Newton’s Laws8th Grade ScienceCreate for BCSD

What will we be examining?

• Newton’s Laws: Three laws that explain the motion of objects caused by forces

• Inertia: The tendency of objects to keep on doing what they are already doing (moving or staying at rest

Why will we be examining Sir Isaac

Newton?• S8P3. Students will investigate

relationship between force, mass, and the motion of objects. • b. Demonstrate the effect of balanced

and unbalanced forces on an object in terms of gravity, inertia, and friction. (DOK 2)

But first… the scientists who came

before…• Newton wasn’t the

first person to come up with this idea…

• Aristotle (384-322 B.C.) started the conversation with the proposal that force is required to keep an object moving

• However, his concept was proven incorrect…

• Galileo (1564-1642) was next…

• Concluded that moving objects not subjected to friction or other forces would continue to move indefinitely

• Descartes (1596-1650) was certainly on Newton’s list of study.

• Published "Principles of Philosophy”

• "that each thing, as far as is in its power, always remains in the same state; and that consequently, when it is once moved, it always continues to move."

Newton’s 1st Law of Motion

• The state of motion of an object does not change as long as the net force is zero

• Known as the Law of Inertia

• First Law - Law of InertiaA moving object moves in a straight line with constant speed unless a force acts on it.

• The tendency of an object at rest to remain at rest and an object in motin to remain in motin unless acted upon by an unbalanced force.

What is Inertia?

• An object's tendency to resist a change in motion is inertia .

• What is included what you discuss the motion of an object?

Inertia and mass

• The more mass an object has, the more inertia it has.

• Why?

This means that the more mass an object has, the harder it is to move, stop, or change the speed or direction of the object.• An object will not start moving unless a

force acts on it• An object will not stop moving unless a

force acts on it• An object will not change speed unless a

force acts on it• An object will not change direction unless a

force acts on it

• Inertia is the tendency of an object to not change it's motion• If it is moving, it keeps moving in the

same direction• If it is at rest, it stays at rest

• Objects do not change their motion unless a force acts on them

Recap…

• Let’s summarize…

• An object moving wants to remain in motion with the same speed and direction

• An object at rest wants to stay at rest

• Things want to keep doing what they are already doing!

Newton’s 2nd Law of Motion

• If a ping pong ball and a basketball were both dropped at the same time from the roof of our school, which would hit the ground with a greater force? Common sense tells us that the basketball ball would. The difference in forces would be caused by the different masses of the balls. Newton stated this relationship in his second law, the force of an object is equal to its mass times its acceleration.

• Units for force, mass, and acceleration

• Units for F=ma• Force is measured in Newtons, N.• Mass is measured in kilograms, kg.• Acceleration is measured in meters per

second squared, m/s2.

• It could be said that mass and acceleration have a direct proportional relationship to the force of an object. Look at the equation. What does that mean?

• If acceleration is directly proportional to the applied net force, then by whatever factor acceleration changes, force changes by the same factor. To see this we will need to consider an object with constant mass. Let's consider an object with a mass of 5 kg.

• Suppose this object has a mass of 5 kg and an acceleration of 3 m/s2. Let's call this a1. So:

a1 = 3 m/s2

• What force is necessary to cause this acceleration to our 5 kg object? Let's figure that out and call it F1:

• F1 = ma1

• F1 = (5 kg)(3 m/s2)

• F1 = 15 N

• Now let's think about this object when it is moving at twice this acceleration. That is, we will change the acceleration by a factor of 2. Let's call this new acceleration a2. So:

• What force is now necessary to cause this new acceleration, a2? We will call this force F2, and here is its calculation:

• Would the force increase?

• Would the force decrease?

• Why?

• F2 = ma2

• F2 = (5 kg)(6 m/s2)

• F2 = 30 N

• The second acceleration, 6 m/s2, is twice the first acceleration, 3 m/s2, therefore,

• The second force, 30 N, is twice the first force, 15 N.

• As acceleration increases, so does force at the same rate.

• Clearly, both the acceleration and the force on this object change by the same factor, 2. These matching factor changes would occur for any factor change and for any constant mass. So the equation F=ma contains the direct proportion between acceleration and the applied net force.

Examine closely…

The inverse proportion

between acceleration

and mass• Now, what about the inverse

proportion between acceleration and mass, is that contained within F=ma?

• Let's consider a situation in which an applied net force is 6 N. That is, object 1 will have a net force of 6 N applied to it.

• We will say our object has a mass of 3 kg. So:

• What will be the acceleration of this object? Let's call that acceleration a1, and let's solve for it using F=ma. So:

• Now let's cut the mass to one-third. That is, we will consider a change in mass by a factor of 1/3. That would make the mass of object 2 to be 1 kg, or m2 = 1 kg. If we apply the same force, 6 N, to object 2, what will be its acceleration?

Here is the answer

• So the second mass is 1/3 the first mass, since:• m2 = (1/3)m1

• 1 kg = (1/3)(3 kg)• 1 kg = 1 kg

• And the second acceleration is three times the first acceleration, as in:• a2 = (3)a1

• 6 m/s2 = (3)(2 m/s2)• 6 m/s2 = 6 m/s2

• Clearly, the acceleration and mass change by reciprocal (or inverse) factors. The factors are 3 and 1/3, respectively. Since these two quantities change by reciprocal (inverse) factors, these two quantities are in an inverse proportion.

Let’s Examine closely…

Newton’s 3rd Law of Motion

• Imagine a rocket is being launched from the earth. Hot gases are pushed out from the bottom of the rocket as the rocket is pushed upward. The force of the gases pushing against the surface of the earth is equal and opposite to the force with which the rocket moves upward. The motion of the rocket can be explained by Newton's third law, for every action there is an equal and opposite reaction. In other words, when one object exerts a force on another object, the second object exerts a force of equal strength in the opposite direction on the first object.

Newton’s 3rd Law of Motion

• The Third Law is concerned with how objects push on each other and exchange momentum when they interact.

• The third law states that for every force there is an equal and opposite force. For example, if you push on a wall, it will push back on you as hard as you are pushing on it.

• Equal and opposite forces

• Equal and opposite forces are present when two objects collide, as shown in the following animation.

• http://ezeducationorama.com/ezMedia/physics/forces/newton/threeLaws/thirdLaw/forces/thirdLaw1.mp4

• Forces always act in pairs. The two forces act in opposite directions. When you push on an object, the object pushes back with an equal force. Think of a pile of books on a table. The weight of the books exerts a downward force on the table. This is the action force. The table exerts an equal upward force on the books. This is the reaction force. Note that the two forces act on different objects. The action force acts on the table. The reaction force acts on the books.

Let’s look at another example

• There is a force from the boy on the dog's toy, and there is a force from the dog's toy on the boy. Forces always come in pairs similar to this example. Consider the boy (A) as one system and the toy (B) as another. 

• There can never be a single force acting alone, without its action-reaction partner. Forces only come in action-reaction pairs. Think carefully about propelling a skateboard with your foot. Your foot presses backward against the ground. The force acts on the ground. However, you move, so a force must act on you, too. Why do you move? What force acts on you? 

• You move because the action force of your foot against the ground creates a reaction force of the ground against your foot. You 'feel' the ground because you sense the reaction force pressing on your foot. The reaction force is what makes you move because it acts on you.

Draw Diagrams

• When sorting out action and reaction forces, it is helpful to draw diagrams. Draw each object apart from the other. Represent each force as an arrow in the appropriate direction. Here are some guidelines to help you sort out action and reaction forces:

Drawing Diagrams

You try!

• Consider the situation of holding a book in your hand. Draw one diagram for the example. Are there any interaction pairs? When identifying interaction pairs, keep in mind that they always occur in two different diagrams and they always will have the symmetry of subscripts noted earlier. In this case, the interaction pair is F(book)on hand and F(hand) on book.

• In this case, the interaction pair is F(book)on hand and F(hand) on book.