#### Transcript What Is a Force?

```12.1 Forces
Bellringer 2/22
1. What do you think is a
force?
1. Name some forces you
can think of off the top of
1. How do you think we
measure forces?
12.1 Forces
What Is a Force?
A force is a push or a pull that acts on an
object.
Forces can cause resting objects to move, or it
can accelerate a moving object by changing the
object’s speed or direction.
12.1 Forces
What Is a Force?
Measuring Force
You may have measured
forces using a spring
scale.
12.1 Forces
What Is a Force?
Units of Force
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
12.1 Forces
What Is a Force?
Representing Force
Arrows can represent a
force. The lengths of the
arrows show relative
amounts of force.
12.1 Forces
Combining Forces
When the forces on an object are balanced,
the net force is zero, and there is no change
in the object’s motion.
When an unbalanced force acts on an
object, the object accelerates.
12.1 Forces
Combining Forces
The net force is the overall force acting on an object
after all the forces are combined.
• Forces in the same direction add together.
• Forces in opposite directions subtract from one another.
12.1 Forces
Combining Forces
Forces can add together or subtract from one
another.
12.1 Forces
Combining Forces
Forces can add together or subtract from one
another.
Subtracting forces
12.1 Forces
Combining Forces
Forces can add together or subtract from one
another.
Subtracting forces
Equal and opposite forces
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
Bellringer 2/23
1. What is a force?
1. What happens when a balanced force acts
upon an object?
1. What happens when an unbalanced force acts
upon an object?
12.1 Forces
Calculating Forces
• To calculate any force, we use the formula:
Force= mass x acceleration
OR
F=ma
12.1 Forces
Important units
• Force:
1 N = 1 kg•m/s2
• Mass:
– Base unit is grams
g
• Acceleration:
m/s2
– Remember!
1000g=1kg
12.1 Forces
Guided Practice
Label the following units as force, mass, or
acceleration:
• Grams
• m/s2 x kg
• m/s2
• kg x m/s2
• N
• kg:
12.1 Forces
Guided Practice
A man hits a golf ball (0.2 kg) which
accelerates at a rate of 20 m/s2. What
amount of force acted on the ball?
12.1 Forces
Guided Practice
You push a friend sitting on a swing. She has
a mass of 50 kg and accelerates at a rate of
4 m/s2. Find the force you exerted.
12.1 Forces
Guided Practice
The wind pushes a paper cup along the sand
at a beach. The cup has a mass of 0.025 kg
and accelerates at a rate of 5 m/s2. How
much force is the wind exerting on the cup?
12.1 Forces
FUN STUFF
• Weekly Writing
Due Friday 2/26
• Homework Due
Friday 2/26
12.1 Forces
Friction
Friction is a force that opposes the motion of
objects that touch as they move past each
other.
12.1 Forces
Gravity
In what direction does Earth’s gravity act?
Earth’s gravity acts downward toward the
center of Earth.
12.1 Forces
Gravity
How do gravity and air resistance affect a
falling object?
Gravity causes objects to accelerate
downward, whereas air resistance acts in
the direction opposite to the motion and
reduces acceleration.
12.1 Forces
Gravity
Gravity is a force that acts between any two
masses.
• Gravity is an attractive force.
• Gravity can act over large distances.
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
Gravity
Falling Objects
• Both gravity and air resistance affect the motion
of a falling object.
• As objects fall to the ground, they accelerate
and gain speed.
• 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