Newton’s Three Laws Of Motion &

Newton’s Law of Gravitation

Objective 4.05 Determine factors that affect motion including:

Force. Friction. Inertia.

Momentum.

Newton’s Three Laws Of Motion(Review)

1. First Law of Motion - An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law is often called "the law of inertia".

What does this mean?

• This means that there is a natural tendency of objects to keep on doing what they're doing. All objects resist changes in their state of motion. In the absence of an unbalanced force, an object in motion will maintain this state of motion.

Let's study the "skater" to understand

this a little better.

What is the motion in this picture?

What is the unbalanced force in this picture?

What happened to the skater in this picture?

This law is the same reason why you

should always wear your seatbelt.

2. Second Law of Motion - Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

What does this mean? • Everyone unconsciously knows the

Second Law. Everyone knows that heavier objects require more force to move the same distance as lighter objects.

• However, the Second Law gives us an exact relationship between force, mass, and acceleration. It can be expressed as a mathematical equation:

F = m x a

or

Force = Mass times Acceleration

This is an example of how Newton's Second Law works:

Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s2. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

Answer = 50 Newtons

Force is measured in Newtons!

3. Third Law of Motion - For every action there is an equal and opposite re-action.

What does this mean?

This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard.

Let's study how a rocket works to understand Newton's Third Law.

The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.

When one object applies a force to a second object, that force is called the action.

The speed of a moving object together with its direction of travel gives the object’s velocity.

Newton’s Law of Gravitation

• Air offers resistance to the motion of objects through it. Since there is no air resistance on the moon, a ball and a feather would fall at the same rate!

• Scientists have learned that when the effects of air resistance are removed, objects of different weights do, indeed, fall at the same rate. In addition, air resistance may be too small to matter for objects that are fairly compact. Over short distances such objects fall at the same rate even in air.

• An object is pulled to Earth by gravity, an attraction between the mass of Earth and the mass of the object. Objects with a large mass are pulled on by gravity with more force, but they also have more inertia. (Remember that an object’s inertia is its resistance to a change in motion.) This extra resistance to motion exactly offsets the greater pull of gravity on them. Therefore, objects with greater mass fall with the same acceleration as less massive objects!

Acceleration of Gravity

• A free-falling object is an object which is falling under the sole influence of gravity. A free-falling object has an acceleration of 9.8 m/s/s, downward (on Earth). This numerical value for the acceleration of a free-falling object is such an important value that it is given a special name. It is known as the acceleration of gravity - the acceleration for any object moving under the sole influence of gravity. A matter of fact, this quantity known as the acceleration of gravity is such an important quantity that physicists have a special symbol to denote it - the symbol g. The numerical value for the acceleration of gravity is most accurately known as 9.8 m/s/s.

––– Look at the graph on page F36. –––

Review

1. The tendency of an object to resist any change of motion is known as ______.

A. force

B. Mass

C. inertia

D. balance

2. The amount of matter in an object is called its ________.

A. mass

B. balance

C. force

D. weight

3. The greater the mass of an object,A. the less force it can exert.

B. the more space it takes up.

C. the more balanced it is.

D. the greater the inertia.

4. A book is sitting on a dashboard of a car that's stopped at a traffic light. As the car starts to move forward, the book slides off the dashboard. This is an example of which law?

A. Newton’s First Law of Motion

B. Newton’s Second Law of Motion

C. Newton’s Third Law of Motion

D. None of the Above

5. When you blow up a balloon and the pinch the open end closed and then let it go sending the balloon zooming all over the room this is an example of which law?

A. Newton’s First Law of Motion

B. Newton’s Second Law of Motion

C. Newton’s Third Law of Motion

D. None of the Above

6. A fish swimming through water pushes water backwards with its fins. The water propels the fish forward. Which law does this represent?

A. Newton’s First Law of Motion

B. Newton’s Second Law of Motion

C. Newton’s Third Law of Motion

D. None of the Above

7. A male giraffe has a large heavy head that is used to strike other males with tremendous force during confrontations. This is an example of which law?

A. Newton’s First Law of Motion

B. Newton’s Second Law of Motion

C. Newton’s Third Law of Motion

D. None of the Above

8. Seatbelts were invented to protect people. This is a result of which law?

A. Newton’s First Law of Motion

B. Newton’s Second Law of Motion

C. Newton’s Third Law of Motion

D. None of the Above

9. According to Newton's Third Law of Motion, when a hammer strikes and exerts a force on a nail, the nail

A. creates a balanced force.

B. disappears into the wood.

C. moves at a constant speed.

D. exerts and equal and opposite force back on the hammer.

10. An example of a balanced force is

A. a car sliding on ice.

B. a tug-of-war game in which no one wins.

C. a car hitting a telephone pole.

D. a roller coaster going down the first drop.