fast people run, etc
Speed is calculated by dividing the distance
traveled by the time it takes to travel the
distance.
Speed = distance / time
Page 13
Velocity is the rate that an object moves in a
particular direction.
Velocity is measured in meters per second
(m/s), Kilometers per hour (km/hr) and miles per
hour (mph) with a direction. For example:
40 m/s west
Velocity is useful for airplanes, helicopters and
boats.
Velocity is calculated by dividing the distance
traveled by the time it takes to travel the
distance.
Velocity = distance / time
SPEED AND VELOCITY ARE
NOT THE SAME
On page 12 of your INB, make two columns and categorize the following
descriptions as speed or velocity:
car moving at 35mph
bird flying at 40mph north
jogger running at 10m/s
jogger running at 10m/s south
a vehicle racing at 125mph
Speed
Velocity
Page 15
S8P3a.
Motion
9/5/12
What is the relationship between velocity and acceleration?
Acceleration is the rate at which an object’s velocity
changes.
There are 3 ways that acceleration can occur:
1. speeding up (acceleration or positive acceleration)
2. slowing down (deceleration or negative
acceleration)
3. Changing direction
Acceleration is expressed in units like meters per
second squared (m/s2)
The formula for calculating acceleration is:
Average acceleration= final velocity-initial velocity
time
Summary
On the top half of pg.14 of your Science NoteBook
Identify which is an example of acceleration and which is not:
Acceleration
Not Acceleration
A train traveling 65 miles per hour north
A boat sailing west at 5 knots
A person jogging at 3 meters/second along a curved path
A car stopped at a stop light
A car stopping at a stop light
A truck speeding up from 55mph to 75mph
A person jogging at a constant speed along a straight path
On the bottom half of pg.14 of your Science NoteBook
Complete the practice acceleration problems provided and paste into
bottom half of page 14 leaving room for a summary.
On the top of pg.16 of your Science NoteBook
Write the following standard neatly.
S8P3b. Students will demonstrate the effect of
balanced and unbalanced forces on an object in
terms of gravity, inertia and friction
Page 17
S8P3b.
Forces
9/7/12
How are forces related to the motion of objects?
All motion is due to forces acting on objects. A
force is a push or pull. Force is measured in a
unit called Newtons (N).
The net force is a total combination (sum) of all
forces acting on an object.
A balanced force is one in which the net forces
equal zero. There is no motion.
An unbalanced force has a net force greater
than zero. The object acted on shows
movement in one direction. Only an
unbalanced force can change the motion of an
object.
On the bottom of pg.16 of your Science NoteBook
Underneath the standard
Page 19
S8P3b.
Forces
9/10/12
How are forces related to the motion of objects?
An unbalanced force acting on an object at rest will
cause it to __________________.
An unbalanced force that acts on an object already in
motion can change the speed OR direction of an
object.
Unbalanced forces can act in the same direction or in
opposite directions. If forces are in the same direction,
you add them. If forces are acting in the opposite
direction, you subtract them.
Summary
On pg.18 of your Science NoteBook
During the game of tug-of-war
What happens when the two teams pull with equal force?
What type of forces are these called?
How would you represent this with force arrows?
What happens when one team pulls with a greater force?
What type of forces are these called?
How would you represent this with force arrows and an illustration?
Page 21
S8P3b.
Forces
9/17/12
How are forces in nature related to the motion of objects?
There are many different types of forces in nature
that act on objects. They include: gravity, friction
and inertia.
Gravity is an attractive force that works to pull
objects together.
The law of universal gravitation states that a
force of gravity acts between all objects in the
universe.
There are two factors that affect the force of
gravity between objects: 1. the objects’ masses
and 2. the distance between them. (mass is the
amount of matter in an object).
Page 23
S8P3b.
Forces
9/17/12
How are forces in nature related to the motion of objects?
The larger the mass, the more the gravitational
attraction.
The shorter the distance between the objects,
the more the gravitational attraction.
Inertia is the tendency of an object to resist a
change in motion.
Mass affects inertia. An object with a large mass
has more inertia than an object with a small
mass.
Summary
On page 20
Draw/write the following 3 questions and determine which has a greater
gravitational “pull” and WHY.
1.
25kg
3.
25kg
25kg
35kg
25kg
2.
25kg
Set A (red balls)
25kg
Set B (green balls)
5kg
On page 22
Earth
Moon
Actual Orbit
Gravity and inertia work together to keep the Moon orbiting Earth. First, the
Moon’s inertia pushes it to travel continuously in a straight line. At the same
time, Earth’s gravity is pulling the Moon toward Earth. These combined forces
cause the Moon to move in a curved path (orbit) around Earth. Without gravity,
the Moon would fly off into space. The Moon is able to continuously orbit Earth
because the Moon’s inertia and the Earth’s gravity are balanced.
Question 1
• What happens to the motion of an object
when the forces are balanced?
– A. The motion changes.
– B. The motion does not change.
– C. The motion speeds up.
– D. The motion slows down.
Question 2
• What happens to an object if the forces acting
upon it are unbalanced?
– A. Its motion will not change.
– B. The motion will come to a stop.
– C. The net force will be zero.
– D. Its motion will change.
Question 3
• What happens to an object moving in the
same direction as the net force?
– A. It will speed up.
– B. Its motion will not change.
– C. It will stop moving.
– D. It will slow down.
Question 4
• Which pair of objects will have the greatest
gravitational attraction to each other?
– A. two cinder blocks 6 centimeters apart
– B. two marbles 12 centimeters apart
– C. a cinder block and marble 6 centimeters apart
– D. a cinder block and marble 12 centimeters apart
Page 25
S8P3b.
Laws of Motion
9/19/12
Since gravity works to pull objects toward each other, what
keeps the planets from crashing into the Sun?
Newton’s first law of motion states that an
objects at rest will stay at rest, and an object in
motion will continue in motion in a straight line
unless an unbalanced force acts on the object.
This law is sometimes called the Law of Inertia.
This means if the net force acting on an object is
zero, the object remains at rest, or if the object is
already moving, continues to move in a straight
line with constant speed.
Summary
Page 27
S8P3b.
Laws of Motion
9/19/12
If Newton’s first law of motion is accurate, why do moving
objects sometimes come to a stop?
The force that brings nearly everything to a stop
is called friction.
Friction is the force that opposes the motion of
an object. This contact force acts whenever an
object in motion rubs against a surface. Friction
opposes motion when two surfaces touch. The
contact reduces the speed of the moving object
and releases heat.
There are 3 types of friction: sliding friction, rolling
friction and static friction
Page 29
S8P3b.
Laws of Motion
9/20/12
If Newton’s first law of motion is accurate, why do moving
objects sometimes come to a stop?
Sliding friction is friction that slows down an
object that slides. Example: brake pads on a
bicycle that rub against the wheel
Static friction is friction that acts on an object at
rest. Example: trying to move a refrigerator
Rolling friction is friction that acts when an object
rolls across a surface. Example: rolling a cart
around
Summary
On page 26
Draw an illustration of friction in action. Your
illustration must be neat and colored. You
must identify the two surfaces that are
interacting and what is occurring in order for
it to be friction. You must also identify the
type of friction.
On page 28
Identify an example of each type of friction (not those provided in class).
Illustrate, color and label the friction interaction of each using the table format
below.
Type of Friction
Example
Illustration with Labels
Page 31
S8P3b.
Newton’s laws
9/25/12
What is the relationship between force, mass and
acceleration?
Newton’s second law of motion states that
the acceleration of an object depends on
the mass of the object and the amount of
force applied.
This law can be expressed mathematically
with:
Force = mass x acceleration
(newton) = (kilogram) x (meters/second2)
Summary
On page 30
Write the question and answer:
Name two ways to increase the acceleration of an
object.
1.
2.
What force is necessary to accelerate a 1,250kg
car at a rate of 40m/s2?
Page 33
S8P3b.
Laws of Motion
9/25/12
When a person pushes on a wall, the wall pushes back. How
does this relate to Newton’s 3rd law?
Newton’s third law of motion states that for
every action there is an opposite and equal
reaction. Forces act in equal and opposite pairs.
Example: pushing on a wall, a rocket launching,
diving off the side of a pool.
Summary
Page 35
S8P3c.
Simple Machines
9/27/12
What is a simple machine and how do they make work easier?
Work is the application of a force to an object to
move a certain distance in the direction of the
force.
Work requires two things:
•a force applied to an object
•the object must move in the direction of the
force.
Example: pushing a lawnmower
The equation for work is:
work = force x distance
The unit for work is the Joule (J).
Page 37
S8P3c.
Simple Machines
9/27/12
What is a simple machine and how do they make work easier?
A simple machine is a device that makes work
easier by changing the size of a force OR
changing the direction of a force.
The six simple machines are:
Pulley
Inclined plane
Wedge
Wheel and Axle
Screw
Lever
Summary
On page 34
An inclined plane is a straight, slanted surface.
They make work easier because it is easier to push
an object up a ramp than to lift the same object
straight up to the same height. Ex. a ramp
On page 34
A wedge is an inclined plane that is wider or thicker
at one end than at the other. A wedge makes work
easier because when moved, a wedge is used to
cut, split, or pry apart objects. Ex. knife blade or
axe.
On page 34
A screw is an inclined plane that is wrapped around
a cylinder. When a screw is turned, a small force is
applied over the long distance of the screw’s
threads.
On page 36
A lever is a simple machine that consists of a bar
that pivots at a fixed point called a fulcrum. The
force applied to a lever is called the effort. The
object being moved is called a load. There are
three classes of levers: 1st class, 2nd class and 3rd
class.
On page 38
A wheel and axle is a simple machine that consists
of a shaft called the axle, inside the middle of a
wheel. Ex. bicycle wheel
Any force that is applied to the wheel gets
transferred to the axle and vice versa. When force
is applied to the wheel, the difference in size
between the wheel and axle causes the force to
increase as it is transferred to the axle.
On pages 38-39
A pulley is a rope or chain wrapped around a
wheel. A load is attached to one end of the rope. A
force is applied to the other end of the rope.
There are 2 kinds of pulleys: fixed and movable.
A fixed pulley is one that does not move. They are
often used to lift something. A fixed pulley changes
the direction of force, but does not reduce the
amount of force needed to lift the load.
On pages 38-39
A movable pulley is a pulley that moves. One end
of the rope is tied to a stationary object and the
other is free for you to pull on. The load is attached
directly to the pulley. The pulley moves along the
rope as the free end is pulled. They reduce the
amount of force needed to move an object, but you
must pull the rope farther.
On pages 38-39
A block and tackle is a system (combination) of
pulleys.
On pages 38-39
Mechanical advantage = output force (Newtons)
input force (Newtons)
MA = F out
F in
Efficiency = output work (joules)
input work (joules)
Eff = W out
W in
X
100%
X
100%