Force, Mass, and Acceleration

Newton’s Second Law States:

• Acceleration is produced when a force acts on a mass.

• The greater the mass of the object being accelerated, the greater the amount of force needed to accelerate the object.

• the Second Law gives us an exact relationship between force, mass, and acceleration

What does this mean?• Everyone intuitively knows the Second Law.

• Everyone knows that heavier objects require more force to move the same distance as lighter objects.

Vocabulary Terms:

• Force is a push or a pull.

• The net force is the overall force acting on an object

•Acceleration is when the motion of an object

changes.

Examples:

Speed up

Slow down

Changes direction

•Mass is the amount of matter in an object.

Acceleration:

Newton’s Second Law of Motion

•Stated a little differently – “When a net force acts on an object the object will accelerate in the direction of the net force”.

Newton’s 2nd Law

Units:

• Units of Force = Newtons (N)

SI units for mass is kg

SI units for acceleration is m/s2

1 Newton (N) = 1kg * m/s2

Gravity & Weight

• Gravity is the force of attraction that exists between any two objects that have mass.

• The force of gravity depends on the mass of the objects and the distance between them.

Gravity & Weight

• Weight is a force, like the push of your hand is a force, and is measured in Newtons.

• The force of gravity causes all objects near Earth’s surface to fall with an acceleration of 9.8 m/s².

• Your weight on Earth is the gravitational force between you and Earth.

Gravity and Weight

• How are weight and mass different?

• Weight is a force, like the push of your hand is a force, and is measured in newtons.

• Mass is the amount of matter in an object, and doesn’t depend on location.

• Weight will vary with location, but mass will remain constant.

Calculating Force• So, Force = mass x acceleration

• Let’s work a problem: What force is needed to accelerate a 10 kg shopping cart 3m/s2

• What do you know? m = 10kg a = 3m/s

• What do you want to find out? Force

• Write the formula: F = ma

• Substitute the formula: F = 10kg x 3m/s

• Calculate: F = 30N

So what happens if…?

• What if we make the shopping cart have a whole bunch of groceries, so that it goes from 10kg to 25 kg?

• Does it take more or less force to move (accelerate) that cart?

• More!

The acceleration of an object increases with increased force

• To move the box:

Directly Proportional

• Force and acceleration are Directly proportional: • As the force increases,

the acceleration increases

Push the shopping cart

The acceleration of an object decreases with increased mass of that object

• Look at the boxes now:

Inversely Proportional

• Mass and acceleration are Inversely proportional:• As the mass increases,

the acceleration decreases

Newton’s 2nd Law

• Newton’s second law of motion can be used to calculate acceleration.

• For example, suppose you pull a 10-kg sled so that the net force on the sled is 5 N.

• The acceleration can be found as follows:

A note about motion:

• If a net force is applied to an object, the object moves in the direct of the largest force

• But not all types of motion are linear (straight)

• What other types of motion can you think of?

Circular Motion

• A rider on a merry-go-round ride moves in a circle.

• This type of motion is called circular motion. • If you are in circular

motion, your direction of motion is constantly changing.

• This means you are constantly accelerating.

Circular Motion

• If you are constantly accelerating, there must be a force acting on you the entire time.

• The force exerted is the centripetal forceand always points toward the center of the circle.

• In circular motion the centripetal force is always perpendicular to the motion.

Lab Page: Force and Acceleration Lab

Lab Expectations and Instructions:

• Lab Behavior: Principled and safe

• Lab Instructions: Follow the procedure as written, if you have questions, ask before doing!

• What to complete at home: the graph and analysis questions. Graph can be worked on in class if your group completes the lab and cleans up first

• (When is the lab due?)

Lab Clean Up:• Place supplies back in the same order that you

found them

• Leave it tidy so that the next group can be ready to start on time – just like YOU did.

• Straighten the room, chairs pushed in, any trash thrown away