I wonder if other dogs think poodles are members of a weird religious cult. - Rita Rudner

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Lesson 1: Combining Forces

Lesson 2: Types of Force

Lesson 3: Unbalanced Forces and Acceleration

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force

net force

unbalanced forces

balanced forces

Newton’s first law of motion

2.1 Combining Forces

Force

2.1 Combining Forces

• A push or a pull on an object is a force.

I wonder if other dogs think poodles are members of a weird religious cult. - Rita Rudner

Force

A. Force is a Vector

B. Force has a direction and a magnitude.

C. Force is measured in Newtons.

2.1 Combining Forces

Combining Forces

A. The force on an object is the combination of all forces on that object

B. The resulting force of all the forces is called net force.

2.1 Combining Forces

Forces in the Same Direction

A. When forces in the same direction combine, 1. the net force is in the same direction

2. the size of the net force are the forces added together.

2.1 Combining Forces

Combining Forces in the Same Direction (cont.)

• If two people push the dresser to the right, the forces are added together.

2.1 Combining Forces

Forces in Opposite DirectionsA. Combining forces in opposite direction=

1. the net force is in the direction of the larger force.

2. The size of the net force is the difference between the two forces.

2.1 Combining Forces

Combining Forces in Opposite Directions (cont.)

• Two people exerting a force in opposite directions show how vectors combine to create net force.

2.1 Combining Forces

Unbalanced and Balanced Forces

A. If the net force on an object is not zero, it is an unbalanced force.

B. If the net force on an object equals zero, it is a balanced force.

2.1 Combining Forces

ONLY UNBALANCED FORCES CAUSE CHANGE IN MOTION!

Balanced Forces

2.1 Combining Forces

Balanced Forces and Motion

• If two people push with the same force, the door does not move.

2.1 Combining Forces

• The forces have the same magnitude.

• The forces are in opposite directions.

• The net force on the door is zero and the forces are balanced.

• The motion of the door does not change.

Unbalanced Forces and Motion

• Unbalanced forces change motion.

• If two people push with different force, the door moves.

• The forces have a different magnitude and are in opposite directions.

• The net force on the door is not zero and the forces are unbalanced.

• The door moves in the direction of the larger force.

2.1 Combining Forces

Which of the following changes occur when an unbalanced force acts on an object?

A motion

B mass

C inertia

D weight

2.1 Combining Forces

What is the net force acting on a door if one person pushes to the right with a force of 2000 N and a second person pushes with a force of 1500 N to the right?

A 3500 N to the right

B 3500 N to the left

C 500 N to the left

D 500 N to the right

2.1 Combining Forces

You are riding a bike. In which of the following situations are the forces acting on the bike balanced?

A You pedal to speed up.

B You turn at constant speed.

C You coast to slow down.

D You pedal at constant speed.

2.1 Combining Forces

gravity

law of universal gravitation

weight

friction

elastic force

tension force

2.2 Types of Forces

compression force

normal force

Which is fundamental force we all know?

1. Gravity is an attractive force between all objects that have mass in the entire UNIVERSE,

2. Also known as Law of Universal Gravitation

Predict the forces acting on the object?

2.2 Types of Forces

Gravity (cont.)

• The ball does not travel in a straight line because of the unbalanced force of gravity acting on it.

2.2 Types of Forces

2. Gravity rules

1. Gravity is stronger when objects are closer

2. Weaker when farther apart

3. The more mass, the more gravity

4. Smaller mass, less gravity

5. F= Gm1m2/r2 (G=6.673x10-11 )

2.2 Types of Forces

The Law of Universal Gravitation (cont.)

• The gravitational force becomes stronger as either or both objects increase in mass.

• The gravitational force becomes weaker as the distance between the objects increases.

2.2 Types of Forces

3. What’s does gravity have to do with Weight and Mass?

• NOTE: Mass is the amount of matter in an object and does not change with location.

A. Weight is the gravitational force on a object and changes with location.

2.2 Types of Forces

Where would you weigh less?

Question Time

1. Give an example of something that has gravity.

2. What are the two ways to increase the size of the force of gravity?

3. What changes as you get farther from the Earth, mass or weight?

4. What is the formula for speed?

B. Friction

1. Friction is a force that opposes motion• Matter isn’t smooth, so things get caught on

each other

2.2 Types of Forces

Friction increase as motion/force increases!

Friction increases with rougher surfaces!

Lubricants reduce friction

Surface area doesn’t matter! (except fluids)

Friction

i. Usually friction is present and an unbalancing force must be applied to keep an object moving.

ii. When there is very little friction, objects move with near constant velocity, with NO other forces required

2. Static Friction

A. Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.

2.2 Types of Forces

If the object doesn’t move as you push on it with more and more force

Then the forces are balanced

Which means that static friction increases to match your force on the book

Keeps things in place! Always a Balanced forces

2. Sliding Friction

2.2 Types of Forces

• The frictional force that acts on the sliding object is called sliding friction. (duh)

Sliding Friction (cont.)

• The book pushed across the table slows down because of sliding friction.

• Without sliding friction, the book would continue moving without a force being applied.

2.2 Types of Forces

Friction

2.2 Types of Forces

Other types of Friction:

3. Rolling Friction

4. Fluid Friction (air and water resistance)

Question time

i. Friction is a force that ______ motion

ii. What kind of friction keeps a car from moving when you push on it?

iii. What is one way to increase friction?

iv. What are the four types of friction?

C. Elastic Forces

1. An elastic force occurs when a material is stretched or compressed.

2.2 Types of Forces

3 Types of Elastic Force1. Tension

2. Compression

3. Normal

2. Tension

• A tension force is a pulling force exerted by an object when it is stretched, such as a rubber band.

2.2 Types of Forces

3. Compression

• A compression force is a pushing force exerted by a material when it is squeezed or compressed.

• Compression force always balances the force pushing on it

2.2 Types of Forces

4. Normal Forces

A normal force is the force exerted by an object that is perpendicular to the surface of the object.

2.2 Types of Forces

Normal Forces (cont.)

• The cup is exerting a downward force on the table, caused by gravity.

• The table is exerting an upward normal force on the cup, caused by compression.

2.2 Types of Forces

Question time

I. What kind of force does a rubber band exert when you stretch it?

II. Give an example of a compression force?

III. What are elastic forces?

IV. What is the “normal” force?

V. Why is tension important to a bridge?

VI. Why is compression important to a bridge?

Forces in the Horizontal Direction

• Friction balances forces applied in a horizontal direction, usually

• Friction equals the horizontal force on an object that is not changing motion.

2.2 Types of Forces

Forces in the Vertical Direction

• Upward normal force balances the downward force of gravity on an object that is not moving vertically.

2.2 Types of Forces

Forces in the Vertical Direction (cont.)

2.2 Types of Forces

A(n) ____ force is the force exerted by an object that is perpendicular to the surface of the object.

A compression

B elastic

C normal

D tension

2.2 Types of Forces

Which force causes a rolling ball to slow down?

A sliding friction

B static friction

C normal force

D gravity

2.2 Types of Forces

The gravitational force between two objects ____ as ____ increases.

A increases; distance

B decreases; mass

C increases; velocity

D decreases; distance

2.2 Types of Forces

Chapter Assessment

California Standards Practice

Concepts in Motion

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If a student pushes a book across a table with a force of 10 N and the force of friction is 2 N, what is the net force on the book?

A 0 N

B 2 N

C 8 N

D 12 N

What would cause an object to have a larger acceleration?

A increasing the net force applied

B decreasing the net force applied

C increasing the mass and keeping the net force constant

D none of the above

What causes a change in motion on an object?

A zero net force

B unbalanced force

C weight

D balanced force

A car is driving at a constant velocity. Which of the following statements is true?

A A balanced force keeps it moving.

B The car is moving in a circle.

C An unbalanced force keeps it moving.

D Friction keeps the car moving.

For every action force, there is a reaction force that is ____.

A in the same direction

B equal to the two objects

C equal and in the opposite direction

D unequal and in the direction of the object with less mass

Which of the following best describes why force is a vector?

A speed and size

B magnitude

C direction and friction

D magnitude and direction

SCI 2.a

Two people are pushing in opposite directions on a box. One person is pushing to the left with a force of 250 N, the other is pushing to the right with a force of 400 N. What is the resulting force vector?

A 150 N to the right

B 650 N to the right

C 150 N to the left

D 400 N to the right

SCI 2.b

You throw a 0.9 kg ball with a force of 17 N. What is the ball’s acceleration?

A 0.05 N

B 15.3 N

C 13.3 m/s2

D 18.9 m/s2

SCI 2.e, 2.f

A tennis ball hit by a racket has an acceleration of 2000 m/s2. If the net force on the tennis ball is 125 N, what is the tennis ball’s mass?

A 16 kg

B 0.25 kg

C 0.06 kg

D 0.43 kg

SCI 2.f

What balanced forces are acting on a book sliding across a table?

A gravity

B gravity and friction

C sliding friction, normal force, and gravity

D gravity and normal force

SCI 2.d

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