Forces

What are Forces?

October 20, 2014

What forces do you think are acting on you right now while you’re sitting in your seat?

There are forces acting on you all the time whether you know it or not!

• Gravity is pulling you down • The ground is pushing you up • Air is squeezing you from every

direction • Your body is pushing back at the air

When forces are balanced, they cancel out and you don’t notice them. But when forces

don’t balance, things happen…

THINK! What would happen if we took the force of gravity away?

A force is a PUSH or a PULL

• Forces can cause an object to move, change speed, change direction, change shape, or stop

• Forces have size and direction

• When you drop your pencil, gravity is pulling it to the ground

• When you are walking around, your feet are pushing down onto the ground

Force Facts

• Measured in Newtons (N) • Always act in pairs • Cannot be seen, but the effect can

be seen • Measured using a spring scale

Measuring Force

• How many Newtons will it take for me to drag your science binder across the desk?

Net Force

Net force: the size of all the forces acting on an object • If two forces acting on an object are

in the SAME direction, you ADD the forces together

• If two forces acting on an object are in the OPPOSITE direction, you SUBTRACT the forces

Written with a magnitude and direction

Ex: Force Net = 30 N

Net force =

50 N 75 N

If two forces acting on an object are in the SAME direction, you ADD the forces together

Net force =

500 N

100 N

If two forces acting on an object are in the OPPOSITE direction, you SUBTRACT the forces

Balanced Forces

• Forces that cancel each other out• Net force = ZERO • There is no change in motion

Balancing your science textbook on your desk

Force net =

Net force =

Unbalanced Forces

• When forces acting on an object do not cancel out

• Net force does not = zero• Produce a change in motion

Net force =

Checkpoint

1. What is the unit used for measuring a force?

Newtons

2. What tool is used to measure forces?

Spring Scale

3. Find the net force

950 N 625 N

Review

1. Find the net force

600 N 600 N

2. Find the net force

525 N 50 N

Forces that act against motion…

If you kick a soccer ball on grass, what will start to slow it down?

The enemy of motion

• Friction: force that occurs when 2 objects rub against each other

• Always acts in the direction opposite to the direction of motion

• Will slow the motion of an object or stop it

• Creates heat

• Every object causes friction, some more than others

• It may look smooth, but it is covered in tiny bumps and dents

• Friction can be beneficial

• Everyday life would be a lot different without it

• THINK! What would happen if the floor in the classroom was ice instead of carpet?

When would decreasing friction be beneficial?

Is it possible to pick up a bottle full of rice with just a pencil?

YES!

4 Types of Friction

• Static friction• Sliding friction • Rolling friction • Fluid friction

Fluid Friction- Air Resistance

• Caused by friction of the air against an object

• Objects with more surface area experience greater air resistance and move more slowly

Gravity

• Force that pulls objectstowards each other

• Law of gravitational force: The force of gravity depends on the distance between the objects and their masses

Which objects have the greatest force of gravity?

A B C D

A and CD and B B and C A and D

• Mass: the amount of matter in an object (g)

• Weight: measure of the force of gravity on an object (N)

• All objects fall at the same rate towards the earth because gravity pulls on every object the same amount

• Air resistance slows objects down

What would hit the ground first, a pencil or a ball?

What about a sheet of paper and a balled up sheet?

An object’s weight can change if you went from the earth to the moon, but

your mass would stay the same.

Ex: Gravity on moon is 1.622 m/s2

THINK! What would happen if we drop a feather and a hammer on earth?

What if we were on the moon?

Hammer-feather drop

What is happening?

Zero G Experience

http://www.gozerog.com

Checkpoint

1. Is your mass the same on earth and the moon?

2. Is your weight the same on the earth and moon?

3. What forces will effect a soccer ball that has been kicked?

StudyJams Video