 # Chapter 12.  Force: a push or pull that acts on an object  Key Point: a force can cause a resting object to move or it can accelerate a moving object.

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Chapter 12

 Force: a push or pull that acts on an object  Key Point: a force can cause a resting object to move or it can accelerate a moving object by changing it’s speed or direction.

 Forces are measures in Newtons (N)  Can use an arrow to represent the direction and strength of a force  The length represents the strength or magnitude of the force

 Forces in the same direction add together  Forces in opposite directions are subtracted  The net force is the overall force once the forces are combined  A. Adding B. Subtracting = =

BALANCED FORCESUNBALANCED FORCES  Combine to produce a net force of zero  Key Point: When the forces are balanced the net force is zero, no change in the motion  Ex. Beginning of arm wrestling  Result is a net force that is not equal to zero  Key Point: When an unbalanced force acts on an object the object accelerates  Ex. Winning team of tug of war

 All moving objects are subject to friction  Friction is the force that opposes motion  Acts at the surface where objects are in contact

1. Static Friction  Acts on objects not moving 2. Sliding Friction  Opposes motion as an object slides over a surface 3. Rolling Friction  When rolling the object and the floor are slightly bend out of shape  Acts on objects rolling 4. Fluid Friction  Opposes the motion of an object in a fluid  Gases or liquids

 Force that acts between any two masses  Is an attractive force, it pulls objects together  Key Point: Earth gravity acts downward toward the center of the Earth

 Two forces affect falling objects 1. Gravity 2. Air resistance  Gravity causes objects to accelerate downward  Air resistance acts in the direction opposite to the motion and reduces acceleration  Larger surface area the more air resistance

 The constant velocity of a falling object, when the force or air resistance equals the force of gravity

 A thrown object follows a curves path  This curved path is projectile motion  Key Point: the combination of an initial forward velocity and the downward vertical force of gravity causes the curved path

 Newton’s First Law  The state of motion of an object does not change as long as the net force acting on the object is zero.  Unless an unbalanced force acts an object at rest remains at rest  An object in motion remains in motion with the same speed and direction unless an unbalanced force acts on it

 Inertia – the tendency of an object to resist the change in motion  Examples 1. Soccer Ball– remains at rest unless someone kicks it 2. Car collision – people move forward until the seat belt stops them

 An unbalanced force causes an objects velocity to change so the object accelerates  Acceleration is directly proportional to the net force acting on it  As force increase acceleration increases

 Acceleration also depends on the mass  Mass – measure of the inertia of an object and depends on the amount of matter  Acceleration depends on force and mass  Key Point: The acceleration of an object is equal to the net force divided by the mass

 Acceleration = force/mass  Force = mass x acceleration  Mass = force/acceleration

1. A car with a mass of 200 kg accelerates when a traffic light turns green. If the net force on the car is 8000 N, what is the car’s acceleration? 2. A car accelerates at a rate of 30 m/s 2 and its mass is 1375 kg. What is the force acting on the car? 3. A 29 – N force accelerates a boy on a skateboard at 0.4 m/s 2. What was the mass of the boy?

WEIGHTMASS  Measure of the force of gravity  Measure of the amount of inertia  Amount of matter

 Weight = mass x acceleration due to gravity (9.8)  Fg = ma 1. An astronaut has a mass of 40 kg. What would his mass be on earth?

 Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.  “Action/Reaction”

 1. Swimming  2. Hammer and Nail

 A product of an object’s mass and velocity  Key Point: An object has a large momentum if the product of it’s mass and velocity is large.

 If no net force acts on a systems, then the total momentum does not change  Key Point: In a closed system, the loss of momentum in one equals the gain of momentum in another.

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