Transcript Newton`s First Law

Newton’s First Law
Law of Inertia
What You Will Learn
► Distinguish
between balanced and net
forces
► Describe Newton’s First Law of motion
► Explain how friction affects motion
Why Is It Important
► Forces
can cause the motion of an object to
change
Review Vocabulary
► Motion-
a change in position
► Velocity-the speed and direction of a
moving object
► Inertia- is the tendency of an object to
resist any change in its motion
What is force?
►Force-is a push or a pull
When you throw a ball you expert a force on
the ball.
After the ball leaves your hand, gravity exerts
a force on the ball bringing it to the ground.
When the ball hits the ground, the ground
exerts a force on the ball bringing it to a
stop.
Examples of Forces
► Magnetic
Force
► Gravitational Force
► Force of a bat on a baseball
Can you give examples of force?
Combining Forces
► More
than one force can act on an object
► If you hold a magnet near a paper clip, you,
the magnet, and gravity all exert forces on
the paper clip.
► Net Force- is the combination of all forces
acting on an object. It determines how the
object moves.
► If motion of an object changes, then its
velocity changes.
Balanced Forces
► Balanced
Forces-
two or more forces
exerted on an object
cancel each other out
and they do not
change the objects
velocity.
► Net Forces is zero.
Unbalanced Forces
► Unbalanced
Forces-
If the effects of the
forces do not cancel
each other out and the
object changes
velocity, then the
forces are unbalanced.
► Net forces is then the
difference between the
forces.
Newton’s First Law
►A
force does not need to be continually
applied for an object to keep moving, giving
it a net force of zero.
► The 1st Law
If the net force acting on an object is zero,
then the object remains at rest, or if the
object is already moving, it continues to
move in a straight line with constant speed.
Newton’s
st
1
Law-Simplified
Friction
►Friction- is the force than brings nearly
everything to a stop.
► It is the force that acts to resist sliding
between two touching surfaces
► Friction also acts on objects that are sliding
or moving through substances such as air or
water.
Examples of Friction
Friction (cont.)
►
►
►
►
To keep an object moving, a force must be applied to
overcome the effects of friction.
If friction is removed, an object will continue to move in a
straight line at a constant speed.
What effect does the air have on the puck in the game
below?
How would the puck move if there were no layer of air?
Types of Friction
► Sliding- the force between two touching surfaces.
The bonds between the two surfaces are broken
and form again as the surfaces pass each other.
Types of Friction (cont.)
► Static- the force that prevents an object from
moving when force is applied and is caused by the
attraction between the atoms on the two surfaces
that are in contact. Example is moving a
refrigerator.
Types of Friction (cont.)
► Rolling- occurs when an object rolls across a
surface.
► The amount of force when rolling across a surface
is much less than the force needed when sliding
between two surfaces.
Test Yourself
►What
do the different forms of
friction have in common?
They all slow an object down!
What is the difference between static
friction and sliding friction?
► Static
friction prevents two objects at rest
from moving past one another.
► Sliding friction slows down two objects
moving against each other
Test Yourself
► What
type of friction is it?
A wagon carrying a small child
Rolling
A basketball player slipping of a wet floor
Sliding
A box at rest on a ramp
static
Test Yourself
Explain whether the forces involved are
balanced or unbalanced?
You push a box until it moves
Unbalanced, the box starts moving
You push a box but it does not move
Balanced, the box does not move
You stop pushing a box and it slows down
Unbalanced, the box slows down
Test Yourself
What two factors cause static friction between
two surfaces to increase?
The surface is rougher or the object is heavier
Can a moving object have a net force of zero?
Yes if an object is already moving, it will
continue at constant velocity until acted
upon by an outside force