Forces and Motion

Forces and MotionChapter 23

What is motion?

Motion- an object changing position

Types of Motion

There are 2 types of motion:

Distance

Displacement

DistanceDistance is the total path traveled. Look at the map- Is the dotted line or the straight line distance?

Displacement

This is the distance and direction between the starting and ending point.

It is the straight line distance between start and finish.

Relative Motion

How do you know what direction something is?

Is Florida north or south?

Is Alaska north or south?

Relative Motion

Motion must have a reference point. This means you must compare the motion of an object to something else.

Describing Motion

There are 3 main ways to describe the motion of an object.

Speed

Velocity

Acceleration

Speed

Speed is distance traveled divided by the time taken to travel the distance.

speed (in meters/second)= distance/time

s= d/t

Speed

Constant Speed= the speed at any given time will be the same.

When can we achieve this?

Changing Speed- When your speed is changing, not constant. We call this instantaneous speed.

What measures this for us?

Calculating Speed

Here is a simple way to solve speed equations!

Practice Problems!!!

Velocity

Velocity= speed with a direction

15 mph NE

It is calculated just like speed except with a direction!

Acceleration

Acceleration is the change in velocity.

Acceleration can occur 3 ways:

Speed Up

Slow Down

Change Directions

Calculating Acceleration

To calculate acceleration you must know:

Final Speed

Starting or Initial Speed

Time

Acceleration Equation

The equation looks like this:

a= sf - si

t

Forces

An object motion changes because of forces.

A force is a push or a pull

A force has a size and a direction.

Why must we have both?

Forces

Contact Forces- a force that is exerted when two objects are touching each other.

Long range forces- a force exerted when the objects are NOT in contact.

Can you think of any examples.

Examples- Gravity, Magnetism, Electricity

Unit of Forces

Forces are measured in newton (N).

It is named after Isaac Newton.

One Newton is equal to the force it takes to lift half a cup of water.

Newton’s First Law of Motion

This law states: “An object remain at rest or move in a straight line with constant speed unless it is acted upon by a force”

Inertia and Mass

The first law is often called the law of inertia.

Inertia= an object’s tendency to resist a change in motion. It wants to keep moving or sit still.

What causes inertia?

inertia=mass

Adding Forces

Motion depends upon the size and direction of all forces acting on an object.

Balanced Forces- Forces that are equal but in opposite directions.

No motion will occur

Unbalanced Forces

Unbalanced Forces- Forces of an object are not equal and opposite.

Motion will occur in the direction of the largest force!

Newton’s Second Law

This is used to predict where and how motion will occur!

Newton’s Second Law

This law states: “An object acted on by an unbalanced force will accelerate in the direction of the force.”

Newton’s Second Law

Newton’s Second Law says that the acceleration of an object can be calculated by the following:

Acceleration= force (N)mass (kg)

How can you use the Second Law?

You can determine the acceleration of an object if you know the mass and the force.

OR

You can determine the force that is needed to make an object accelerate if you know the acceleration and mass!

Gravity

Every object exerts an attractive force on every other object!

Gravitational Force- the attractive force between two objects, it depends on two things:

Mass (More Mass=More Gravity)

Distance (Closer Together=More Gravity)

Gravity

The acceleration due to gravity is ALWAYS 9.8 m/s2

Mass vs. Weight

What is the difference between mass and weight?

Mass

The amount of matter in an object

This cannot change!

Weight

This is the force of gravity on an object!

This can change...

On Earth, 150 lbs.

On other planets

Mercury- 56.7 Jupiter- 394

Venus- 136 Saturn- 179

Moon- 24 Uranus 133

Mars- 56.5 Neptune- 168.7

Friction

Friction- a force that resists sliding motion between surfaces that are touching.

There are 4 types of friction.

Static Friction

Static friction is keeps an object at rest.

Sliding Friction

This is the friction that occurs while an object is in motion.

It typically slows down objects.

When would you want to increase or decrease sliding friction?

How would you do this?

Rolling Friction

Rolling Friction allows a wheel to roll on a surface.

Walking is an example of rolling friction.

Air Resistance

This is also called fluid friction.

Molecules in the air collide with forward moving objects. This will slow down motion.

This is called air resistance.

It acts as a force in the opposite direction of motion.

Newton’s 3rd Law

This law states- “Forces always act in equal but opposite pairs”

You probably know it better, “For every action there is an equal and opposite reaction!”

Action and Reaction Forces

Any time there is motion there are 2 forces!

When you jump, what are the forces?

You push on the ground, it pushes up on you.

Using the 3rd Law

These are not balanced forces.

These forces act on different objects!

When you kick a ball a force is put on your foot and an equal one on the ball.

That is why the harder you kick the more it hurts.

Think about this...

If Newton’s 3rd Law is true, then why don’t we slide backwards when we push a door closed?

Because of friction. When the door pushes you it is also pushing on the Earth!

What about Gravity?

Do we exert the same force on the Earth as the Earth’s gravity exerts on us?

So why don’t we move when we jump into a swimming pool?

Because we have so little mass!