1 forces chapters 2.3-3 physical science. 2 forces objectives 1. explain how force and motion are...

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Forces

Chapters 2.3-3Physical Science

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Forces Objectives

1. Explain how force and motion are related.2. Distinguish between balanced and unbalanced

forces.3. Describe the different types of forces, including

frictional force, gravitational force, and centripetal force.4. Describe the four types of frictional force.5. Describe inertia and how it is related to Newton’s

1st law of motion.6. Define and apply Newton’s 2nd law of motion.7. Calculate force, mass, and acceleration with

Newton’s second law.8. Distinguish between mass and weight.

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9. State Newton’s 3rd law of motion and use it to describe and

calculate momentum.10. Identify action/reaction forces.

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Force

force: cause of acceleration, or change in an object’s velocity

-push or pull that gives energy to an object. -a force can cause an object to stop moving, start

moving, or change its direction -net force: combination of all forces acting on an

object ♦forces in the same direction add together, while forces in the opposite direction subtract from one another -measured in newtons (N), kg·m/s2

The description of force includes both a size and direction, making it a vector.

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Forces

Forces will do one of two things:

1. Cancel each other out completely (balanced forces)

-net force = zero

-no change in object’s motion

2. Do not cancel each other out completely (unbalanced forces)

-net force ≠ zero

-change in object’s motion

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Forces

= 0

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Forces

A. Forces add together when they are acting in the same direction

= 0

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Forces


= 0

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Forces


= 0

B. Forces subtract from each other when they are acting in opposite directions

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Forces


= 0


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Forces


= 0


C. Forces that are opposite in direction and equal in size cancel each other out

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ForcesHere is an example of adding and subtracting

forces to get a net force.

Is this a balanced or unbalanced net force?

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Forces Review 11. The overall force acting on an object after all

forces are combined is the ______________.2. How do balanced and unbalanced forces affect

the motion of an object?3. What are the two ways to write the units for

force?4. How is force a vector?5. What is the net force of the following: 5N 2N6. a. What are the balanced forces?

b. Will the refrigerator move? If so, in

which direction?

Forces Quiz 1

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Types of Forces

There are three types of forces:

1. frictional force (friction)

2. gravitational force (gravity)

3. centrepetal force

We will discuss frictional force first.

friction: force between two objects in contact that opposes the motion of either object

-type of unbalanced force

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Friction

Q: What does frictional force depending upon? A: the surfaces in contact

-the rougher the surface, the greater the frictional force

Example: Imagine you are wearing a pair of dress shoes. What would happen if you were to walk across ice in winter

and grass in summer?

Which is easier?

Why?

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Frictional Force (Friction)

Q: How do we overcome friction? A: a constant force must be applied to overcome

the friction

vehicle tires are designed to use friction to increase grip

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Types of Friction

1. static: friction that acts on objects that are not in motion

-acts in the direction opposite of the applied force

-ex: trying to push a box across the floor and it stays put

2. sliding: force that opposes the direction of motion of an object as it slides over a surface

-ex: actually pushing the plant across the floor

(static friction not longer acts on the plant)

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3. rolling: force that acts on rolling objects

-friction is 100-1000 times less than static or sliding

-exs: using a dolly to move large objects;

ball bearings in skates, skateboards,

and automobiles

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4. fluid: force that opposes the motion of an object as it moves through a fluid (liquid or gas)

-air resistance: fluid friction acting on an object moving through air

-ex: airplane flying through

the air; submarine

moving through the

water

-also called viscous friction

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Air Resistance

Air resistance occurs between the surface of a moving object and the air molecules.

Q: How can objects experience greater air resistance?

A: 1. increase surface area

2. increase the object’s speed

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Air Resistance

Since friction is a force, you can add and subtract

the vectors to get a net force:

Is this a balanced or unbalanced net force?

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Forces Review 21. A box is in the back of a truck. If the truck

accelerates quickly, what happens to the box?2. Can an object continue moving and have a net

force of zero?3. What is the net force equal to if unbalanced forces

exist?4. Is the following statement true or false? Why or

why not?“Friction is a force that helps objects that are touching move past each other mores slowly.”

5. What can we do to the surfaces of objects to increase friction?

6. What are the two ways to write the units for force?7. Define what we mean by a fluid.8. How do we increase air resistance?

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Gravitational Force (Gravity)

Introduction: Isaac Newton made a discovery about

gravity: Legend has it that he sat under an apple

tree one evening and saw an apple fall. After giving it some thought, Newton realized that a force pulled the apple to the ground.

-called it the Universal Law of Gravitation

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Gravity

gravity: force of attraction between objects/particles of matter due to their mass.

-all objects exert gravitational forces on all other objects, even when they do not touch

-can act over large distances

Acts on: 1. falling objects -causes objects to accelerate downward, where air resistance reduces acceleration ♦terminal velocity: constant velocity of a falling

object when the force of air resistance = force of gravity

http://www.mcasco.com/p1mot2d.html

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2. projectile motion: motion of a falling object after it is given an initial forward velocity

-follows a curved path -gravity and air resistance act on the object

Objects fall and undergo projectile motion due to the attraction between objects.

http://apcentral.collegeboard.com/apc/members/courses/teachers_corner/30827.html

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Gravity

The attraction between objects depend on two things1. the mass of the objects -greater the mass, the greater the attraction2. the distance between the objects -shorter the distance, the greater the attraction

The moon is closer to the earth than the sun, therefore the earth has a greater pull on it, keeping it in orbit.

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Centripetal Force

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

-ex: Earth’s gravity pulls the

moon into a nearly circular

path around the Earth; satellites in orbit -reason for the tide changes

at the ocean

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Forces Review 3

1.What do we call an y force that keeps an object in a circle?

2. How do we overcome friction?3. List and describe the types of friction?4. How does gravity and air resistance affect

the motion of a falling object?5. Define what we mean by a fluid.6. How do we increase air resistance?7. Define gravity. What is gravity dependent

on?8. What is meant by terminal velocity?

Forces Quiz 2

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Newton’s Laws of Motion

Isaac Newton summarized his study of motion and force into three laws that we now call Newton’s Laws of Motion.

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Newton’s First Law

Newton’s 1st Law of motion: an object at rest will stay at rest and an object in motion will stay in motion until acted upon by an outside unbalanced force. -net force = 0; keeps the object in unchanging

motion -A car will continue to stay in motion until a force,

such as friction, slows it down.

-Ex: You keep moving forward, even when the car

you are riding in comes to a stop.

Law of Inertia

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Also called the “Law of Inertia”, where inertia is the tendency of an object to remain at rest or in motion with a constant velocity

An object’s velocity will change if an unbalanced force acts on it, causing the object to accelerate.

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Newton’s 2nd Law of Motion states that the force is proportional to the mass and is also proportional to the acceleration of an object. -can be stated as the equation:

-more mass requires more force to accelerate the object -more force causes more acceleration

F= ma (measured in newtons, N = kgm/s2)

This law means:

Forces cause acceleration and mass resists acceleration.

Newton’s 2nd Law of Motion

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Newton’s Second Law: Example 1Zookeepers lift a stretcher that holds a sedated lion.

The total mass of the lion and stretcher is 175 kg and the lion’s upward acceleration is 0.657 m/s2. What is the unbalanced force necessary to produce this acceleration?

Known variables: F = ?m = 175 kg

Unknown variable:a = 0.657 m/s2

Formula: F = ma

= (175 kg)(0.657 m/s2)

= 115 kgm/s2

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Newton’s Second Law: Example 2If a 50 N force pulls on a 10 kg object, how

much acceleration will occur?

Known variables: F = 50 N m = 10 kgUnknown variable:a = ? m/s2

Formula: F = ma 50 N = (10 kg)a

= 5 m/s2

50 kgm/s2 = (10 kg)a

50 kgm/s2 = (10 kg)a 10 kg 10 kg

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Newton’s Second Law Practice 1

1. What is the net force on a dragster with a mass of 900 kg if is acceleration is 32.0m/s2?

2. What is the force on a helicopter that has a mass of 4500 kg if it’s acceleration is 4m/s2?

3. If a car with a mass of 1500 kg has an acceleration of 2.0m/s2, what is the force needed to accelerate the car?

4. You push a friend on a sled. Your friend and the sled together has a mass of 70 kg. If the net force on the sled is 35 N, what is it’s acceleration?

Forces Homework 1

1. How much force is needed to accelerate a 1000 kg car at a rate of 3 m/s2?

2. If a 70 kg swimmer pushes off a pool wall with a force of 250 N, what is her acceleration?

3. If a person is pushing a cart with a force of 40 N and it accelerates at 0.5 m/s2, what is the mass of the object?

4. What is the acceleration of a 3 kg rock that is thrown with a force of 18N?

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Forces Review 4

1. Describe the two factors that affect the acceleration of an object?

2. Are these statements true or false? Why or why not? a. “According to Newton’s 1st law of motion, an object’s motion does not change as long as the net force

acting on it is zero.” b. “The law of inertia states that an object in motion

will eventually slow down and stop if it travels far

enough.”3. Describe Newton’s 2nd law of motion. 4. A 70N force pushes on 3 objects: 2kg, 4kg and 6 kg a. Which will have the greatest acceleration? b. Which will push back with the greatest force?

Forces Quiz 3

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Free Fall

free fall: the motion of a body when only the force of gravity is acting on it

-its directed toward the center of the earth

-often abbreviated as the letter ‘g’ due to the acceleration due to gravity

Free-fall acceleration near Earth’s surface is constant.

-in the absence of air resistance, all objects near Earth’s surface accelerate at the same rate, 9.8 m/s2

-in our calculations, we will disregard air resistance

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Weight

weight: force of gravity on a mass -weight changes as gravity changes

-unit is the N, since it is a force

-formula is W = mg

~‘g’ is different on different planets and in space

(it is 0 in space)

~use 9.8 m/s2 unless stated differently

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Newton’s Third Law

Newton’s 3rd law of motion: for every action, there is an equal and opposite reaction.

-forces always occur in pairs, though they may act on different objects

~Ex: kicking a soccer ball – the action force acts on the ball, but the reaction force acts on the foot

-action/reaction forces always occur at the same time

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Momentum

If a tractor trailer truck and a car are moving in the same direction at the same speed, why is it more difficult for the tractor trailer to stop?

-it has more mass, therefore a greater momentum.

momentum (p): quantity defined as the product of an objects mass and velocity

momentum = mass x speed

p = mv

-an object must be moving to have momentum

-the more momentum, the harder it is to stop

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Momentum

Law of conservation of momentum: the total amount of momentum in a system does not change.

-can use it to describe what happens between pairs of forces, as mentioned in Newton’s 3rd Law

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Momentum: Example 1

Ex. Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley.

p = ? m = 6.00kg v = 10.0m/s p = mv

p = (6.00 kg)(10.0 m/s)

p = 60.0 kgm/s

REMEMBER- law of conservation of momentum: the total amount of momentum in a system is conserved

You MUST show the units in both the problem and the answer!!

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Momentum: Example 2

Ex. What is the mass of an object going 3 m/s with a momentum of 36 kg•m/s

p = 36 kg•m/s m = ? kg v = 3 m/s 36 kg•m/s = mv

36 kg•m/s = m(3 m/s)

36 kg•m/s = m(3 m/s)

3 m/s 3 m/s

12 kg = m

You MUST show the units in both the problem and the answer!!

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Momentum (and Force) Practice1. What is the momentum of a 2kg toy truck that

moves at 10m/s?2. What is the speed of a 25kg object if the

momentum of that object is 300kg•m/s? 3. What is the mass of an object that goes 25m/s

and has a momentum of 500kg•m/s?4. If a 500kg thoroughbred at the Kentucky Derby

is galloping with a momentum of 8220kg•m/s, what is its speed?

5. An automobile with a mass of 1000 kg accelerates when the traffic light turns green. If the net force on the car is 4000N, what is the car’s acceleration?

Forces Homework 2

1. How much momentum does a 30 kg object going 4 m/s have?

2. How fast is a 15 kg object going if it has a momentum of 45 kg•m/s?

3. A 2N and 6N force pull on an object to the right and a 4N force pulls to the left. What is the net force of the object? Draw a diagram.

4. A 20 kg bike accelerates at 10 m /s. With what force was the person pedaling?

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Forces Review 5

1. Are these statements true or false? Why or why not? a. “The loss of momentum of one object equal the

gain in momentum of the other.” b. “Action reaction forces do not cancel each other out because the action force is always greater than

the reaction force.”2. How are mass and weight different?3. What is the unit for momentum?4. Describe the two factors that affect the momentum of

an object? 5. Which of Newton’s law’s applies? a. How a rocket flies in space. b. Braking hard wears out your breaks faster. c. A car turns & objects in the car keep going

straight.

Forces Quiz 4

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Forces Practice 21. An automobile with a mass of 1200kg accelerates

at a rate of 3.0m/s2 in the forward direction. What is the net force acting on the automobile?

2. How much force was exerted on a sled that has a mass of 70kg and accelerated at a rate of 0.5m/s2?

3. What is the force on a dragster with a mass of 900kg if its acceleration is 32.0m/s2?

4. During a crash test, an airbag inflates to stop a 75 kg dummy’s forward motion. If the net force on the dummy is 825 N, what is the dummy’s acceleration?

5. A car is being pulled by a tow truck. What is the car’s mass if the net force on the car is 3000N and has an acceleration of 2.0m/s2?

Forces Homework 3

Forces Homework Worksheet

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1 forces chapters 2.3-3 physical science. 2 forces objectives 1. explain how force and motion are...

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