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Chapter 3 Forces & Newton’s Laws

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Chapter 3 Forces & 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.

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  • 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
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 d1
Forces cont’d
  • Unbalanced Forces—when the size and direction of multiple forces acting on an object are unequal.
    • cause objects to move.
  • 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 1 st law of motion
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.

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 II
  • Punkin Chunkin: Inertia

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
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
  • Air Resistance: Collisions of an 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 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
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
  • 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.
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.

Weight vs. Mass

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 4 min 40
gravity centripetal force
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

7 min 50 sec

falling objects on earth
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? 3 min
  • Without air resistance, all objects hit the ground at the same time, mass has no effect.

dropping objects on the moon 3 3 min

projectile motion
Projectile Motion

An object shot horizontally & one dropped from the same height at the same time, will hit the ground at the same time.

  • Projectile—Anything dropped, thrown, or shot through the air
  • Because of gravity and inertia, projectiles moving horizontally have a curved path.

Juggling & Projectile Motion2minmin

Mythbusters Bullet Fired and Dropped

3 min

monkey and the zookeeper
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 zookeeper1
Monkey and the Zookeeper

Where should the zookeeper aim?

Taking Gravity into effect The zookeeper aims above the monkey

Does 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.


In a low gravity environment, where should the zookeeper shoot the banana?

newton s 3 rd law for every action there is an equal and opposite reaction
NEWTON’S 3rd LAW:Forevery action, there is an equal and opposite reaction
  • “Action” and “Reaction” arenamesofforces
  • Forces ALWAYS occur in pairs
  • Single forces NEVER happen
  •'s+third+law&FORM=VIRE1#view=detail&mid=DAE8FBF3714FD0D979B6DAE8FBF3714FD0D979B6 6 min
equal opposite
In Newton’s 3rd Law, “equal” means:

Equal in size

Equal in time.

occur at EXACTLYthe same time.

In Newton’s 3rd Law, “opposite” means:

Opposite in direction

forces are EXACTLY180oapart in direction.

“Equal & Opposite”
  • 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 statements

Bowling ball hits the pin to the left.


Pin hits the bowling ball to the right

Bozeman science 3rd law 6 minutes

Also: find 3rd law action reaction pairs: how to write

Writing Action Reaction Statements
  • A property of how much force is required to change the object’s motion
    • The object’s mass times its velocity
    • SI UNIT for momentum = Kg m/s
  • p = m×v
    • p = momentum (kg m/s)
    • m = mass (kg)
    • v = velocity (m/s)
  • If an object has no velocity, it has no momentum.
law of conservation of momentum
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
Types of Collisions
  • Elastic collisions
    • the total kinetic energy (motion) before the collision is equal to the total kinetic energy (motion) 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
the truck and the ball
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
  • physclipsnewton's laws