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12.1 Forces
12.1 Key Concepts• How do forces affect the motion of an object?
• What are the four main types of friction?
• How do gravity and air resistance
affect a falling object?
• Why does a projectile follow a
curved path?
12.1 Forces
The wind pushes against the man and his umbrella. The push from the wind is a _____________force
12.1 Forces
A force is a _________ or a _________ that acts on an object.
A force can cause…• a resting object to ______________• a moving object to __________________
What Is a Force?
push pull
move
accelerate
12.1 Forces
Measuring Force
You may have measured forces using a _________ ___________
The stretch of the spring in the scale depends on the amount of ____________ (a type of force) acting on it.
What Is a Force?
springscale
weight
12.1 Forces
Units of Force
Force is measured in newtons, abbreviated as ___
One newton (N) is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2).
1 N =
What Is a Force?
N
1 kg · m/s2
12.1 Forces
Representing Force
The ___________ of the arrows show relative amounts of force.
In which direction does the weight (force) of the apples point?
In which picture is the weight (force) greater?
What Is a Force?
length
downward
picture B
12.1 Forces
Representing Force
What upward force is keeping the apples from plummeting to the ground?
What Is a Force?
The spring
12.1 Forces
When the forces on an object are balanced, the net force is _____________, and there is no change in the object’s motion.
Combining Forces
When an unbalanced force acts on an object, the object _________________.
zero
accelerates
12.1 Forces
The net force is the ___________________ acting on an object after all the forces are combined.
• Forces in the same direction ___________ together.• Forces in opposite directions _________________ from
one another or ________________ each other out.
Combining Forces
overall force
addsubtract
cancel
12.1 Forces
Forces can add together or subtract from one another.
Combining Forces
Adding forces Subtracting forces
Equal and opposite forces
12.1 Forces
Are the forces shown equal?
Are they moving towards each other or in opposite directions?
What is the net force?
Combining Forces
yes
oppositezero (canceled out)
12.1 Forces
Friction is a force that ________________ the motion of objects that touch as they move past each other.
Friction
There are four main types of friction: • ______________ friction• ______________ friction• ______________ friction• ______________ friction
opposes
Static
Sliding
Rolling
Fluid
12.1 Forces
Static Friction
Static friction is the friction force that acts on objects that are ________________.
Static friction always acts in the direction opposite to that of the applied force.
push
Static friction
not moving
12.1 Forces
Sliding Friction
Sliding friction is a force that opposes the direction of motion of an object as it _______________ over a surface.
Sliding friction is ___________ than static friction.
push
Sliding friction
movement
slides
less
12.1 Forces
Rolling FrictionRolling friction is the friction force that acts on ________________ objects.
The force of rolling friction is about 100 to 1000 times ______________ than the force of static or sliding friction.
______________________ in these wheels greatly reduce friction by replacing sliding friction with rolling friction.
rolling
less
Ball bearings
12.1 Forces
Fluid Friction• Fluid friction opposes the motion of an object
through a _______________.
– Fluids are anything that _____________• Gases, liquids
– Fluid friction acting on an object moving through the air is known as _________ ___________________.
fluid
flows
air resistance
12.1 Forces
Gravity
Gravity causes objects to accelerate ________________, whereas air resistance acts in the direction __________________ to the motion and reduces acceleration.
Gravity is a force that acts between any _______ ________________.
• Gravity is an attractive force. • Gravity can act over _______________ distances.
downwardopposite
twomasses
large
12.1 Forces
Earth exerts an attractive, downward force on this boulder. The supporting rock exerts an upward force on the boulder. The forces are balanced.
Gravity
12.1 Forces
Falling Objects• Terminal velocity is the ______________
velocity of a falling object when the force of air resistance ____________ the force of gravity.
Gravity
Has the flying squirrel reached terminal velocity? Check the size ofthe arrows!
constant
equals
No…forces are not equal
12.1 Forces
A thrown ball follows a ____________ path.
Projectile motion is the motion of a falling object (projectile) after it is given an initial _______________ velocity.
Projectile Motion
curved
forward
12.1 Forces
Assessment Questions
1. If an object is at rest, which of the following statements must be true? a. There are no forces acting on the object.
b. There is no friction acting on the object.
c. The forces acting on the object are unbalanced.
d. The net force acting on the object is zero.
12.1 Forces
Assessment Questions
2. Which of the following is not a type of friction?a. static frictionb. sliding frictionc. fluid frictiond. pull friction
12.1 Forces
Assessment Questions
3. In which direction does Earth’s gravitational force act? a. opposite the direction of motion
b. downward toward the center of Earth
c. upward away from the center of Earth
d. in the direction of motion
12.1 Forces
Assessment Questions
4. A ball thrown into the air follows a projectile course due to the initial velocity and thea. force of gravity.
b. effect of air resistance.
c. motion of Earth beneath it.
d. mass of the ball.