Forces & Motion Chapter 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 kgm/s 2  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.  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 friction 1. 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... 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

12.2 Newton’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

Brainstorm What 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? 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

2 nd Law of Motion  The acceleration of an object is equal to the net force acting on it divided by the object’s mass Acceleration = Force Mass F a = m

Acceleration  Always in the same direction as the net force  REMEMBER  N = 1kgm/s 2

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 2 nd Law Examples #1 A 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? #2 An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s 2 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

12.3 Newton’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 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

12.4 Universal 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 1 st 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