FORCES
CHAPTER 19
M. OSTMANN
7th Grade Science
Kate Bond Middle School
WHAT IS A FORCE?
•What causes these cyclists to move forward?
•What prevents these cyclists from moving as
fast as a race car?
WIND
GRAVIT
Y
FORCES
FORCE…
IS A PUSH OR PULL THAT ACTS ON AN OBJECT.
Measuring Force… is easy.
You can measure weight (force)…
with Spring scales like these.
UNITS OF FORCE: NEWTON
•Sir Isaac Newton is
the eponymous
scientist who
discovered how force,
mass and acceleration
are related.
1 Newton is the FORCE needed to accelerate a
1kg object 1 meter per second every second
OR
1N = 1kg * m/s2
HOW DO YOU REPRESENT FORCES?
DRAW ARROWS!
•Direction of arrow = direction
of force
•Length of arrow = strength of
force
QUESTIONS?
1) How many forces are represented here?
2) Which force has the largest magnitude? The smallest?
3) Is this wood block moving or at rest? How do you know?
NET FORCE
ADD FORCES IN THE SAME DIRECTION
SUBTRACT FORCES IN THE OPPOSITE DIRECTION
NET FORCE =
•SAME DIRECTION
= Wave force + Surfer force
= 1,000 N + 800 N = 1,800N
PUSHING FORCE OF
WAVE = 1,000 N
•OPPOSING DIRCETION
= Weight force – Water
force
= 757 N – 757 N = 0 N
NET FORCE = 1,800 N
PUSHING FORCE
OF SURFER =
800 N
UPWARD
FORCE OF
WATER = 757
N
WEIGHT = 757N
BALANCED FORCES
Skater does not
move!!!
NET FORCE = Gravitational force – upward force from the ground =
680N -680 N = 0 N
UNBALANCED FORCE
The Skater is
moving in a forward
direction.
Net Force = 120N skater - 100 N air resistance = 20 N forward force
YOU DECIDE…
IS THE SKATER MOVING?
NO!
BALANCED OR UNBALANCED?
BALANCED.
FRICTION
•ALL MOVING OBJECTS ARE SUBJECT TO FRICTION
•FORCE THAT OPPOSES THE MOTION OF MOVING OBJECTS.
•WITHOUT FRICTION, ALL SURFACES WOULD BE MORE
SLIPPERY THAN A SHEET OF ICE
THERE ARE FOUR MAIN TYPES OF FRICTION:
Midtown Memphis, 2010
STATIC
SLIDING
ROLLING
FLUID
STATIC FRICTION
Friction force that prevents object from moving.
Acts on objects that are not moving.
Static friction force acts in the direction
opposite to applied force.
SLIDING FRICTION
Force that slows the motion of a sliding object.
Acts on objects that are moving in a sliding motion.
A weaker force than static friction.
Less force is needed to keep an object moving than it takes to
start it moving.
ROLLING FRICTION
Force that slows the rolling motion of a round object.
100 to 1,000 times less strong than the force of static or
sliding motion
•This is why round objects are easier to move
•Invention of wheel made it easier to move heavy things
•Human advancement rooted in principle of frictional force
FLUID FRICTION
Force that slows the motion of an object through a fluid
Fluids include water, air, and other viscous fluids such as molasses, mashe
You can feel fluid friction when you stir thick mashed potatoes this batch.
Fluid friction increases as the object’s speed increases.
The faster you stir, the stronger the fluid friction is, and thus, the
harder it is to stir.
AIR RESISTANCE
IS A TYPE OF FLUID FRICTION THAT DECREASES THE SPEED OF A FALLING
OBJECT.
GRAVITY
An attractive force that pulls objects together.
THE EARTH HAS THE STRONGEST GRAVITATIONAL FORCE.
THIS GOALIE IS PULLED TOWARS THE GROUND
DUE TO THE EARTH’S GRAVITATIONAL FORCE.
Acceleration due to gravity = 9.8 m/s2
Veritasium Video 1
Veristasium Video 2
FALLING OBJECTS
GRAVITY : DOWNWARD FORCE
WHAT FORCES AFFECT THE MOTION OF THESE SKYDIVERS?
AIR RESISTANCE: OPPOSING FORCE
WHICH FORCE IS GREATER? IN WHAT DIRECTION ARE THE SKYDIVERS MOVING?
PROJECTILE MOTION
ALL THROWN OBJECTS WILL FOLLOW A CURVED PATH
ONCE THROWN, THE OBJECT FEELS TWO FORCES:
1) GRAVITY
2) AIR RESISTANCE
G
AIR RESISTANCE
G
G
G
G
G
G
G
G
PROJECTILE MOTION: A CASE STUDY
THE RED BALL WAS DROPPED
G
THE YELLOW BALL WAS THROWN
G
WHICH BALL HITS THE GROUND FIRST?
JA JA. TRICK QUESTION.
BOTH BALLS HIT THE GROUND AT THE SAME TIME
WHY???
EXTRA CREDIT OPPORTUNITY!!!!!
WHO WROTE THIS SONG?