Forces & MotionChapter 12
TUG-O-WAR TIME!!!
What is a force A push of pull that acts on an object
Forces can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction
Measuring Force Spring Scales
The stretch of a spring scale depend on the weight (type of force) acting on it
Units of Force Measured in newtons (N)
1 N = the force that causes a 1 kilogram mass to accelerate at a rate of 1 meter per second squared 1 N = 1 km•m/s2
This unit is named after Sir Isaac Newton (1642-1727) Scientist who explained how force, mass and
acceleration are related
Force Diagrams Use arrows to represent the direction
and strength of a force (like a vector!)
Spring Scale ActivityChoose five objects on your table. Attach a string to your objects if necessary. Use the spring scale to determine the weight (in newtons) of your objects. Draw a force arrow for each object that is to scale relative to each other force arrow.
Combining Forces Back to tug-o-war…
You can combine force arrows to show the result of how forces combine
Forces in the same direction add together
Forces in the opposite direction subtract from one another
Net Force The overall force acting on an object
after all the forces are combined
Balanced Forces Sometimes the net force acting on an
object is zero.
Balanced Forces Forces that combine to produce a net
force of zero Results in NO CHANGE in an object’s
motion
Common Example Two people locked in an arm wrestling
match
Tug-o-War match with two evenly matched teams!
Two football players pushing against one another at the line of scrimmage
Unbalanced Forces Results when the net force acting on an
object is NOT equal to zero
When an unbalanced force acts on an object, the object accelerates
Combining Forces
= Adding Forces
= 0 Equal and opposite
forces
=Subtracting Forces
Friction A force that opposes motion of objects that
touch as they move past each other.
Acts at the surface where objects are in contact (includes all solids, liquids, and gases)
Friction is important! Without friction every surface would be
impossibly slippery Food would slide right off your fork Walking would be impossible Cars would slide around with their wheels spinning
Four main types of friction1. Static Friction
2. Sliding Friction
3. Rolling Friction
4. Fluid Friction
Static Friction The friction force that acts on objects
that are not moving
Always acts in a direction opposite to that of the applied force
Sliding Friction A force that opposes the direction of
motion of an object as it slides over a surface
LESS than static friction This means that once an object is moving,
less force is needed to keep the object moving than to start it moving
Rolling Friction The friction force that acts on rolling objects
When a round object rolls across a flat floor, both the object and the floor are bent slightly out of shape at the point of contact
100 – 1000 times less than static or sliding friction This is why movers use wheeled dollies to
move heavy objects!
Fluid Friction Liquids and mixtures of air are known as
fluids
Fluid friction results when fluids (like liquids and air) oppose motion of an object Example, when you stir cake batter you can
feel fluid friction
Fluid friction increases as the speed of the object moving through the object increases
Air Resistance Fluid friction acting on an object moving
through the air
At higher speeds air resistance is a significant force
For example, swimmers, cyclists and even runners wear slick racing suits to reduce air resistance
http://Wing Suits...(Start @ 1:00 min
Types of Friction Foldable Activity!
Gravity A force that acts between any two
masses
An attractive force (it pulls objects together)
Unlike friction, gravity can act over large distances (think skydiving!)
Gravity (continued) Earth’s gravity acts downward toward
the center of the Earth
Falling Objects Both gravity and air resistance affect
the motion of a falling object
Gravity causes objects to accelerate downward
Air resistance acts in the direction opposite to the motion, reducing acceleration
Flying Squirrels… As objects fall they accelerate (gain speed)
As speed increases, air resistance increases
If an object falls long enough, the upward force of air resistance eventually will equal the downward force of gravity Forces are balanced, acceleration is zero and
the object continues falling at a constant velocity
Terminal Velocity Constant velocity of a falling object
when the force of air resistance equals the force of gravity
Projectile Motion The motion of a falling object (projectile)
after it is given an initial forward velocity
The only forces acting on a projectile are gravity and air resistance
Projectile Motion (Continued) The combination of an initial forward
velocity and the downward vertical force of gravity causes the ball to follow a curved path