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Physics Day 1 of Force
LO: Difference between Weight and Mass
LO: Explain Newton's first law of Motion
Agenda
Do Now
Notes
Worksheet
HW # 10 Read page 120, page 125-129
Do Pr. 1, 2and 5 on page 129
Newton’s First Law of Motion
Think about:


Is A force required to keep an object in
motion?
Would it require a force to set an object
in motion?
Force


A force is a push or pull
A force can:
 Start an object moving
 Stop an object
 Speed up an object
 Slow down an object
 Exerted施加on an object can
change the velocity.
 Change the shape of an object
Force
A force is a vector, therefore it
has a magnitude and direction
 A force follows the rules of
vector addition
 Force is measured in Newtons
(N)

Why do objects move?

Is a force required to keep an object in
motion?


Only if there is another outside force acting
on the object such as friction.
Galileo found this out


An object moving horizontal moves with a
constant velocity on a smooth surface.
A ball has this quality
Net Force


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When forces are applied to an
object, you take the sum of the
force vectors to find the total force
on the object
This total force is called the net
force.
When the net force equals zero
then all the forces are said to be
balanced.
Force are balanced
Net Force
Normal force
The net force equals
zero because the
weight equals the
normal force.
Weight
Normal Force


The normal Force is the force
placed on an object by the surface
it is on.
The normal Force is always
perpendicular to the surface.
Weight

The weight of an object is determined
by two things



Gravity
Mass
Formula:
W = mg
(g= 9.81 m/s2)
Net Force


When a force is applied to an object it
has an effect on the object.
What happens when two forces of equal
magnitude are applied on an object, but
are in opposite directions?
10 Newtons
平
衡

-10 Newtons
(The net force
equals zero)
Newton’s First Law
1.
2.

An object at rest stays at rest
An object in motion stays in
motion and continues in a straight
line.
Condition 1 and 2 are true if there
is no outside force (unbalanced)
acting on the object.
Write in you own words

An object at rest remains at rest, and
an object in motion continue in motion
with constant velocity( that is constant
speed in a straight line) unless the
object experience a net external force.
Forces Don't Keep Objects Moving

Newton's first law of motion declares
that a force is not needed to keep an
object in motion. Slide a book across a
table and it slide to a rest position.
Newton’s First Law


Also called the law of inertia.
Inertia:


The more mass an object has the more
force it takes to change the motion of the
object.
The faster the object moves the more force
it takes to change the motion of the object.
A train has a lot of inertia
Mass


Mass is essentially the measure of
the inertia 惯性 an object has.
The more mass the more inertia.
Does an object have same mass
on the Earth and Jupiter?



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W= mg ( g earth = 9.81m/s/s)
W= mg ( g Jupiter =26m/s/s)
W= mg ( g moon =1.6m/s/s)
http://phet.colorado.edu/sims/massspring-lab/mass-spring-lab_en.html
Inertia



Inertia: the resistance an object has to
a change in its state of motion.
Mass as a Measure of the Amount
of Inertia
Newton's first law of motion sometimes referred to as the law
of inertia.
Question?

Have you ever experienced inertia
(resisting changes in your state of
motion) in an automobile while it is
braking to a stop?
Inertia


Inertia: tendency趋势 of an object
to resist抵制changes in its velocity.
Inertia: tendency of an object to
resist accelerations.
Why does he continue
moving?
Which one has more inertia?
Truck or car ?
What happen with ladder?
Does he have a seat belt?
Can you explain Why it
happens?


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Blood rushes from your head to your feet
while quickly stopping when riding on a
descending elevator.
The head of a hammer can be tightened
onto the wooden handle by banging the
bottom of the handle against a hard
surface.
A brick is painlessly broken over the hand of
a physics teacher by slamming it with a
hammer. (CAUTION: do not attempt this at
home!)
Think about:

Will an object change its state of
motion with balanced force on it?

Will an object resist changes in its
velocity with balanced force? Why?
Applications

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1. Imagine a place in the cosmos far
from all gravitational and frictional
influences. Suppose that you visit that
place (just suppose) and throw a rock.
The rock will
a. gradually逐步stop.
b. continue in motion in the same
direction at constant speed.
Applications

2. A 2-kg object is moving horizontally
with a speed of 4 m/s. How much net
force is required to keep the object
moving at this speed and in this
direction?
What is its mass in space?

3. Supposing you were in space in a
weightless environment, would it
require a force to set an object in
motion?
Inertia depend upon…?

4. Mac and Tosh are arguing in the
cafeteria. Mac says that if he flings the
Jell-O with a greater speed it will have a
greater inertia. Tosh argues that inertia
does not depend upon speed, but
rather upon mass. Who do you agree
with? Explain why.


Check Your Understanding
1. A group of physics teachers is taking
some time off for a little putt-putt golf.
The 15th hole at the Hole-In-One PuttPutt Golf Course has a large metal rim
that putters must use to guide their ball
towards the hole. Mr. S guides a golf ball
around the metal rim When the ball
leaves the rim, which path (1, 2, or 3)
will the golf ball follow?