12.2 Newton’s First and Second Laws of Motion

12.2 Key Concepts• How does Newton’s first law relate change in

motion to a zero net force?

• How does Newton’s second law relate force, mass, and acceleration?

• How are weight and mass related?

12.2 Newton’s First and Second Laws of Motion

Aristotle _______________ thought that a force was required to __________ an object moving

Almost 2000 years later ______________ work helped to correct this misconception Galileo Galilei

(1564 – 1642)

incorrectly

keep

Galileo

12.2 Newton’s First and Second Laws of Motion

Galileo

By rolling balls down wood ramps, Galileo studied how gravity produces _________________ acceleration.

• He concluded that moving objects NOT subjected to ______________ or any other force would continue to move ___________________.

Aristotle, Galileo, and Newton

constant

frictionindefinitely

12.2 Newton’s First and Second Laws of Motion

Newton

Newton built on the work of scientists such as Galileo.

• He published his results in book called _________________

• In this book, Newton defined _________ and _____________ and introduced his laws of motion.

Aristotle, Galileo, and Newton

Principia

mass force

12.2 Newton’s First and Second Laws of Motion

Newton’s First Law of Motion

According to Newton’s first law of motion, the state of motion of an object does ______ ______________ as long as the net force acting on the object is ________.

• So an object at rest ______________ at rest• A moving object ________________ moving

• This law is also called the law of ______________ -

the tendency of an object to resist a change in

motion

not change

zero

remains

continues

inertia

12.2 Newton’s First and Second Laws of Motion

A strong kick or ___________ will overcome static friction and make the ball move.

Without friction,

the ball would

continue to move

Newton’s First Law of Motion

push friction

Unbalanced forceschange motion

push

12.2 Newton’s First and Second Laws of Motion

This crash sequence illustrates inertia. The test dummy continues its ______________ motion as the car slows and stops.

Newton’s First Law of Motion

forward

12.2 Newton’s First and Second Laws of Motion

Newton’s Second Law of MotionAccording to Newton’s second law of motion, the acceleration of an object is equal to the ________ _________ acting on it divided by the object’s ___________.

net

forcemass

12.2 Newton’s First and Second Laws of Motion

So the acceleration of an object is ______________ proportional to the net force acting on it.

Example: The _____________ you throw a ball, the more it accelerates

Newton’s Second Law of Motion

directly

harder

12.2 Newton’s First and Second Laws of Motion

The acceleration of an object also depends ______________________upon its mass - measure of the ____________ of an object.

Example: Which will have more acceleration when you throw it…a baseball or a bowling ball?

Newton’s Second Law of Motion

inertia

A baseball – less mass

inversely

12.2 Newton’s First and Second Laws of Motion

The acceleration of an object is always in the _____________ direction as the net force.

When a net force acts in the direction opposite to the object’s motion, the force produces a _____________________

Newton’s Second Law of Motion

same

deceleration

12.2 Newton’s First and Second Laws of Motion

An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

Newton’s Second Law of Motion

4000 N

1000 kg= 4 N/kg

12.2 Newton’s First and Second Laws of Motion

Recall that one newton is the force that causes a _______ mass to accelerate at a rate of _________ each second

1 N =

So 4 N = 4 =

Newton’s Second Law of Motion

kg·m s2

kg·m s2

kg kg

1 kg1 m/s

= m/s2

12.2 Newton’s First and Second Laws of Motion

1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

Newton’s Second Law of Motion

60. 0 N

40.0 kg= 1.5 N/kg = 1.5 m/s2

12.2 Newton’s First and Second Laws of Motion

2. A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2. What is the mass of the boy and the wheelchair? (Hint: Solve Newton’s second law for mass.)

Newton’s Second Law of Motion

0.5 m/s2 = _25 N_ M

(M)(M) (M) 0.5 m/s2 = 25 N

0.5 m/s2 0.5 m/s2 =

M = 50 kg

12.2 Newton’s First and Second Laws of Motion

Mass and weight are related but are NOT the same.

• Mass is the measure of the ____________ of material an object contains.

• Weight is the force of gravity _____________ on the mass of an object.

Weight and Mass

amount

acting

12.2 Newton’s First and Second Laws of Motion

If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?

Weight and Mass

Weight = 112 kg x 9.8 m/s2

= 1098 kg·m/s2

= 1098 N

12.2 Newton’s First and Second Laws of Motion

On the moon, the acceleration due to gravity is only about ___________ that on Earth.

Weight and Mass

Astronaut on EarthMass = 88.0 kg, Weight = 863 N

Astronaut on MoonMass = 88.0 kg, Weight = 141 N

1/6

12.2 Newton’s First and Second Laws of Motion

Assessment Questions

1. What is inertia? a. the force of gravity acting on an object

b. forces of friction slowing an object’s motion

c. the mass of an object

d. the tendency of an object to resist change in its motion

12.2 Newton’s First and Second Laws of Motion

Assessment Questions

2. A 3600-N force causes a car to accelerate at a rate of 4 m/s2. What is the mass of the car? a. 600 kgb. 900 kgc. 14,400 kgd. 1200 kg

12.2 Newton’s First and Second Laws of Motion

Assessment Questions

3. How would your mass and weight change if you were on the moon’s surface? a. They wouldn’t change.

b. Your mass would remain constant, and your weight would increase.

c. Your mass and weight would decrease.

d. Your mass would remain constant, and your weight would decrease.