Chapter 10 Newton’s Laws of Motion

8th Grade

Learning Targets

Today students will learn:•To identify Newton’s First Law and Newton’s Second Law of Motion•To understand what inertia is.•To understand the relationship between force, mass, and acceleration.

Activity 1 : Jot Notes

• You will complete Jot Notes on Ch. 10 Sec. 3

• Be sure to write down any BOLD words

• Be sure to write down anything in RED.

• Galileo Galilei– Italian Astronomer– Suggested that once an object is in motion, no

force is needed to keep it moving.

• Force – Needed to change the motion of the object.

http://www.3m.co.uk/intl/uk//3mstreetwiseuk/pupils-force.htm

Newton’s 1st Law of Motion• Restates Galileo’s ideas about force and

motion

• An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity unless it is acted upon by an unbalanced force.

Newton’s 1st Law of Motion• This means that if an object is not moving, it

will not move until a force acts on it.

• If an object is already moving, it will continue to move until a force changes its speed or direction.

• Gravity and friction are unbalanced forces that often change an object’s motion.

Newton’s 1st Law of Motion

Inertia• An object moving or not moving it resists

any change to its motion.

• Inertia– The tendency of an object to resist a change

in motion

Inertia• Newton’s 1st Law

– Also called the Law of Inertia

• The greater mass of an object, the greater its inertia, and the greater the force needed to change its motion.

Newton’s 1st Law

Newton’s 2nd Law of Motion• Acceleration

– depends on the object’s MASS and the NET FORCE acting on the object.

– Net force / mass– Units = Newtons (N)

Newton’s 2nd Law of MotionFormulas:

**Force = mass x acceleration– The greater the mass the greater the force

needed to accelerate the object.

**Acceleration = net force

mass– The greater the mass, the less the object will

accelerate.

Newton’s 2nd Law of Motion

• How do you increase acceleration?– If mass is constant, increase force = increase

acceleration – If force is constant, decrease in mass = increase

in acceleration

Activity 2: Demonstrations

• Each group will participate in the following demonstrations.

• You must correctly discuss with your partner, – What is the force acting on the object at start?– What is the force acting on the object at stop? – Which law is this identifying?

Activity 2: Demonstrations

• Egg Spin– Consider the motion of a boiled egg & raw

egg after you stop them from spinning.

• Penny Drop– Can you get the penny to drop into the

beaker?

• Ball Race– Which ball will win the race? Ping pong?

Foam ball? Golf ball?

Activity 3: Question Builder

• You will have 10 minutes to complete your Ch. 10 Sec. 3 review questions

– Two Level 1 Questions (RECALL)– Two Level 2 Questions (INFERENCE)– One Level 3 Question (APPLICATION)

Newton’s 2nd Law of Motion

Newton’s 2nd Law of Motion

Learning Targets

Did you learn today:•To identify Newton’s First Law and Newton’s Second Law of Motion•To understand what inertia is.•To understand the relationship between force, mass, and acceleration.

Section 10.3 Assessment • What does Newton’s 1st Law of motion state?

– An object at rest will remain at rest and an object in motion will remain in motion, unless acted upon by an unbalanced force.

• Why is Newton’s 1st law of motion sometimes called the law of intertia?– Inertia is a measure of an object’s tendency to resist a

change in its motion. • Use what you know about inertia to explain why you feel

pressed back into the seat of a car when it accelerates?– Because of your inertia, your body tends to remain in

place. The car seat causes you to accelerate, therefore, by exerting a force on your back.

Section 10.3 Assessment• State Newton’s 2nd law of motion in your own

words.– The greater the force= the greater the

acceleration.– The greater the acceleration or the greater the

object’s mass = the lower the acceleration

• How could you keep an object’s acceleration the same if the force acting on the object were doubled?– You could double the object’s mass

Section 10.3 Assessment• Why does a car with a large mass might use

more fuel than a car with a smaller mass.– A greater force is required to accelerate a more

massive car, therefore, more engine power is required and more fuel is needed to produce that power.

• Find the force it would take to accelerate an 800kg car at a rate of 5 m/s2– F=ma– 800 kg X 5m/s2= 4,000 N

Section 10.3 Assessment• What is the net force acting on a 0.15 kg

hockey puck accelerating at a rate of 12 m/s2?– F=ma– (0.15 kg x 12 m/s2) = 1.8 N

Learning Targets

• Identify Newton’s 3rd Law of Motion• Discuss momentum, and how it

relates to mass and velocity.

Activity 1: Say Something

• After watching the following video, turn to your partner and discuss what you understood from the video.

Activity 1: What is momentum?

Activity 1: Say Something

• Now, turn and say something to your neighbor about what you think the definition of momentum is.

• WHAT IS MOMENTUM?

Activity 2 : Jot Notes

• You will complete Jot Notes on Ch. 10 Sec. 4 “NEWTONS 3rd LAW”

• Be sure to write down any BOLD words

• Be sure to write down anything in RED.

Newton’s 3rd Law of Motion• Whenever one object exerts a force on a

second object, the second object exerts a force of equal strength in the opposite direction on the first object

Newton’s 3rd Law of Motion• “Action-Reaction” forces

• Ex. When air rushes out of a balloon “action”, the balloon is pushed up with a force “reaction”

Momentum• “mass in motion”

• Every object has mass, so if that mass is in motion then that object has momentum.

Momentum• It is the characteristic of a moving object

related to the mass and velocity of the object

• Momentum = mass x velocity

• Units = kg* m/s

Momentum• The more mass an object has the more

momentum it has, so the harder it is to stop the object.

• Ex. A car moving 20 m/s is harder to stop than a baseball moving at 20 m/s because the car has a greater mass and greater momentum.

Momentum• Amount of momentum objects have is

conserved when they collide.

• It can be transferred from one object to another, but none is lost.

• Law of conservation of momentum– The total amount of momentum of objects that

interact does not change in the absence of outside forces.

– Ex. Friction

Momentum

Momentum

Did you learn?

• Identify Newton’s 3rd Law of Motion• Discuss momentum, and how it

relates to mass and velocity.

Determining Acceleration Word Problems

• 1. A speedboat pulls a 55-kg water skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.

Determining Acceleration Word Problems

• 2. What is the net force on a 1,000kg object accelerating at 3m/s2?

Determining Acceleration Word Problems

• 3. What net force is needed to accelerate a 25 kg cart at 14 m/s2?

Determining Acceleration Word Problems• 4. Find the force it would take to accelerate

an 800 kg car at a rate of 5 m/s2?

Determining Acceleration Word Problems

• 5. What is the net force acting on a 0.15 kg hockey puck accelerating at a rate of 12 m.s2?

Momentum Word Problems• Which has more momentum: a 3.0- kg

sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?

• Momentum = Mass X Velocity

• Smaller : 3.0 kg X 1.5 m/s = 4.5 kg x m/s

• Larger : 4.0 kg X 0.9 m/s = 3.6 kg x m/s

Momentum Word Problems• A golf ball travels at 16 m/s, while a

baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has a greater momentum?

• Golf Ball: 0.045 kg x 16 m/s = 0.72 kg x m/s

• Baseball: 0.14 kg x 7 m/s = 0.98 kg x m/s

• The baseball has greater momentum

Momentum Word Problems• What is the momentum of a bird with a mass

of 0.018 kg flying at 15 m/s?

• 0.018 kg x 15 m/s = 0.27 kg x m/s

Momentum Word Problems• What is the momentum of a 920 kg car

moving at a speed of 25 m/s?

• 920 kg x 25 m/s = 23,000 kg x m/s

Momentum Word Problems• Which has more momentum? A 250 kg

dolphin swimming at 4 m/s or a 350 kg manatee swimming at 2 m/s?

• Dolphin : 250 x 4 = 1,000 kg x m/s

• Manatee: 350 x 2 = 700 kg x m/s

• The dolphin has more momentum!