Standard 8.2

Unbalanced forces cause changes In velocity.

Forces

Anticipatory Set

How do you win at tug-o-war? What does it take?

Standard 8.2 (a-c)

A. Students know a force has both direction and magnitude.

B. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

C. Students know when the forces on an object are balanced, the motion of the object does not change.

LANGUAGE OF THE DISCIPLINE

• Force: is a push or pull. Every force has a magnitude and a direction.

• Newton: (N) is the unit of measurement for the magnitude of a force

• Balanced Forces: will not change the motion of the object

• Unbalanced Forces: causes a change in the motion of an object.

• Net Force: the sum of all of the forces acting on the object

Forces• What causes MOTION• Forces have a magnitude and direction. (Just

like velocity) • Magnitude for Force= NEWTONS (N)• Use ARROWS to indicate direction of forces

and length of arrow to indicate magnitude– Example:

Net Forces

• NET means total – You combine all forces on

an object to get NET

• It determines if an object moves, which direction it moves, and how far.

Unbalanced Forces

• If you have a net force (not = to 0) then it is unbalanced forces.

• It will cause a change in velocity (direction or speed)

Balanced Forces

• There will be NO change in velocity of object (won’t move)

Check for Understanding

• If two girls push a box the same direction, both with 5 N of force. What is the net force?

10 N• If two boys push a wagon with 2 N each and in opposite

directions what is the net force?

0 N• In a tug of war game if team A pulls the rope with 7 N and

team B moves the rope with 3 N. What is the net force?

4 N (towards team A)

Practice & HW

• Guided Practice:– Read through Guided instruction on page 41-42 and

highlight important information– Complete the Guided questions in Measuring up on

page 41-42– Raise hand for a stamp

• Independent Practice: – Complete questions 1-4 on page 42

• HW: – complete questions 1-4 on page 43.

Anticipatory Set

How would these surfaces affect an obejct’s motion?

Standard 8.2 (d)

D. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.

LANGUAGE OF THE DISCIPLINE

• Gravity: force that pulls objects together and pulls toward earth.

• Compression: is a force that develops when the molecules of an object are squeezed together.

• Tension: force that develops when molecules are stretched apart.

• Friction: force between objects that works against their movement past each other

Gravity• Mass: amount of matter in an object• Weight: amount of gravitational force on an

object– So mass never changes, but weight depends on

where you are in space

This is why when you step on a scale it tells your weight, because it is how much gravity is pulling down on you on to the scale

Gravity and Mass

• Gravity= Mass x 10 m/s2

• Gravity amount doesn’t change if you put another force on it.

• the more mass the move gravity pulls• Gravity pulls down and the surface an object is

on pushes up

Elastic Forces

• Matter is elastic if it returns to its original shape after being squeezed or stretches.

• Compression- SQUEEZING• Tension- PULLING

• These are the forces that work against gravity to keep forces BALANCED

• Gravity, Compression, and Tension are forces that act “up and down”

Friction

• Friction depends on the types of surfaces and how hard they push together.

• Example: lightly put your hands together and rub them. Now push your hands together firmly and try to rub them. Notice the Difference?

• Friction is a SIDE to SIDE force– So if you place a sideways force on an object it is

and equal force of friction that will keep it stationary

Friction Problem

• Ex: a boy is pulling a box along the floor with 40 N. The force of friction is 10 N between the floor and the box. What is the net force?– 40 N (from boy) MINUS (-) 10 N (of friction)

• Net force: 30 N

• ALWAYS SUBTRACT THE FRICTION FORCE

Check for Understanding

• What forces are up and down forces?–Compression, tension, gravity

• What is a force that deals with surfaces?–friction

• What is the net force of a 6 N weight hanging from a spring and then is pulled down 4 N?

–6+4= 10

Practice & HW

• Guided Practice:– Read through Guided instruction on page 45-47 and

highlight important information– Complete the Guided questions in Measuring up on

page 45-47– Raise hand for a stamp

• Independent Practice: – Complete questions 1-6 on page 47

• HW: – complete questions 1-4 on page 48.

Standard 8.2 (e-g)

E. Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).

F. Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion.

G. Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system.

LANGUAGE OF THE DISCIPLINE

• Newton’s 2nd law of motion: motion of an object will change when unbalanced forces work upon it

• Inversely proportion: as one quantity increases, the other quantity will decrease.

• Universal force: force that works on all objects• Unbalanced forces: (see slide)

Newton’s Second Law

• Acceleration depends on the net force acting on the object and the object’s mass

Acceleration= Net force Mass

• So to speed up acceleration you need to either increase force or decrease mass

• To slow down acceleration you need to decrease force or increase mass

• SO MASS IS INVERSERLY PROPORTIONAL To ACCELERATION

Newton’s 2nd Law

Calculating Force

Acceleration= Net force Mass

So Force= Mass x Acceleration

(1 N is = to 1kg x m/s2)

Calculating Acceleration

• ex: a wagon has a mass of 10 kg. Sarah pushes it with 3 N and Brandon pushes it with 8 N in the same direction. There is a force of 9 N of friction acting on it. What is the acceleration? – ADD UP FORCES SUBTRACT FRICTION (3+8-9)– Divide it by mass (10)– Acceleration = .2 m/s2

Calculating Force

• Example: What force is needed to accelerate a box has a mass of 40 kg at 5 m/s2?

• F= M x A• 40 x 5 = 200 N

UNBALANCED FORCES

• Unbalanced forces will cause an object to either– Accelerate– Slow down– Or stop

• Other forces that will change acceleration– Gravity- will pull objects down (especially if in the air)– Friction- will slow objects down if touching surface

Reading Informational Text• Read pages 55-56- ANSWER QUESTIONS GUIDED QUESTIONS AS YOU READ

• Why is gravity a universal force?– It exists between all objects in the universe

• Why don’t you float away when you jump or skip?– Force of gravity pulls you to earth

• What influences gravitational attraction between objects– Mass and distance

• What is weight– Measure of gravity on an object

Gravity between OBJECTS• The force of gravity between two objects– increases with greater mass – decreases with greater distance.

• Gravitational force= Mass of object A X Mass of object b

Distance Between objects• Example: Which pair of objects has the greatest gravitational

force on each other?

• The 2nd and 3rd

6 kg 10 kg 7 kg

4 m 2m

Calculating weight

• On the moon acceleration due to gravity is 1.6 m/s2. On the earth it is 9.8 m/s2. If something weighs 784 kg on earth, how much will it weigh on the moon?

• USE CROSS MULTIPLICATION!1.6 = 9.8 X 784

1.6 (784)= 9.8 x Solve for XX= 128

Check for Understanding

• What is the equation for force– F= M x A

• What is the equation for acceleration– A= F/M

• Three tennis balls are sitting on a table, ball A and B are 3 feet apart and ball B and C are 10 feet part. Which has a greater gravitational force between them?– Ball A and B

Practice & HW

• Guided Practice:– Read through Guided instruction on page 50-52 and

highlight important information– Complete the Guided questions in Measuring up on

page 50-52– Raise hand for a stamp

• Independent Practice: – Complete questions 1-6 on page 52

• HW: – complete questions on page 53,56,57.