Post on 13-Jan-2016
Chapter 3Forces & Newton’s
Laws
Forces Force—a push or pull that one
body exerts on another Forces can be shown as
vectors Vector –an arrow showing the
direction and amount of force.– The longer the arrow, the larger
the force.
Forces cont’d Net Force—when 2 or
more forces act on an object at the same time
Balanced Forces—multiple forces that are equal in size and opposite in direction.–An object will remain
still
Forces cont’d Unbalanced Forces—when the
size and direction of multiple forces acting on an object are unequal.–cause objects to move.
Balanced & Unbalanced Forces
Inertia Inertia—the tendency of an object
to resist a change in its motion–The velocity of an object only changes
if a force changes it.
The inertia of an object is related to mass– The greater the mass, the greater its
inertia.
Newton’s 1st Law of Motion Newton’s 1st Law: The Law of
Inertia–An object at rest stays at rest and an
object in motion stays in motion with the same speed and direction unless acted on by an unbalanced force.
Newton’s 1st Law of Motion
Shooting Objects at RestThe object’s inertia causes it to remain still
even when a bullet passes through it.
Punkin Chunkin: Inertia II
Punkin Chunkin: Inertia
Friction Friction: A force that opposes motion
between two surfaces that are touching each other
The amount of friction is dependent upon– Type of Surfaces– Amount of force pressing the surfaces together
Types of Friction Static Friction
– Friction between two objects that are NOT moving.
Sliding Friction– Friction between two surfaces moving past
each other. Rolling Friction
– Friction between a rolling object & the surface it is rolling on.
Fluid Friction – Friction between an object and a gas (like air)
or liquid (like water)
Air Resistance Air Resistance: Collisions of the object's leading
surface with air molecules. Dependent upon a variety of factors, most
importantly– the speed of the object
Increased speeds = increased air resistance. – the cross-sectional area of the object
The more spread out, the more air resistance
Newton’s Second Law Newton’s 2nd Law: The acceleration of an object is
dependent upon the force acting upon the object and the mass of the object.
Newton’s Second Law as an equation: F = m × a F = force (N) m = mass (kg) a = acceleration (m/s/s or m/s2)
Force Unit The SI unit for force is the Newton (N) The Newton is a combined unit Since F=ma, the unit for force
combines the mass unit (kg) with the acceleration unit (m/s2) N = kg•m/s2
Gravity Universal law of Gravity:
– Any 2 objects will exert an attractive force on each other
– The size of the force is dependant on 2 things Mass of both objects Distance between the objects
– The range of gravity never disappears Therefore it is not possible for an
object with mass to be weightless.
http://www.youtube.com/watch?v=p_o4aY7xkXg
Weight vs. Mass Weight: Force of gravity pulling you
toward the earth (or moon or other object)– F = ma becomes W = mg – Weight = mass × gravitational acceleration
g on Earth is 9.8m/s2
– SI Unit for Weight = Newton (N)– Weight can change with a change in
location.
Mass: A measure of how much matter an object has.–All objects have mass because they have
inertia–SI Unit for Mass = Kilograms (kg)
Pounds is a unit of mass NOT weight.
Centripetal Force Centripetal Force: The force exerted toward
the center of a curved path. The smaller the radius, the larger the
centripetal force– The radius could be the length of rope or distance
between 2 objects– A tighter turn has more force
Video: Roller coaster Loops
Gravity & Centripetal Force Earth’s gravity exerts
a centripetal force on the Moon that keeps it in a nearly circular orbit.
Real World/space station & centripetal force
Falling Objects on Earth Near Earth’s surface acceleration
due to gravity is 9.8 m/s2
Video: If 2 objects with different masses are dropped from the same height, which would hit the ground first?
Without air resistance, all objects hit the ground at the same time, mass has no effect.
http://www.youtube.com/watch?v=aRhkQTQxm4w&feature=plcp
dropping objects on the moon
Projectile Motion Projectile—Anything dropped, thrown, or shot through
the air Because of gravity and inertia, projectiles moving
horizontally have a curved path.
Juggling & Projectile Motion
Mythbusters Bullet Fired and Dropped
An object shot horizontally & one dropped from the same height at the same time, will hit the ground at the same time.
Monkey and the Zookeeper The zookeeper must shoot the
banana from the banana cannon to the monkey who hangs from the limb of a tree.
This particular monkey has a habit of dropping from the tree the moment that the banana leaves the muzzle of the cannon.
The zookeeper is faced with the dilemma of where to aim the banana cannon in order to hit the monkey.
If the monkey lets go of the tree the moment that the banana is fired, then where should she aim the banana cannon?
Monkey and the ZookeeperWhere should the zookeeper aim?Taking Gravity into effect The zookeeper aims above the monkeyDoes the speed of the banana change the effect?The zookeeper aims at the monkey and shoots the banana very fast The zookeeper aims at the monkey, yet shoots the banana very slow. ZOO in SPACEIn a low gravity environment, where should the zookeeper shoot the banana?
NEWTON’S 3rd LAW:
For every action, there is an equal and opposite reaction
“Action” and “Reaction” are names of forces
Forces ALWAYS occur in pairs Single forces NEVER happen
“Equal & Opposite” In Newton’s 3rd Law,
“equal” means: Equal in size Equal in time.
– occur at EXACTLY the same time.
In Newton’s 3rd Law, “opposite” means:
Opposite in direction– forces are EXACTLY
180o apart in direction.
Only forces that act on the same object can cancel.
Newton’s 3rd Law action and reaction forces act on different objects, so they DON’T cancel.
Writing Action Reaction Forces–ACTION
Bowling ball hits the pin to the left.
–REACTION
Pin hits the bowling ball to the right
Momentum: A property of how much force is required
to change the object’s motion – The object’s mass times its velocity
p = mv– p = momentum– m = mass– v = velocity
SI UNIT = Kg m / s
Law of Conservation of Momentum Momentum may be
transferred to another object
The total momentum before a collision is equal to the total momentum after the collision
m1v1 = m2v2
Types of Collisions Elastic collisions
– the total kinetic energy before the collision is equal to the total kinetic energy after the collision.
– Usually the objects bounce off of each other.
Inelastic collisions– the total kinetic energy before the collision is NOT
equal to the total kinetic energy after the collision.– Usually the objects stick together
interactive
The Truck and The Ball A pickup truck is moving with a constant speed. In the course of its motion, a ball is projected straight
upwards by a launcher located in the bed of the truck. Assume the ball does not encounter a significant amount
of air resistance. What will be the path of the ball and where will it be
located with respect to the pickup truck?
Many would insist that there is a horizontal force acting upon the ball since it has a horizontal motion.
Yet this is simply not the case. The horizontal motion of the ball is the result of its own inertia.
When projected from the truck, the ball already possessed a horizontal motion, and thus will maintain this state of horizontal motion unless acted upon by a horizontal force.
An object in motion will continue in motion with the same speed and in the same direction ... (Newton's first law).
Newton's Law Review physclips newton's laws http://www.phys.unsw.edu.au/einsteinlight/