#### Transcript Unbalanced Forces & Acceleration

```2.3 Unbalanced Forces
and Acceleration
centripetal force
Newton’s second law of motion
Newton’s third law of motion
Acceleration
Force pairs
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity
• Unbalanced forces cause accelerations.
• Without the upward normal force exerted by
the hand, the ball accelerates downward
because of the unbalanced force of gravity.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• An unbalanced force applied to a moving
object in the same direction as the motion
speeds the object up.
• An unbalanced force applied to a moving
object in the opposite direction as the
motion slows the object down.
• Friction is applied in the opposite direction
and slows the sled down.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• Unbalanced forces can also change the
direction of motion.
• A thrown ball curves downward because of
the unbalanced force of gravity.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• A ball tied to a string and swung in a
horizontal circle has circular motion.
• The velocity of the ball
changes because the
direction of motion
changes.
• The string exerts an
unbalanced force on
the ball called
centripetal force.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• Centripetal force is the force perpendicular
to the velocity and pulls toward the center
of the circle.
• The string keeps the
ball moving in a circle.
• Unbalanced forces
cause an object to
speed up, slow down,
or change direction.
2.3 Unbalanced Forces and Acceleration
Newton’s Second Law of Motion
• The acceleration of an object equals the
net force divided by the object’s mass.
• Unbalanced forces cause an object to
accelerate.
What is Newton’s
second law of
motion?
2.3 Unbalanced Forces and Acceleration
Balanced Forces, Unbalanced Forces,
and Newton’s Second Law
• If the net force is equal to zero:
– the forces are balanced.
– The acceleration is zero.
– The object’s motion does not change.
2.3 Unbalanced Forces and Acceleration
Balanced Forces, Unbalanced Forces,
and Newton’s Second Law (cont.)
• If the net force is not equal to zero:
– the forces are unbalanced.
– The acceleration is not zero.
– The object changes motion.
2.3 Unbalanced Forces and Acceleration
Newton’s Second Law and
Planetary Motion
• According the Newton’s second law, there
must be an unbalanced force acting on
Earth and the other planets.
• The Sun’s gravity is the
centripetal force that
keeps Earth and the
planets moving in a
circular path around
the Sun.
2.3 Unbalanced Forces and Acceleration
Newton’s Third Law of Motion
• When one object exerts a force on another,
the second object exerts an equal force in
the opposite direction.
• The feet exert a force on the ground and
the ground exerts an equal force on the feet
of a jumper, causing the jumper to
accelerate upward.
2.3 Unbalanced Forces and Acceleration
Force Pairs
• The forces that two objects exert on each
other are called force pairs.
• The pairs are opposite in direction.
• The pairs are equal in magnitude.
• The forces do not cancel out because they
act on different objects.
2.3 Unbalanced Forces and Acceleration
Action and Reaction
• One force in a force pair is called the action
force.
• The second force in a force pair is called
the reaction force.
• For every action force there is an opposite
and equal reaction force.
2.3 Unbalanced Forces and Acceleration
In what direction does the net force
of an object moving with constant
speed and in a circular motion
occur?
A downward
B opposite to the object’s motion
C toward the center of the circle
D in the direction of the object’s
velocity
2.3 Unbalanced Forces and Acceleration
Acceleration depends on both the
force applied and ____.
A distance from the object
B speed of the object
C mass of the object
D normal force of the object
2.3 Unbalanced Forces and Acceleration
How does the motion of an object
traveling in a straight line at 5 m/s
change if a balanced force is applied?
A The motion does not change.
B The object speeds up.
C The object slows down.
D The object travels in a horizontal
circle.
```