Chapter 12 Forces and Motion Name: _______________________________ Class: ____ 12.2 Newton’s 1st & 2nd Laws of Motion Chapter 12 Forces and Motion

What are we going to learn? Newton’s Laws of Motion (Imagine a toy car with an unanchored load – what will change its motion?)

What is a Scientific Law? A statement of what will happen in a given set of circumstances. FYI: Newton developed the Law of Universal Gravitation in addition to the Laws of Motion

Newton’s First Law of Motion An object at rest will remain at rest unless acted upon by a nonzero net force An object moving at a constant velocity will continue moving at a constant velocity unless acted upon by a nonzero net force

Inertia The tendency of an object (moving or not) to resist a change in its motion Newton’s 1st Law is also known as the Law of Inertia Ex: You move forward in your seat when your car stops suddenly Relate inertia to safety bars on a roller coaster _______________________________________

Inertia Relate inertia to safety bars on a roller coaster They keep you in your seat when changes in the coaster’s motion moves you out of your seat

Mass and Inertia The greater the mass of an object, the greater its inertia...and the greater the force required to change its motion

Use this information to explain why you feel tossed around whenever a roller coaster goes over a hill or through a loop...

Use this information to explain why you feel tossed around whenever a roller coaster goes over a hill or through a loop Because of inertia, you resist changes in motion. When the coaster accelerates over a hill, inertia keeps you moving at a constant velocity. You feel tossed around.

Example What keeps a ball rolling down a hill?

Example What keeps a ball rolling down a hill? Gravity and Inertia

What force is acting on the ball, resisting motion?

What force is acting on the ball, resisting motion? Friction between the ball and the ground

If you put a ball on the floor and roll it across the floor so it hits the wall, why did the ball stop moving in the direction it was rolled?

If you put a ball on the floor and roll it across the floor so it hits the wall, why did the ball stop moving in the direction it was rolled? It rolled into the wall, so it could not keep going in the direction it was rolled

Now I get it! Now I know that Newton’s first law of motion states that _____________________________

Now I get it! Now I know that Newton’s first law of motion states that objects only change motion when a nonzero net force acts on them

Newton’s Second Law of Motion An object’s acceleration depends on its mass and on the net force acting on it Ex: Is a full or empty shopping cart easier to push? Ex: Who can cause a greater acceleration on the cart, a small child or grown adult?

Newton’s Second Law of Motion Ex: Is a full or empty shopping cart easier to push? Empty cart – less mass Ex: Who can cause a greater acceleration on the cart, a small child or grown adult? Grown adult – more force

Formula The acceleration of an object is equal to the net force acting on it divided by the object’s mass Acceleration = Net force Mass or Net force = Mass x Acceleration

Mass x Acceleration = Net force x Mass which gives Mass x Acceleration = Net force when flipped gives Net force = Mass x Acceleration

Changes in Force and Mass If you increase the force on a cart without changing its mass, the acceleration will increase If something falls out (decrease mass), it will accelerate faster if you keep pushing just as hard (same force)

If you want to increase the acceleration of a bicycle, what must you increase?

If you want to increase the acceleration of a bicycle, what must you increase? The force you apply to the pedals

Why are racing bicycles made of lightweight metals like titanium?

Why are racing bicycles made of lightweight metals like titanium? The less mass a bicycle has, the greater the acceleration the rider can produce

Suppose 4 dogs pull a sled carrying 2 people Suppose 4 dogs pull a sled carrying 2 people. Change the dog/person arrangement to change the sled’s acceleration in the 2 questions.

Suppose 4 dogs pull a sled carrying 2 people Suppose 4 dogs pull a sled carrying 2 people. Change the dog/person arrangement to change the sled’s acceleration in the 2 questions.

Which part of the formula would you change if you added dogs to the team?

Which part of the formula would you change if you added dogs to the team? Net force

How does changing the top number of a fraction affect the value of the answer?

How does changing the top number of a fraction affect the value of the answer? It increases the value

Write an “if-then” sentence to show the effect of adding more dogs

Write an “if-then” sentence to show the effect of adding more dogs If you add dogs to the team, then you increase the acceleration

What else could be done to increase acceleration?

What else could be done to increase acceleration? By removing some people from the sled or a combination of adding dogs and removing people

Remember: Mass units are kg Acceleration units are m/s2 Force units are N (kg · m/s2)

Force = Mass x Acceleration

2 kg x 2 m/s2 = 4N

It would be cut in half to become 1 m/s2

I get it! Now I know that Newton’s second law of motion describes the relationship ________ ____________________________________

I get it! Now I know that Newton’s second law of motion describes the relationship among force, mass and acceleration