Warm - Up What is Inertia?

The resistance an object has to changes in motion.

Mass is used to measure inertia. How is Newton’s 1st Law used to get ketchup out

of a glass bottle?When the bottle is turned upside down, the

ketchup slowly falls to the bottom. If you shake the bottle, the bottle suddenly stops at the bottom of the path, but the ketchup continues to move.

7-3 – Newton’s 2nd Law Describe Newton’s second law of motion and use

it to calculate acceleration, force, and mass values.

Check your understanding? Suppose that an astronaut in that space throws a

rock. The rock will

a) gradually stop.

b) continue in motion in the same direction at constant speed.

Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why.

Force & mass & acceleration The relationship between mass, force and

acceleration is Newton’s 2nd Law.

Two objects with different masses. Object with the greater mass will experience

smaller acceleration. Acceleration due to gravity

All objects will accelerate down at 9.8 m/s2

g = 9.8 m/s2

F ma

Weight vs. Mass Weight and mass are NOT the same. Mass is how much matter is present Weight is a measurement of the force of gravity.

W mg

An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile?

Known

m = 1200 kg

a = 3.0 m/s2 forward

Fnet= ?

Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2 kg book toward Roberto. If the net external force acting on the book is 2.6 N to the right, what is the book’s acceleration?

m = 2.2 kg

Fnet= 2.6 N to the right

a = ?

A soccer ball is kicked with a force of 13.5 N and accelerates at 6.5 m/s2 to the right. What is the mass of the ball?

F = 13.5 Na = 6.5 m/s2

m = ?

Concept Challenge The force of gravity is twice as great on a 2 kg

rock as it is on a 1 kg rock. Why doesn’t the 2 kg rock have greater free-fall acceleration?A greater force acts on the heavier rock, but the

heavier rock has greater mass, More resistance to changes in motion.

So the acceleration is the same. Free-fall acceleration is independent of mass.

Concept Challenge A truck loaded with sand accelerates at 0.5 m/s2

on the highway. If the driving force on the truck remains constant, what happens to the truck’s acceleration if sand leaks at a constant rate from a hole in the bed?The acceleration will increase as the mass

decreases

Summary On note card:

1. List one topic about Newton’s first law.

2. List one topic about Newton’s second law.