NEWTON’S LAWS OF MOTION

1st Law Inertia An object at rest will stay at rest.

1st Law Inertia An object in motion will stay in motion

1st Law Inertia An object will stay in its current condition until acted upon by another force.

Balanced Force Equal forces in opposite directions produce no motion

Unbalanced Forces Unequal opposing forces produce an unbalanced force causing motion

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon them. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

Newton’s First Law – Law of Inertia Mass is the measure of the amount of matter in an object. Inertia is a property of an object that describes how much it will resist change to the motion of the object More mass = more Inertia

Newton’s First Law – Law of Inertia Unless acted upon by an unbalanced force, this soccer ball would sit on the ground forever.

What is this unbalanced force that acts on an object in motion? Friction! There are four main types of friction: Sliding friction: ice skating Rolling friction: bowling Fluid friction (air or liquid): air or water resistance Static friction: initial friction when moving an object

Newton’s First Law – Law of Inertia Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction) the soccer ball would never stop!

Inertia

How To Apply Newton’s 1st Law of Motion

How To Apply Newton’s 1st Law of Motion Estimated 19 Miles/Gallon HWY Estimated 9 Miles/Gallon City WHY?????

Newton’s 2nd Law of Motion Once an object is moving, it will move in a straight line until acted upon by another force. Force = ma (F=ma)

Force = Mass x Acceleration (F=ma) Newton’s Second Law Force = Mass x Acceleration (F=ma) ACCELERATION of GRAVITY (Earth) = 9.8 m/s2 Weight (force) = mass x gravity (Earth) Moon’s gravity is 1/6 of the Earth’s If you weigh 420 Newtons on earth, what will you weigh on the Moon? 70 Newtons

Newton’s Second Law One rock weighs 5 Newtons. The other rock weighs 0.5 Newtons. How much more force will be required to accelerate the first rock at the same rate as the second rock? Ten times as much

How To Apply Newton’s 2nd Law of Motion

How To Apply Newton’s 2nd Law of Motion

How to apply F=ma Before…

How to apply F=ma After!

Newton’s 3rd Law of Motion For every action there is an EQUAL and OPPOSITE reaction.

Newton’s 3rd Law of Motion Table to book Book to earth

Action and Reaction on Different Masses Action: earth pulls on you Reaction: you pull on earth

Action: rocket pushes on gases Reaction: gases push on rocket Action: rocket pushes on gases

Think about it . . . What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

Newton’s 3rd Law of Motion A bug with a mass of 5 grams flies into the windshield of a moving 1000kg bus. Which will have the most force? The bug on the bus The bus on the bug

Newton’s 3rd Law of Motion Without the acceleration, it is impossible to know! Force (bug)= m x a Force (bus)= m x a

How To Apply Newton’s 3rd Law of Motion

How To Apply Newton’s 3rd Law of Motion

Consider hitting a baseball with a bat Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force. (a) the force applied to the bat by the hands (b) the force applied to the bat by the ball (c) the force the ball carries with it in flight (d) the centrifugal force in the swing (b)

Newton’s 3rd Law of Motion A rich merchant finds himself stranded in the middle of a frozen lake. Don't ask how he got there. Think Twilight Zone... The surface is perfectly frictionless. All he has is his clothing and a large bag of gold coins. How can he save himself?

What Laws are represented?

Review Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law: Force equals mass times acceleration (F = ma). Newton’s Third Law: For every action there is an equal and opposite reaction.

Newton's Laws 1stlaw: Homer is large and has much mass, therefore he has much inertia. Friction and gravity oppose his motion. 2nd law: Homer’s mass x 9.8 m/s/s equals his weight, which is a force. 3rd law: Homer pushes against the ground and it pushes back.

A person is traveling at 20 m/s in a car when the car hits a tree A person is traveling at 20 m/s in a car when the car hits a tree. The person comes to a complete stop in 0.4 seconds. What was the person’s acceleration?

A person is traveling at 20 m/s in a car when the car hits a tree A person is traveling at 20 m/s in a car when the car hits a tree. The person comes to a complete stop in 0.4 seconds. What was the person’s acceleration? a = (0 m/s – 20 m/s)/0.4 s = –50 m/s2

You throw a ball to a friend. If the ball has a mass of0 You throw a ball to a friend. If the ball has a mass of0.15kg and it accelerates at 20m/s, what force did you exert on the ball.

You throw a ball to a friend. If the ball has a mass of0 You throw a ball to a friend. If the ball has a mass of0.15kg and it accelerates at 20m/s, what force did you exert on the ball. F = m x a F = 0.15 x 20 = 3N

You throw a 0. 5 kg ball with a force of 10N You throw a 0.5 kg ball with a force of 10N. What is the ball’s acceleration?

M = 0.5 F = 10 N Acceleration = a A = F/m 10 N / 0.5 kg = 20m/s2 You throw a 0.5 kg ball with a force of 10N. What is the ball’s acceleration? M = 0.5 F = 10 N Acceleration = a A = F/m 10 N / 0.5 kg = 20m/s2

Force & Motion Thinking Questions

Can an object’s acceleration be a negative number? Why or why not?

Can an object’s acceleration be a negative number? Why or why not? Yes. When an object slows down and the final speed is less than the initial speed, the object’s acceleration will be a negative number.

If forces occur in equal but opposite pairs, how can anything ever move?

Read more about this here: If forces occur in equal but opposite pairs, how can anything ever move? According to Newton’s third law, the equal and opposite forces work on different objects. Read more about this here: http://www.mansfieldct.org/schools/mms/staff/hand/Lawshowcananythingmove.htm

Two crates, one heavy and one light, are at rest on a waxed floor Two crates, one heavy and one light, are at rest on a waxed floor. Which crate will need a greater force to provide the same acceleration? Use the concept of inertia to explain your answer.

Two crates, one heavy and one light, are at rest on a waxed floor Two crates, one heavy and one light, are at rest on a waxed floor. Which crate will need a greater force to provide the same acceleration? Use the concept of inertia to explain your answer. The heavy crate has more inertia, so it requires more force to make it accelerate.

Explain the role of friction in walking on a sidewalk versus walking on a sidewalk covered with ice.

Explain the role of friction in walking on a sidewalk versus walking on a sidewalk covered with ice. Friction between your shoe and the sidewalk allows you to push against the sidewalk. Ice decreases this friction.

You sit on a rock. Your weight is a force acting on the rock You sit on a rock. Your weight is a force acting on the rock. What force acts on you?

You sit on a rock. Your weight is a force acting on the rock You sit on a rock. Your weight is a force acting on the rock. What force acts on you? The rock provides an upward force that is equal to your weight since you are not moving.

How is acceleration related to change in speed?

How is acceleration related to change in speed? Acceleration is the change in velocity divided by the time needed for the change to occur, and velocity changes when speed changes.

Forces & Motion Review

Describe Acceleration

Describe Acceleration A change in velocity – which may be: A change in speed Starting Stopping Speeding up Slowing down A change in direction Acceleration is caused by unbalanced forces More

Describe Acceleration Deceleration is also called negative acceleration - it means an object is slowing down When acceleration is calculated, it may be a negative number

Describe Speed

Describe Speed A way to describe motion Average speed - Rate of motion calculated by dividing the distance traveled by the amount of time it takes to travel that distance Constant speed - Speed that does not change Instantaneous speed - Speed of an object at any given time

What is the formula used to calculate speed?

What is the formula for calculating speed? Speed is calculated by dividing distance by time –

Calculate This Speed A football field is about 100 m long. If it takes a person 20 seconds to run its length, how fast was the football player running?

Remember to include the UNITS!! Calculate this Speed: A football field is about 100 m long. If it takes a person 20 seconds to run its length, how fast was the football player running? Speed = Distance ÷ Time Speed = 100 m ÷ 20 s Speed = 5m/s Remember to include the UNITS!!

Explain Balanced Forces

Explain Balanced Forces When all the forces acting on an object balance each other Balanced forces do not cause a change in motion

Describe Friction

Describe Friction Force that resists motion between two touching surfaces Acts in the opposite direction of the object’s motion Produces heat

Explain Inertia

Explain Inertia Moving objects tend to continue moving unless acted upon by an unbalanced force Objects at rest tend to stay at rest unless acted upon by an unbalanced force The more mass an object has, the more inertia it has More massive objects are harder to start moving and stop moving Smaller objects are easier to start and stop moving More

Explain Inertia Newton’s First Law on Motion describes the idea of inertia An object at rest or in constant motion is acted upon by balanced forces – an unbalanced force will change the motion Acceleration of an object at rest or in constant motion is 0 m/s/s (no motion)

Explain Newton’s First Law of Motion

Explain Newton’s First Law of Motion Describes the idea of inertia Click the link below to observe the law http://archive.ncsa.uiuc.edu/Cyberia/VideoTestbed/Projects/NewPhysics/newtons_1.html When you are finished, click the back button on your browser to return to this tutorial

Explain Newton’s Second Law of Motion

Explain Newton’s Second Law of Motion Describes motion created by unbalanced forces Mass and acceleration change in opposite ways The more mass an object has, the more force it take to accelerate the object, the slower it accelerates The less mass an object has, the less force it take to accelerate the object, the faster it accelerates More

Explain Newton’s Second Law of Motion Click on the link below to observe the law: http://archive.ncsa.uiuc.edu/Cyberia/VideoTestbed/Projects/NewPhysics/newtons_2.html When you are finished, click the back button on your browser to return to this tutorial

Explain Newton’s Third Law of Motion

Explain Newton’s Third Law of Motion Describes why forces act in pairs For every action there is an equal and opposite reaction Action and reaction forces are equal forces acting in opposite directions on different objects

Explain Newton’s Third Law of Motion Click the link below to observe the law http://archive.ncsa.uiuc.edu/Cyberia/VideoTestbed/Projects/NewPhysics/newtons_3.html

Distinguish Between Speed & Velocity

Distinguish Between Speed and Velocity Speed describes distance and time Velocity describes distance, time, and direction

How Can Forces Affect Objects?

How Can Forces Affect Objects? Slow them down Speed them up Stop them Start them Change their direction Change their shape

Click here for more review: http://www.usoe.k12.ut.us/curr/science/sciber00/8th/forces/sciber/intro.htm