### 12.1 Forces

12.1 Forces
Notes Titles:
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What is a Force?
Combining Forces
Friction
Gravity
Projectile Motion
12.1 Forces
The wind pushes against
the man and his umbrella.
The push from the wind is a
force.
12.1 Forces
I.
Forces
A. Force- a push or a pull that acts on an object.
1. A force can cause a resting object to move, or it can
accelerate a moving object by changing the object’s speed
or direction.
2. Measuring Force
a. You may have measured forces using a spring scale.
b. The stretch of the spring in the scale depends on the
amount of weight (a type of force) acting on it.
12.1 Forces
3. Classification of Forces
a. Contact
1) normal forces
2) frictional forces
3) applied forces
b. Non-contact
1) gravitational forces
2) electrical forces
3) magnetic force
12.1 Forces
What Is a Force?
12.1 Forces
4. Units of Force
a. One newton (N) is the force that causes a 1kilogram mass to accelerate at a rate of 1 meter per
second each second (1 m/s2).
1 N = 1 kg•m/s2
5. Representing Force
a. Arrows can represent a force.
1) The direction of the arrow represents the
direction of the force
2) The lengths of the arrows show the strength, or
magnitude of the force
12.1 Forces
What Is a Force?
12.1 Forces
B. Combining Forces
1. net force- the overall force acting on an object after all
the forces are combined.
a. Forces in the same direction add together.
b. Forces in opposite directions subtract from one
another.
12.1 Forces
2. Balanced Forces
a. When the forces on an object are balanced, the net
force is zero, and there is no change in the object’s
motion
b. An unlimited number of individual forces can act
on an object to produce a net force of zero.
12.1 Forces
Combining Forces
The two groups pull with equal forces in opposite
directions. The forces combine to make a net force
of zero.
12.1 Forces
3. Unbalanced Forces
a. An unbalanced force is a force that results when the
net force acting on an object is not equal to zero.
b. When an unbalanced force acts on an object, the
object accelerates.
1) Speeds up
2) Slows down
3) Changes direction
12.1 Forces
Combining Forces
c. Forces can add together or subtract from one another.
12.1 Forces
Combining Forces
Forces can add together or subtract from one
another.
Same direction
Subtracting forcesOpposite direction
12.1 Forces
Combining Forces
Forces can add together or subtract from one
another.
Same direction
Subtracting forcesOpposite direction
Equal and opposite forcesResults in no net force
12.1 Forces
C. Friction- force that opposes the motion of objects that touch as
they move past each other.
1. The strength of the force of friction depends on two
factors.
a. How hard the surfaces push together
1) Rubbing hands together
b. The types of surfaces involved
1) Sand vs snow
12.1 Forces
D. Gravity - a force that acts between any two masses.
1. Gravity is an attractive force among all objects
2. Gravity can act over large distances.
a. Moon and Earth distance
b. Earth and Sun distance
3. Earth’s gravity acts downward toward the center of Earth.
12.1 Forces
Gravity
Earth exerts an attractive,
downward force on this
boulder. The supporting rock
exerts an upward force on the
boulder. The forces are
balanced.
12.1 Forces
4. Both gravity and air resistance affect the motion of a
falling object.
a. Gravity causes objects to accelerate downward at a
rate of 9.8 m/s²
b. air resistance acts in the direction opposite to the
motion and reduces acceleration.
1) Terminal velocity is the constant velocity of a
falling object when the force of air resistance
equals the force of gravity.
12.1 Forces
Gravity
This flying squirrel takes
slow its fall and increase the
distance covered in the jump.
feather and hammer
12.1 Forces
Projectile Motion
A. Their masses are different, but the blue and
green balls fall at the same rate.
12.1 Forces
Projectile Motion
projectile motion video
A. Their masses are different, but the blue and
green balls fall at the same rate.
B. The yellow ball is a projectile, following a
curved path.
12.1 Forces
E. Projectile motion - the motion of a falling object (projectile)
after it is given an initial forward velocity.
1. The combination of an initial forward velocity and the
downward vertical force of gravity causes the ball to follow
a curved path.
2. Air resistance and gravity are the only forces acting on a
projectile.
12.1 Forces
Assessment Questions
1. If an object is at rest, which of the following
statements must be true?
a.
b.
c.
d.
There are no forces acting on the object.
There is no friction acting on the object.
The forces acting on the object are unbalanced.
The net force acting on the object is zero.
12.1 Forces
Assessment Questions
1. If an object is at rest, which of the following
statements must be true?
a.
b.
c.
d.
There are no forces acting on the object.
There is no friction acting on the object.
The forces acting on the object are unbalanced.
The net force acting on the object is zero.
ANS: D
12.1 Forces
Assessment Questions
2. Which of the following is not a type of friction?
a.
b.
c.
d.
static friction
sliding friction
fluid friction
pull friction
12.1 Forces
Assessment Questions
2. Which of the following is not a type of friction?
a.
b.
c.
d.
static friction
sliding friction
fluid friction
pull friction
ANS:
D
12.1 Forces
Assessment Questions
3. In which direction does Earth’s gravitational force
act?
a.
b.
c.
d.
opposite the direction of motion
downward toward the center of Earth
upward away from the center of Earth
in the direction of motion
12.1 Forces
Assessment Questions
3. In which direction does Earth’s gravitational force
act?
a.
b.
c.
d.
opposite the direction of motion
downward toward the center of Earth
upward away from the center of Earth
in the direction of motion
ANS: B
12.1 Forces
Assessment Questions
4. A ball thrown into the air follows a projectile
course due to the initial velocity and the
a.
b.
c.
d.
force of gravity.
effect of air resistance.
motion of Earth beneath it.
mass of the ball.
12.1 Forces
Assessment Questions
4. A ball thrown into the air follows a projectile
course due to the initial velocity and the
a.
b.
c.
d.
force of gravity.
effect of air resistance.
motion of Earth beneath it.
mass of the ball.
ANS: A
12.1 Forces
Assessment Questions
1. The SI unit for force is 1 kg•m/s2, also called one
kepler.
True
False
12.1 Forces
Assessment Questions
1. The SI unit for force is 1 kg•m/s2, also called one
kepler.
True
False
ANS:
F, newton