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Physical Science II
Chapter 12 – Part 1: Forces
Force
 What is a force?
 ______________ – a push or pull that acts on an object.
 A force can cause a resting object to move or it can accelerate a moving object by changing the
object’s speed or direction.
 There are two types of Forces:
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 ______________ _____________ – objects are in direct contact when they are exerting forces
on each other.
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 _____________ _____________ – are exerted over distances when objects are not in direct
contact.
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 Measuring force is normally straight forward. If you’ve ever used a spring scale to measure the
weight of an object, you’ve measured a force.
 The units of Force are __________________, which we abbreviate with a capitol letter ____.
 A Newton is the amount of force required to accelerate a 1-kg mass at a rate of 1-m/s2.
 1-N = 1-kgm/s2
 ________________ are used to represent forces.
 A _______________ is an arrow that shows the size and direction of a measurement.
 If forces are in the same direction you __________ them together.
 If forces are in opposite directions you __________________ them from each other.
 When you add or subtract forces to find a total, you are calculating the net force acting on an
object.
 ________ ______________ – the total force acting on an object.
 Example #1 – Find the net force acting on the block below:
10 N
10 N
 Example #2 – Find the net force acting on the block below:
10 N
30 N
 Example #3 – Find the net force acting on the block below:
10 N
10 N
 Sometimes the forces acting on an object are ________________. When this happens the net
force acting on the object is ___________ and the motion of the object is unchanged.
 Other times, the forces acting on an object are _________________. During these situations the
net force is not zero and the object will experience ___________________ in one direction.
 Tug of war example.
 When two teams of people are playing tug of war they are trying to pull each other in
opposite directions.
 If each team pulls with exactly the same force, neither team will move.
 If one team pulls with a greater force than the other, the team pulling with less force
will accelerate in the direction of the team using greater force.
Friction
 Under normal earthly circumstances, all moving objects are subject to friction.
 ______________ – a force that opposes the motion of an object.
 Friction always acts in the ________________ direction of an object’s motion.
 Friction is a _______________ force, thus it only acts between objects that are touching.
 There are four main types of friction:
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 ____________ ______________ – a friction force that acts on objects that are not moving.
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 Detailed Static Friction Example
 Every time you take a step, your foot pushes into the floor.
 The static friction force between your foot and the floor is what keeps your foot from
sliding out from under you.
 Without this static friction force, walking would be impossible.
 ____________ ____________ – a friction force that acts between two objects that are sliding
past one another. (also called sliding friction)
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 ______________ _______________ – a friction force that acts on rolling objects.
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 Detailed Rolling Friction Example
 Many machines use ball bearings to help reduce the friction between moving parts.
Round balls are placed between points of contact for different surfaces.
 Instead of the surfaces sliding past one another, the ball bearings roll as the surfaces
move. This greatly reduces the friction between these surfaces.
 Roller blades, skateboards and automobiles are a few of the many machines that use
ball bearings.
 ____________ ______________ – a force of friction that opposes the motion of an object
moving through a fluid. (Fluids can be liquids or gasses!)
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 Detailed Fluid Friction Example
 Air resistance doesn’t seem like a big deal, but when objects are moving at high rates of
speed air resistance becomes a huge force.
 This is why professional cyclists and speed skaters wear slick racing suits.
 All types of friction forces cause the energy of a moving object to be converted into
___________ energy.
 If you rub your hands together, you feel them get warm. This is an example of transferring
energy into heat by the use of _______________.
Gravity
 Early in life we learn that if you drop an object it falls to the floor.
 Of course this is because of ______________.
 _______________ – is a force that acts between any two masses.
 Gravity is an ________________ force, which means it _________ objects together.
 Gravity is also a ____________ force since objects do not have to be in direct contact to
experience the pull of gravitation.
 Gravity exists anywhere there is ___________, the more massive an object the larger the
gravitational field.
 Earth has a gravitational field that extends far into space. Thus any mass on or near the planet
will experience a gravitational force.
 Earth’s gravity is always directed toward the ________________ of the planet.
 Gravity causes all objects to accelerate at a rate of _______________.
 ________________ – the force of gravity acting on an object.
 The reason we experience weight is because the Earth and our bodies have mass.
 The force of gravity acting between us and the Earth is what we call our ________________.
 As we get farther from the center of the Earth, the force of gravity becomes lower, thus we
would weigh less.
Falling Objects
 If you drop a stone from the top of a tall building, gravity will cause the stone to accelerate
downward.
 However, if the building is tall enough, eventually the stone will reach _____________
______________.
 _______________ _______________ – the maximum speed that can be reached by a falling
object.
 Terminal velocity caused by _________ ___________________.
 When the force of air resistance _____________ the force of gravity acting on an object, the net
force is _____________ and the object will stop accelerating.
 Terminal velocity is different for all objects.
 Both ___________ and _________________ ___________ help to determine the terminal
velocity of an object.
 Detailed Terminal Velocity Example
 A flying squirrel makes use of a large surface area to slow its decent when jumping out
of a tree.
 The squirrel spreads its arms and legs so that its body maximizes the force of air
resistance.
 Since the squirrel can decrease its acceleration to zero after only a short time, it can
travel farther through the air than what would normally be possible.
 Every object that is near Earth experiences the exact same gravitational acceleration. ___________
 However, you may notice that not all objects fall at the same speed.
 This is only because of air resistance.
 In the absence of air resistance, all objects fall at exactly the same speed.
Projectile Motion
 When you throw an object forward, you will notice that it follows a ________________ path.
 During the object’s time in flight, the only forces acting on it are ______________ and ________
____________________.
 If an object is dropped and another is thrown horizontally at the same instant, both objects will
accelerate downward at the same rate and will hit the ground at the same time.
 It does not matter that one object is initially moving horizontally. Both objects are experiencing
the same force of gravity and the same air resistance. Thus they will move downward at exactly
the same rate.