Download Chapter 12: Forces and Motion

12.1
Forces
Objectives:
M 4-1
Describe examples of force and identify appropriate SI units used
to measure force.
10
M 4-2
Explain how the motion of an object is affected when balanced
and unbalanced forces act on it.
7
M 4-3
Compare and contrast the four kinds of friction.
15
M 4-4
Describe how gravity and air resistance affect falling objects.
8
FORCES
 FORCE: a push or pull that
acts on an object
 Measured in Newtons (N)
 = a vector: shows direction
and magnitude
 Net force: is the overall force
acting on an object after all
forces have been combined.
Balanced and Unbalanced Forces
Balanced forces: a net force of 0.
*No Motion
Unbalanced forces: net force not equal to 0.
* Causes motion
Examples of Forces
 Electromagnetic Forces (between magnets,
protons/electrons)
 Nuclear Forces (strong force) (holds the nucleus of an
atom together, nuclear energy, atomic bomb).
 Centripetal Forces (changes the direction of an object and
causes it to move in a circle)
 Friction
 Gravity
Friction
 A force that opposes the
motion of objects that touch
each other
 Friction happens at the
surface of objects (where
they touch)
 4 main types of friction:
 Static, Sliding, Rolling, and
Fluid.
Friction between two surfaces
Static Friction
 Acts on stationary objects.
 Always acts in the direction that is opposite of an
applied force
 Keeps an object from being moved across a surface
 Stops acting on an object once the object is moving
 Ex. Moving a piece of furniture on carpet.
Static Friction
Sliding Friction
 Acts on an object once the object is moving
 Goes against the direction in which the object is
moving as it slides on a surface
 Occurs when two objects rub together
 Less of a force than static friction
Rolling Friction
 Force that acts on rolling (round) objects
 Acts on the surface and at the point where the object is
touching something else (the ground for example)
 Allows the object to touch the ground and not slip
 Slows rolling objects down
Fluid Friction
 Force that opposes an object’s motion through a fluid
(liquid or gas)
 Force increases as the object’s speed through the fluid
increases
 When fluid friction acts on objects moving through the
air, it is called air resistance.
Gravity
 Force of attraction that acts between any two
masses.
 Pulls objects together
 The magnitude of the force is proportional to
the mass of the objects.
Ex. Gravitational pull between sun and earth
Gravitational pull between satellites and the
earth.
Gravity on earth
 Pulls objects towards the earth at a rate of
 9.8 m/s2
 Gravity acts equally on ALL objects, pulling them at the
same rate towards the earth.
 Objects fall towards the earth at different rates due to
air resistance (fluid friction).
Falling objects
TWO Forces acting on a falling object:
1. Gravity (pulling downward)
2. Air resistance (acts in opposition to gravity, reduces
acceleration)
 Terminal velocity: when air resistance = force of
gravity
 The object is no longer accelerating.
 V= a x t speed of a falling object
Puzzle…
 Myth or fact:
If you drop two basketballs with the same shape but
different masses (2 kg and 20kg), they will hit the
ground at the same time.
True or False, and why?