Mass vs. WeightNewton’s Laws of Motion

8th Grade Science

-Mass is defined as the amount of matter an object has.

-does NOT change no matter where you are in universe!

-The weight of an object on earth depends on the force of attraction (gravity) between the object and earth

-DOES change depending on where you are

2

3

Formula to calculate weight: Weight = Mass × Gravity

W = mg

Mass: measured in KgGravity on Earth: 9.8 m/s/s

Practice

-Calculate the weight of a car with a mass of 1500 kg.

-14700N

-Calculate the weight of an object with a mass of 550 kg.

-5390N

4

5

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

6

1st Law

• Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

7

Newton’s First Law is also called the Law of Inertia

Inertia: the tendency of an object to resist changes in its state of motion

The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

8

Examples from Real LifeA powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.

On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

9

If objects in motion tend to stay in motion, why don’t moving objects keep moving

forever?

Things don’t keep moving forever because there’s almost always an unbalanced force

acting upon it.

A book sliding across a table slows down and stops because of the force of friction.

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

10

Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.Don’t let this be you. Wear seat belts.

Because of inertia, objects (including Because of inertia, objects (including you) resist changes in their motion. you) resist changes in their motion. When the car going 80 miles/hour is When the car going 80 miles/hour is stopped by the brick wall, your body stopped by the brick wall, your body keeps moving at 80 miles/hour.keeps moving at 80 miles/hour.

11

2nd Law

Force = mass x acceleration

12

What does F = m x a mean?F = m x a basically means that the force of an

object comes from its mass and its acceleration.

Something very small (low mass) that’s changing speed very quickly

(high acceleration), like a bullet, can still have a great force. Something

very small changing speed very slowly will have a very weak force.

Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.

13

2nd Law (F = m x a)

• How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

• Write the formula• F = m x a• Fill in given numbers and units• F = 1400 kg x 2 meters per second/second• Solve for the unknown

• 2800 kg-meters/second/second or 2800 N

14

3rd Law• For every action, there is an

equal and opposite reaction.

• The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).