Forces

1
Chapter 3

• Forces Motion

2
Newton

• Sir Isaac Newton (1642-1727)
• Found the relationship between force, mass,
  acceleration
• Three laws of motion
• Universal law of gravitation
• Invented calculus

3
Force

• Forces change motion
• Objects will remain doing what they are doing
  until there is a force (push or pull) applied to
  it
• Force push or pull any action that has the
  ability to change motion
• Units pounds and newtons
• Newtons preferred
• 1 pound 4.48 newtons

4
Origin of the newton

• Relates force motion
• 1 newton I kilogram times 1 meter per second
  squared
• A force of 1 newton causes 1 kilogram to have an
  acceleration of 1 meter per second squared
• Force mass have different units
• Force newton
• Mass kilogram

5
Newtons 1st Law of Motion

• Law of inertia
• Inertia property of an object to resist
  changing its state of motion
• Objects will resist change unless a large enough
  force acts upon it
• Amount of inertia depends on mass
• More mass more inertia

6
Newtons 1st Law of Motion

• Inertia developed by Galileo

7
Newtons 1st Law of Motion

• Forces dont keep objects moving
• An objects resistance to inertia depends on the
  objects mass

8
Check Yourself

• Imagine a place far in the cosmos far away from
  frictional and gravitational influences. Suppose
  that an astronaut in that place throws a rock.
  What will the rock do?
• An 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?
• 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.

9
Check Yourself

• The NCHS DES teachers are taking some time off
  for a little putt-putt golf. The 15th hole at the
  Hole-In-One Putt-Putt Golf Course has a large
  metal rim which putters must use to guide their
  ball towards the hole. Mrs. Hastings 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?

10
Newtons Second Law of Motion

• Relates force, mass, and acceleration
• a F/m
• a acceleration in m/s2
• F force in Newtons (N)
• m mass in kg
• Force causes acceleration mass resists
  acceleration acceleration is equal to the ratio
  of force over mass

11
Newtons 2nd Law of Motion

• Force causes acceleration
• Ice skater
• a F/m F ma m F/a
• To use this equation correctly, you NEED to USE
  the correct units!

12
Law 2 How its used today

• Scientists and engineers use it to solve
  technical problems
• Airplanes to fly
• Cars

13
Forces

• Net force ? total of all forces acting upon an
  object
• Motion depends on net force
• Equilibrium ? when the net force is zero
• No change in motion
• Balanced forces ? equal forces acting in
  opposite directions
• Unbalanced forces ? unequal forces acting in
  opposite directions

14
Forces

• Acceleration is caused by unbalanced forces or a
  net force that is not zero

15
Gravity

• Gravity ? force that pulls every mass toward
  every other mass
• Earths gravity pulls everything to the Earths
  center
• Direction of force of gravity is downward
• More mass more force of gravity
• Gravity on earth 9.8 m/s2
• Gravity is different on other planets

16
Mass Weight

• Weight ? force created by gravity on objects
• Weight depends on mass
• Mass is constant but weight changes due to
  gravity
• If you know mass you can calculate weight
• F mg where F is weight force m is mass g is
  acceleration of gravity

17
Law of Universal Gravitation

• Law of universal gravitation the force of
  attraction between two objects is directly
  related to the masses of the objects and
  inversely related to the distance between them
• ? F G m1m2
• R2
• F is force (N)
• m is mass(kg)
• G is gravitational constant (6.67 x
  10-11Nm2/kg2)
• R is distance between mass 1 and mass 2 (m)

18
Law of Universal Gravitation

• Equation used to calculate the gravity force
  between two objects

19
Friction

• Friction ? two surfaces moving against each
  other
• Used to describe forces that result from relative
  motion between objects
• Frictional forces always work against the motion
  that produces them
• Wear ? objects that continuously rub against
  each other

20
Kinds of Friction

• Air friction air moving around objects creates
  an opposing force
• Sliding friction when two surfaces rub against
  each other caused by irregularities in the
  surfaces
• Viscous friction objects that move in water
• Rolling friction one object rolling over another

21
Friction

• Friction opposes motion
• Force of friction is opposite the force causing
  the motion
• Friction reduces acceleration

22
Newtons Third Law of Motion

• For every action, there is an equal and opposite
  reaction
• All forces come in pairs
• Skateboarding your foot and the Earth
• Action/reaction forces act upon different
  objects, so the forces DO NOT cancel out and
  motion occurs

23
Momentum

• Momentum mass of an object multiplies by its
  speed (velocity)
• More mass more momentum
• P mv
• P momentum in kg m/sec
• m mass in kg
• v velocity (speed) in m/s
• Can be positive or negative

24
Law of Conservation of Momentum

• Law of conservation of momentum as long as
  interacting objects are not influenced by outside
  forces, the total amount of momentum cannot
  change
• Momentum before momentum after