Forces & Motion

Forces & MotionChapter 12

TUG-O-WAR TIME!!!

What is a force A push or 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 depends 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 kg•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

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

12.2Newton’s First & Second Laws of Motion

Newton’s First Law of Motion The state of motion of an object does not

change as long as the net force acting on the object is zero

An object at rest remains at rest

An object in motion remains in motion with the same speed and direction (i.e. no acceleration)

An unbalanced force must be acting on an object if the object is accelerating

BrainstormWhat are some everyday events that give evidence of Newton’s first law?

Inertia Newton’s first law

is called the “Law of Inertia”

Inertia is the tendency of an object to resist change in motion

Beach Ball Activity

Crash Test… Seatbelts... To wear or not to wear?

Unbalanced Forces Unbalanced forces cause an object’s velocity to

change

If velocity is change, the object is accelerating (a change in speed or direction)

The acceleration of an object depends on both the force acting on it and the mass of the object

Mass A measure of the inertia of an object; depends on

the amount of matter an object contains

2nd Law of Motion The acceleration of an object is equal to

the net force acting on it divided by the object’s mass

Acceleration = ForceMass

Fa = m

Acceleration Always in the same direction as the net

force

REMEMBER N = 1kg•m/s2

Math Practice (guided) An automobile with a mass of 1000

kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

More 2nd Law Examples#1A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

#2An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile?

Weight vs. Mass Weight is the force of gravity acting on

an object

Weight = Mass x Acceleration due to gravity

Weight is a measure of the force of gravity acting on an object

12.3Newton’s Third Law of Motion and Momentum

Newton’s Third Law Whenever one object exerts a force on a

second object, the second object exerts an equal and opposite force on the first object

For every action there is an equal and opposite reaction Action and reaction forces are equal in

size and opposite in direction

Action-Reaction Forces DO NOT CANCEL Action forces and reaction forces act on

different objects so they do not cancel each other out

Net force only equals zero if opposite forces are acting on the same object

Example 1 A swimmer uses her

arms to push against the water (ACTION FORCE)

The swimmer is propelled forward because the water exerts a force on the swimmer (REACTION FORCE)

Example 2 Hammer hitting a

nail into a piece of wood

Action Force?

Reaction Force?

Example 3 Your bumper car

runs in to another bumper car

Action force?

Reaction force?

Momentum The product of an object’s mass and its

velocity

An object has a large momentum if the product of its mass and velocity is very large

Momentum = Mass (kg) x Velocity (m/s)

The momentum of an object at rest is zero because it has no velocity

Which has more momentum? A 0.046 kilogram golf ball with a speed

of 60.0 meters per second

Or a 7.0 kilogram bowling ball with a speed of 6.0 meters per second?

Conservation of Momentum What happens to momentum when

objects collide? It is conserved (or stays the same)

If no net force acts on a system then the total momentum of the system does not change

The loss of momentum of one object equals the gain in momentum of another object

12.4Universal Forces

4 Universal Forces These four forces exist everywhere in

the universe Electromagnetic Forces Strong Nuclear Forces Weak Nuclear Forces Gravitational Forces

Gravitational Force The weakest universal force

Definition An attractive force that acts between any

two masses

Newton’s law of universal gravitation states that every object in the universe attracts every other object

Examples Your desk is exerting a gravitational force

on you and you are exerting a gravitational force on your desk.

The person next to you is exerting a gravitational force on you, and you are exerting a gravitational force on them

Believable?

Here’s why you don’t feel it… It takes an enormous mass (like Earth’s)

to exert a large gravitational force

The Earth & Moon Newton’s 1st law says that unless a force

acts on an object, the object will continue to move along a straight line path

Earth’s gravitational force on the moon keeps the moon in orbit around Earth

Centripetal Force A center-directed force that continuously

changes the direction of an object to make it move in a circle

Gravity on Earth is a center-directed force, so object that are close enough to earth to be affected by gravity move in a circular path around Earth

Satellites in Orbit Launched into orbit by a rocket or space

shuttle

Because they have an initial velocity (and because there is virtually no friction in space), satellites will move in a circular path around Earth at a constant speed

The centripetal force of gravity results in a circular path around Earth