Motion and Forces

Unit 3 Force and Motion

8.6A – demonstrate and calculate how unbalanced forces change the speed or direction of an object’s motion

Student success statements:

I know how to define and identify a force

I will able to identify how forces work on an object and cause motion

I will practice working with balanced and unbalanced forces.

Motion

What is Motion?

Motion is an object’s change in position over time.

It can be measured on the atomic scale (like solid molecules vibrating in place) or on the larger scale (people and cars).

What is Motion?

Common examples of the idea of motion: pushing a block is an example of linear motion, opening a door is an example of rotation, rotating ball with attached thread is an example of a circular motion.

Linear motion

Circular motion

How is motion described?

Tell me, Where are you right now?

How many ways can you answer that question?

Can you be more precise in describing your location?

Position is the location of an object or person

If you decide to move around, your position changes each time you move.

How is motion described?

Whenever your position changes you are in motion

Motion is defined as an object’s change in position relative to reference point

A reference point is used to determine if an object has moved or not

Imagine this is a video…

How would you be able to tell that the cars on the track are moving?

A different point of view…

Now, pretend you are IN the car. How do you know if the car is moving?

Reference Point

It has to do with comparing the moving object to other objects.

Motion is hard to detect if you have your eyes closed – why?

The easiest way for us to determine if something is moving is to use a still object as a reference point to compare the motion against.

Reference Point

Reference point: A place or object used for comparison to determine if something is in motion

Usually it is a stationary (still) object – but it doesn’t have to be

Example – cars moving down the road

A funny point about Reference Points ☺

In fact, no reference point that we could choose - is in fact fixed.

Our planet Earth spins on its axis, and orbits around our Sun.

Our solar system in turn orbits around the center of the Milky Way Galaxy.

Our galaxy moves in relation to other galaxies.

Forces

Ok, back to basics…

Objects move because there are forces “at work” on them.

Let’s review the concept of forces

What is a Force?

A force is a push, pull or a twist on an object

When one object pushes or pulls another object, the first object is exerting a force on the second object

Forces are described by strength and direction

What is a Force?

Forces are represented by arrows on a page.

The size of the arrow indicates magnitude (size) of the force

The direction the arrows points is the direction that force is moving in

An arrow is called a

“vector”

Forces are measured in Newtons

SI unit of force (SI is the abbreviation for System

Internationale. SI is the name given to the standard metric system of units)

Symbol: N

Commonly measured by using a spring scale

What is Net Force?

We calculate forces by mathematically looking at all of the forces involved.

Net force - The overall force on an object after all the forces acting on it are determined.

Net Force = 3N

Forces occur in pairs

Balanced Forces(Result in no change in

motion)

Balanced Forces and Motion

Are equal forces acting on one object in opposite directions.

Equal forces acting on an object will not change the object’s motion.

The net force is 0 N (zero Newtons).

Net force = 0 (push)

Net force = 0 (pull)

Forces occur in pairs

Unbalanced Forces(Result in a change in

motion)

Unbalanced Forces

The forces acting on an object are not equal (one or more is greater than the other)

The net force is greater than 0 N (it has a number value) One or more force is stronger than others, the object moves in

the direction of the greatest force

A Change in Motion occurs

Unbalanced Forces and Motion

Can cause an object to: start moving

stop moving

change direction or speed.

All of these are considered

“change in motion”

Car started to move Motion stopped Changed direction

Unbalanced Forces and Motion

When two forces act in the same direction, they are added together.

When forces act in opposite directions they are subtracted. The net force = total calculated force

Determining Forces

225 N

Unbalanced

Unbalanced

13 N

Balanced

0 N

=

Online Activity

What to do:

You need your computer and the student pages

Either type the link or go to flippedoutscience.com, Unit 3.1 and look for the embedded website there.

What to do:

Once the PHET site has loaded, follow the instructions for completing each task.

There are summarizing questions at the end –don’t forget these!