Forces and Motion

Chapter 12

Inquiry activity (Individual work)

Follow the directions on page 355 of your textbook

Answer the questions Label it as Penny Lab

and turn it in.

Forces

Sec. 12.1

• A push or a pull exerted on an object

• Measured in Newton, N

•Examples: Wind, Gravity

•Represented with an arrow

•The length of the arrow represents the strength of the force

Balanced Forces

• Equal in size

10 N 10 N 0

Net Force

Combining Forces

•Forces in the same direction add together

•Forces in opposite directions subtract from each other

Net Force = Overall Force

Balanced Forces cont.

• Result in NO movement

Unbalanced Forces

• Not equal in size

10 N 15 N

Ex: Tug-of-War

• Ex: Standing still

150 N

150 N

5 N

Net Force

Unbalanced Forces cont.

• Result in movement

30 N 10 N 20 N

Net force

FrictionForce that opposes motion of objects that touch

Types of Friction

1. Static friction: the force that acts on objects that are not moving

*Always acts in an opposite direction

Ex. Balloon, dryer sheet

Types of Friction

2. Sliding Friction: Caused by two object sliding over each other.

Ex. Slide, Sled, Carpet Burn

Quick Lab on sliding friction

We are now going to do the quick lab found on page 360 in your textbook.

You will be working in pairs that I will assign. Make a data table to record your data that includes

eraser with sticky note and eraser without sticky note and 3 trials and the average distance of each eraser.

Answer the questions Turn this lab in to me

Types of Friction

3. Rolling Friction: Caused by two objects rolling over each other.

Ex. Roller skates, car tires, ball

Types of Friction

4. Fluid Friction: force that opposes the motion of an object through a fluid

Ex. Submarine, Cake Batter

Interactive forces

http://www.engineeringinteract.org/resources.htm

Air ResistanceFluid Friction acting on an

object moving through the air

Examples: Speed Skaters

Gravity

Force of attraction between two objects that have mass.

Attractive Force - pulls objects together

Gravity

On Earth gravity is a constant

9.8 m/s2.

Gravitational Forces on the Planets The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared.

Falling Objects

Gravity causes objects to accelerate

downward, whereas air resistance acts

in the direction opposite to the

motion and reduces acceleration.

Terminal Velocity

Constant velocity of a falling object when the force of air resistance equals the force of gravity.

Projectile Motion

The motion of a falling object after it is given an initial forward velocity.

Air resistance and gravity are the only forces acting on a projectile.

Newton’s three laws of motion demonstrated

http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

Newton’s 1st law of motion

Sec. 12.2

•“An object at rest stays at rest, and an object in motion stays in motion, unless acted upon by an outside force.”

•Law of Inertia

Ex. Resting Soccer Ball

Inertia

Newton’s first law of motion is sometimes called the law of inertia

Inertia is the tendency of an object to resist change in its motion

Youtube newton’s cradles

http://www.youtube.com/watch?v=mFNe_pFZrsA

http://www.youtube.com/watch?v=V87VXA6gPuE

Demos performed

Quarters on a card falling into a jar when card is thumped.

Marker falling into a jar when the card is thumped.

Pen falling into a bottle when hoop is removed.

Egg in the beaker with cookie sheet.

Newton’s 2nd law of motion

•“When a force is applied to an object, the object accelerates in the direction of the force applied.”

Acceleration=Net Force/Mass

Mass is a measure of the inertia of an object and depends on the amount of matter the object contains

Ex. How much force in needed to accelerate a 10 kg object at 2 m/s2?

F = ? m = 10kga = 2 m/s2

Formula: a = F/m

2 = F/10 F = 20 N

Newton’s 2nd law of motion

If a 100 newton force is applied to a 5 kg object, how quickly will it accelerate?

F = 100 N

m = 5 kg

a = ?

a = 100/5

a= 20 m/s2

Formula: a = F/m

Weight A measure of the force of gravity

acting on an object

Weight = mass x acceleration due to gravity

W = mg

If an astronaut has a mass of 112 kilograms, what is his weight on Earth?

Weight = mass x acceleration due to gravity

=112 kg x 9.8 m/s2

=1100 kg m/s2

=1100 N

Math practice problems

1. What is the acceleration of a 1,500 Kg truck with a net force of 7,500 N?

2. A runner with a mass of 60Kg accelerates at 2.2m/s2. What is the runner’s net force?

3. Find the mass of a flying discus that has a net force of 1.05N and accelerates at 3.5m/s2.

4. Ian has a mass of 58Kg. What is his weight?

Newton’s 3rd law of motion

Sec. 12.3

“For every action, there is an equal and opposite reaction.”

Ex. Bumper Cars

The single cart accelerates eight times faster than the chain of eight carts.

Momentum

The product of an object’s mass and its velocity

A large momentum is hard to stopThe momentum of an object at rest

is zeroMomentum = Mass x VelocityUnits are kg m/s

Which has more momentum, a 0.046 kg golf ball with a speed of 60.0 m/s, or a 7.0 kg bowling ball

with a speed of 6.0 m/s?

Momentumgolf ball = 0.046 kg x 60.0 m/s=2.8 kg m/s

Momentumbowling ball = 7.0 kg x 6.0 m/s = 42 kg m/s

The bowling bowl has more momentum.

Law of Conservation of Momentum

If no net force acts on a system, then the total momentum of the system does not change.

The total momentum of the train cars does not change-momentum

is conserved.

Practice problems– How much force is needed to make a 150kg

object accelerate at a rate of 1 m/s2?– How much does a 50 kg person weigh if the

acceleration due to gravity is 9.8 m/s2?– What is the mass of a car that weighs

140,000 N?– What is the acceleration of a 150 kg object

when 1,500 N is applied?– Calculate Dave’s acceleration knowing that

in a race he starts from rest and reaches a speed of 15.3m/s in 4.71 s.

Quizzes on forces and motion

• http://www.neok12.com/php/watch.php?v=zX5b70020d7b510f41440767&t=Laws-of-Motion

After I show the video you need to click on this link and take the 6 practice quizzes.

You also need to take the vocabulary quiz on the same page.

Sec. 12.4 Universal forces

Electromagnetic Forces

Electric Forces-act between charged objects or particles

Magnetic Forces-act on certain metals, on the poles of magnets, and on moving charges

Electric and magnetic forces are the only forces that can both attract and repel

Nuclear forces

Act within the nucleus to hold it together

Strong Nuclear Force- a powerful force of attraction that acts only on the neutrons and protons in the nucleus

Weak Nuclear Force- acts only over a short range

Gravitational Force

An attractive force that acts between any two masses

Newton’s Law of Universal Gravitation- “every object in the universe attracts every other object”

Weakest, but most effective force

Centripetal Force

A center-directed force that continuously changes the direction of an object to make it move in a circle

The result of a centripetal force is a circular path of the object

The moon’s inertia and the gravitational pull of the Earth result

in a circular orbit.

Satellites in Orbit

Satellites need only a centripetal force provided by gravity and its inertia to stay

in orbit

Satellites in low orbit are slowed by friction with the Earth’s atmosphere

TEST ON CHAPTER 12 Next class meeting