Force, Motion, and Inertia

Force, Motion, and Inertia

b. Construct an explanation using Newton’s Laws of Motion to describe the effects of balanced and unbalanced forces on the motion of an object.

c. Construct an argument from evidence to support the claim that the amount of force needed to accelerate an object is proportional to its mass (inertia).

Essential Question:

How do gravity, inertia, and friction affect the balance of objects?

What is a force?

• A force is a push or pull.

• Forces have size and direction and are expressed in Newtons (N).

• Force is always exerted by one object on another object.

Open Ended

Constructive response: Read the question carefully and respond in the space provided:

If you roll or kick a ball, what happens eventually? An unbalanced force is needed to change the speed of a moving object. So, what force is stopping the ball?

Forces: Friction

• Friction is a force that opposes motion between two surfaces.

• Friction occurs because the surface of any object is rough. Even surfaces that feel smooth are covered with microscopic hills and valleys.

Forces: Friction

• When two surfaces are in contact, the hills and valleys of one surface stick to the hills and valleys of the other surface.

Forces: Friction

• The amount of friction depends on factors such as roughness of the surfaces and the force pushing the surfaces together.

Multiple Select

Which of the choices are surfaces that will cause the most friction? More than one may apply. Hint: there are two correct choices.

• sandpaper

• gravel

• ice

• glass

• tile

Multiple Select

Which of the choices are surfaces that will cause the least friction? More than one may apply. Hint: there are three correct choices.

• carpet

• gravel

• ice

• glass

• tile

Forces: Friction

• Friction is both harmful and helpful.

Open Ended

Constructive Response: Type your answer to the question in the space provided below:

Look at the images to the left. Why would air resistance be a form of friction?

Multiple Choice

Example #1: Is friction a friend (helpful) of foe (harmful) in the following example: You are sliding into second based and scratch up your leg.

• Friend

• Foe

Multiple Choice

Example #2: Is friction a friend (helpful) of foe (harmful) in the following example: You are trying to turn your bike around a sharp curve.

• Friend

• Foe

Multiple Choice

Example #3: Is friction a friend (helpful) of foe (harmful) in the following example: You are trying to race down a water slide at Six Flags.

• Friend

• Foe

Multiple Choice

Example #4: Is friction a friend (helpful) of foe (harmful) in the following example: You are trying to push your desk into another room.

• Friend

• Foe

Multiple Choice

Example #5: Is friction a friend (helpful) of foe (harmful) in the following example: You are driving in a car and see a possum walking in front of you.

• Friend

• Foe

Forces: Air Resistance

• Air resistance is a form of friction.

• Air resistance depends on the size, shape, and speed of the object

Forces: Gravity

• Gravity is a force of attraction between objects that is due to their masses.

• All objects experience an attraction toward all other objects. This gravitational force pulls objects toward each other.

Gravity

• Compared with all other objects around you, Earth has a huge mass. Therefore, Earth's gravitational force is very large.

• You must apply forces to overcome Earth's gravitational force any time you lift objects or even parts of your body.

Gravity

• Earth's gravitational force pulls everything toward the center of Earth.

• Because of this force, the books, tables, and chairs in the room stay in place, and dropped objects fall to Earth rather than moving together toward you.

Forces Review

• There are many different forces which act on objects such as gravity and friction.

• If forces are balanced the object doesn't move

• If forces are unbalanced the object will do one of the following things: (1) Begin to move, (2) Stop moving, (3) Change direction, (4) Speed up , and (5) Slow down

Forces occur in pairs and they can be either balanced or unbalanced

Balanced Forces

• When two forces action on an object are equal in size, but are opposite in direction, we say the forces are balanced.

• Balanced forces do not cause a change in motion.

Balanced Forces

The truck in the picture to the rights does not move because the forces acting on it are balanced.

Force Diagrams

• We can show forces acting on an object in a force diagram.

• In a force diagram, the force is shown with an arrow - the larger the arrow, the bigger the force.

• The arrows in a force diagram also show the direction that the force is acting on.

• Sometimes, the arrow will be labeled with the size of the force in Newtons (N). A larger number means a larger force.

Force Diagram Example

Open Ended

Look at the picture the the left, the ship is floating on the water, the forces are balanced. Which forces are acting on the ship?

Balanced Forces

Because Gravity’s force [down] and the water’s force [up] are equal the ship stays afloat.

Open Ended

Look at the picture to the left, the vase is resting on the table. It is not moving, therefore the forces must be balanced. Which forces are acting on the vase?

Balanced Forces

The downward force of gravity and the upward force of the table are balanced therefore the vase does not move.

Unbalanced Forces

• When two forces are acting on an object and are not equal in size, we say the forces are unbalanced.

  
  

• Unbalanced forces cause a change in motion by changing the object’s speed or direction.

Open Ended

Look at the picture to the left, the man and the parachute are slowly falling to the ground.

Which forces are acting on the parachute?

Which force is bigger? How can you tell?

Unbalanced Forces

Gravity is the bigger force.

We can tell this because the direction of movement of the man and the parachute is the same as the direction of the force of gravity.

Unbalanced Forces

The same forces are acting on the elephant, but without a parachute there is much less air resistance.  

Gravity is a much bigger force. Because there is a bigger difference between the force pulling downwards and the force pushing upwards, the elephant falls much more quickly than a man with a parachute.

Open Ended

Look at the picture to the left, The paperclip is jumping up to the magnet.

Which forces are acting on the magnet? Which is the greater force?

Unbalanced Forces

The magnetic force is greater than the force of gravity therefore the paperclip moves in the direction of the biggest force.

Open Ended

Look at the picture to the left, The magnetic force is greater than the force of gravity therefore the paperclip moves in the direction of the biggest force.

What would happen if the team on the left were able to produce a bigger pulling force than the team on the right?

Forces

- If each team is exerting the same pulling force the yellow handkerchief will not move.

- If the team on the left is producing the bigger pulling force the team on the left will pull the team on the right towards the left.

Open Ended

Based on the forces shown in the picture to the left:

Which arm wrestler will likely win?

Which direction will both arms move?

Multiple Choice

Which direction will the rope move?

• left

• right

Unbalanced Forces

The rope will move to the right with the greater force.

Look at the diagram to the right.

Both women are wearing ice skates on an ice rink. If both women push off from one another, which woman will most likely move the furthest? Why?

Imagine that you are playing baseball. The pitch comes in and you hit the ball hard. 

But, instead of flying off the bat, the ball just drops to the ground. Is that normal?

You would probably say no because you know that force and motion are related. 

When you exert a force on a baseball by hitting it with a bat, the baseball should move.

Isaac Newton explained this relationship between force and the motion of an object. 

Law of Inertia

Inertia is the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object.

Law of Inertia

• Because of Inertia, an object at rest will stay at rest unless acted on by an unbalanced force.

  
  

• In other words, objects will not start moving until a push or a pull is exerted on them.

Multiple Choice

Inertia is the tendency of an object to resist a change in its ___________.

• temperature

• shape

• size

• motion

Law of Inertia

• An object in motion

  remains in motion unless

  acted on by an unbalanced force.

• The object will continue to move in the same direction with the same speed unless an unbalanced force occurs.

Mass and Inertia

• Mass is a measure of Inertia

• An object that has a small mass has less inertia than an object that has a large mass

• So, changing the motion of an object that has a small mass is easier than changing the motion of an object that has a large mass.

Multiple Choice

Objects with greater _________ also have greater inertia.

• speed

• mass

• temperature

• friction

Multiple Choice

Objects with greater _________ also have greater inertia.

• speed

• mass

• temperature

• friction

Friction and the Law of Inertia

• Objects in motion remain in motion unless acted on by an unbalanced force. Friction is often the unbalanced force that causes objects to slow down and stop moving.

  
  

• Because of friction, the motion of objects changes.

View the Newton's Law of Motion Cornell Notes in the Forces, Motion, and Inertia Notes and then answer the questions that follow.

Multiple Choice

A person is pushed forward into their seat belt when a car stops.  Which law is this?

• 1st
•  2nd
• 2 and 3
• 3rd 

Multiple Choice

Which of Newton's laws are illustrated with a rocket launch?

• 1st Law, the Law of Inertia. 
• 2nd Law, the Law of Acceleration.
• 3rd Law, the Law of Action Reaction
• The Law of Conservation of Mass

Multiple Choice

Which of Newton's Laws is illustrated with a large mass and a small mass accelerating?

• 1st Law, the Law of Inertia.
• 2nd Law, the Law of Acceleration
• 3rd Law, the Law of Action Reaction
• The Law of Conservation of Mass

Multiple Choice

What is Newton's First Law

• F=ma

• Every action has an equal and opposite reaction

• An object at rest stays at rest, an object in motion stays in motion.

• Friction

Multiple Choice

What is Newton's First Law

• F=ma

• Every action has an equal and opposite reaction

• An object at rest stays at rest, an object in motion stays in motion.

• Friction