1. Unbalanced Forces Cause Changes in Motion That Can Be Predicted and Described

CHAPTER 6

SECTION 1

1.  Unbalanced forces cause changes in motion that can be predicted and described.

Gravity and Falling Objects

1.  Galileo demonstrated in the 1500s that the mass of an object does not affect the time the object takes to fall to the ground.

2.  No matter what the mass of an object is, it will fall to the ground at the same rate.

3.  The rate at which the object falls is due to the acceleration of gravity.

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4.  All objects accelerate toward Earth at a rate of 9.8 m/s2.

5.  For every second that an object falls, its downward velocity increases by 9.8 m/s.

6.  The change in an object’s velocity can be calculated with the following formula:

change in velocity = acceleration due to gravity x time it takes to fall

ΔV = g x t

7.  The change in an object’s velocity is determined by subtracting the final velocity and the initial velocity.

Air Resistance and Falling Objects

1.  Air resistance is defined as the force that opposes the motion of objects through air.

2.  The amount of air resistance that a falling object experiences is determined by its size, shape, and speed.

3.  The greater the object’s surface area, the greater the resistance will be.

4.  As the speed of a falling object increases, air resistance also increases and acts as an upward force.

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5.  As an object falls, the upward force of air resistance continues to increase until it is equal to the downward force of gravity.

6.  When air resistance force equals gravitational force, the object stops accelerating because the net force is zero.

7.  Terminal velocity is the constant velocity that occurs when the force of air resistance is equal to the force of gravity.

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8.  An object is in free fall when no force other than gravity is acting on a falling object.

9.  Free fall occurs only in the absence of air resistance.

Orbiting Objects Are In Free Fall

1.  Objects such as a spacecraft that orbit Earth are in a free fall.

2.  Orbiting objects have 2 motions; they move forward and they are falling toward Earth.

3.  If you are inside of an object that is in a free fall, you will experience weightlessness.

4.  As an object orbits, it is traveling in a circular path. Any object traveling in a circular path is constantly changing direction.

5.  Any object changing direction is being acted on by an unbalanced force.

6.  The unbalanced force that causes objects to move in a circular path is centripetal force.

7.  Centripetal means center seeking.

8.  Gravity provides the centripetal force that keeps objects in orbit around other objects.

Projectile Motion and Gravity

1.  Projectile motion is the curved path an object follows when it is thrown near Earth’s surface.

2.  Projectile motion has 2 independent components; horizontal motion and vertical motion.

3.  When the horizontal component and the vertical component are combined, it causes a curved path.

If there was no gravity, a projectile would move in a straight path.

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SECTION 2

1.  In 1686, Sir Isaac Newton proposed his 3 laws of motion.

Newton’s First Law of Motion

1.  The first law states that an object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.

2.  An object in motion will stay in motion forever unless an unbalanced force acts on it. Friction is just such an unbalanced force.

3.  Newton’s first law is also known as the law of inertia.

4.  Inertia is the tendency of all objects to resist any change in their motion or non-motion.

5.  Mass is a measure of inertia.

6.  The greater the mass, the greater the inertia.

Newton’s Second Law of Motion

1.  The second law states that the acceleration of an object depends on the mass of the object and the amount of force applied.

2.  Newton’s second law describes the motion of an object when an unbalanced force acts on it.

3.  Acceleration decreases as mass increases. Acceleration increases as force increases.

4.  The acceleration of an object will be in the direction of the unbalanced force.

5.  Newton’s second law is expressed in the formula

Acceleration = force

mass

a = F

m

6.  Newton’s second law explains why objects fall to Earth with the same acceleration.

Newton’s Third Law

1.  The third law states whenever one object exerts a force on another object, the second object exerts an equal and opposite force on the first object.

2.  The third law says that for every action, there is an equal and opposite reaction.

3.  Newton’s third law notes that all forces act in pairs.

4.  The way that force pairs interact affects the motion of objects.

5.  Force pairs do not act on the same object. If they did, the net force would be zero and no motion would occur.

SECTION 3

1.  Momentum is a characteristic of a moving object. It is the product of the mass and velocity of the object.

Momentum, Mass, and Velocity

1.  The more momentum an object has, the harder it is to stop or change its direction.

2.  The greater the mass, the greater the momentum. The greater the velocity the greater the momentum.

3.  Momentum can be calculated using the following formula:

momentum = mass x velocity

p = m x v

4.  Just like velocity, momentum has a direction. The direction is always the same as the direction of the object’s velocity.

5.  Units for momentum are kg∙m/s2

The Law of Conservation of Momentum

1.  The law of conservation of momentum states that when objects collide, the total amount of momentum stays the same if no other forces are at work.

2.  When objects collide, momentum can be transferred, but not created or destroyed.