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