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.

2.3 Unbalanced Forces and Acceleration

What is Newton’s second law of motion?

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.

2.3 Unbalanced Forces and Acceleration

• The Sun’s gravity is the centripetal force that keeps Earth and the planets moving in a circular path around the Sun.

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

Lesson 3 Review

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

Lesson 3 Review

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

Lesson 3 Review

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.

2.3 Unbalanced Forces and Acceleration