# Forces & Motion Chapter 12.

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Forces & Motion Chapter 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 ( ) 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 Activity Choose 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.
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

= 0 Equal and opposite forces
Combining Forces = Adding Forces = Subtracting Forces = 0 Equal and opposite 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 friction
Static Friction Sliding Friction Rolling Friction 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

Suits... 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

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