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Forces and Motion

Three Incorrect laws of motion.

• http://www.youtube.com/watch?v=Yf0BN0kq7OU&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

• Why does the Earth Spin?– http://www.youtube.com/watch?v=TQxeutcYP6I

&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

• Second clip?– http://www.youtube.com/watch?v=9zso7ChaQX

Q&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

Vocabulary

•Force - A push or pull on an object.

•Net Force – vector sum of all forces

•Equilibrium:– the state in which the net force on an

object is zero or balanced– no change in velocity

Sir Isaac Newton• 1643 - 1727

2.2.4 State Newton’s first law of motion. 2.2.5 Describe examples of Newton’s first law.

1st Law of Motion (Law of Inertia)

An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

1st Law of Motion (Law of Inertia)

Translation: Objects keep doing what they are doing, in a straight line.

Newton’s 1st Law – Inertia

• Inertia is a measurement of ______________ TO CHANGE

• It is measured in terms of ______. The greater the _____, the greater the inertia.

RESISTANCE

massmass

What is Mass?

• Mass is the amount of material a body/object contains

• You are what you eat

• Bigger does not equal more mass

2.2.2 - Identify the forces acting on an object and draw free-body diagrams representing the

forces acting

Free Body Diagrams

•FBD’s are a neat way to show the forces acting on an object.

•The object is drawn as a dot or a box, and then vectors are drawn to show the forces acting on it.

•Ex: 10 N

6 N

5 N

FBD’s

• Can you draw the FBD for you sitting on your chair?

• Draw the FBD for your friend pushing you in a rolling chair.

FBD’s

• Solve the net force for the following FBD.

10 N

6 N

5 N

FBD

• The picture at the bottom shows a snowman pulling a sled. Draw a free-body diagram for this sled. The magnitudes of the forces acting on the sled are 60N by the string, 130N from gravity, and 90N upward by the ground.

FBD

• Now try doing some of the free-body diagrams on

• pg 124 Practice A

• Pg 124 Section Review # 2-5

2.2.6 - State the condition for translational equilibrium.2.2.7 - Solve problems involving translational equilibrium.

• Equilibrium in a straight line.

• Fnet = 0 so there is constant velocity

2.2.6 - State the condition for translational equilibrium.2.2.7 - Solve problems involving translational equilibrium.

• Derek leaves his physics book on top of a drafting table that is inclined at 35º angle. The FBD to the right shows the forces acting on the book. Find the net force acting on the book.

Vocabulary

• Force - A push or pull on an object.

• Net Force – vector sum of all forces

• Equilibrium:– the state in which the net force on an

object is zero or balanced– no change in velocity

2.2.8 State Newton’s second law of

motion. 2nd Law

The net force of an object is equal to the product of its mass and acceleration, or F=ma.

Newton’s 2nd Law – Simplified

• So, a little easier to read would be:

a = ΣF OR ΣF = ma m

• So… what are the units for force

• Forces are measured in newtons 

• N = (kg)(m/s2).

What does F = ma mean?

• Force is directly proportional to mass and acceleration.

Imagine a ball of a certain massmoving at a certainacceleration. This ball has a certain force.

What does F = ma mean?

Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball.

What does F = ma mean?Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

More about F = ma

• If you double the mass, you double the force. If you double the acceleration, you double the force.

What if you double the mass and the acceleration?

(2m)(2a) = 4F

Doubling the mass and the acceleration quadruples the force.

So . . . what if you decrease the mass by half? How much force would the object have now?

What does F = ma say?• F = ma basically means that the force of an

object comes from its mass and its acceleration.

Something very small (low mass) that’s changing speed very quickly

(high acceleration), like a bullet, can still have a great force. Something

very small changing speed very slowly will have a very weak force.

Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.

Newton’s 2nd Law – Vectors!

• Remember, acceleration is a vector quantity, so Force will be too

• Some equations to remember:– Pythagorean Theorem– Sin θ– Cos θ– Tan θ

2.2.1 Calculate the weight of a body using the expression W = mg.

Gravity causes weight• The weight of an object is caused by the

force of attraction between the earth and objects on the surface of the earth.

Gravitation force is equal to weight, therefore…

How much does a person with a mass of 70.0 kg weigh on

the earth?

Weight = mass X 9.8 m/s2

Weight = 70.0 kg X 9.8 m/s2

Weight = 686 N

Remember that 1 N = (1 kg) (1m/s2)

Mass vs. WeightYour MASS is the amount of material in your body.Your MASS doesn’t depend on where you are.• Your WEIGHT is how much your body pushes down on a scale.• Your WEIGHT depends on how much MASS you have and where you are.• Weight is caused by gravity.

Martym = 68 kg

• G(sun)= 275 m/s2

G(pluto)= 0.654 m/s2

What does Marty weigh?

18,700 N

44 N

2.2.9 Solve problems involving Newton’s second

law. 1. Determine the accelerations that result

when a 12-N net force is applied to a 3-kg object.

2. A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. What is the mass of the encyclopedia?

2.2.9 Solve problems involving Newton’s second

law. • Now try answering some problems on

pg 132 practice C # 1-4

• There is a net force of 12N acting on a desk which has a gravitational force of 40N. How much is the desk accelerating?

• A 50N crate experiences a horizontally applied force of 10N from a student and a frictional force of 4N. What is the acceleration of the crate?

Newton’s 2nd Law – Let’s practice

1. You have run out of gas and have to push your car. Your car weighs 560 N and you push with a force of 395 N, and your friend helps with 275 N. What is the acceleration of the car?

2. You push a rolling chair (m=20kg) with a force of 17 N. A friend pushes with 15 N at 30o. What is the net force? What is the acceleration of the chair?

Quick Quiz

Consider the following free-body diagram. Ignoring air resistance and friction, can this object be moving in a horizontal direction?

We’ll come back to this question.

2.2.14 State Newton’s third law of motion.

2.2.15 Discuss examples of Newton’s third law.

http://www.youtube.com/watch?v=8bTdMmNZm2M&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

3rd Law• For every action, there is an

equal and opposite reaction.

What does this mean?

• For every force acting on an object, there is an equal force acting in the opposite direction.

Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving.

• There is a balanced force acting on you– gravity pulling down, your seat pushing up.

•Put really simply: F = F

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 Third Law

• It states:Whenever one object exerts a force on a

second object, the second object exerts an equal and opposite force on the first object

• It requires two forces:– _____________ Force– ______________ Force

ActionReaction

Newton’s Third Law

• Does it matter which force we call the ________ and which is the _________?

• Forces always occur in ________. Therefore, it ________ matter which is the action and the reaction, as long as you can identify both.

action reaction

pairsdoesn’t

ReviewNewton’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.

Quick Quiz

Consider the following free-body diagram. Ignoring air resistance and friction, can this object be moving in a horizontal direction?

YES!!!! IT CAN!!!

Quick Quiz

Think about this situation.What are the forces on the boy on the sled between points B and C?

I hope you got something that looked a lot like this.

Quick Quiz

This means a force is not required to keep a moving object in motion.

Forces

• Types of forces• Contact• Non-Contact

Forces• Contact

• Normal• Frictional• Tension• Air Resistance• Spring

Forces• Non-Contact

• Electrical• Magnetic• Nuclear

Normal Force

• http://www.youtube.com/watch?v=aJc4DEkSq4I&list=EC772556F1EFC4D01C&safety_mode=true&persist_safety_mode=1

The Normal Force

• The normal force (FN) is one component of the force that a surface exerts on an object with which it is in contact

• Namely, the component that is ____________ to the surface.

Perpendicular

The Normal Force

• How do I find the normal force?• Easy, it is equal and opposite to the

weight of an object if the object is on a horizontal surface.

• The Fn is not always horizontal.

• Lets look at Fig 12 (pg136)

• The Fn can be found by

• Fn=mg cosΘ (where Θ is the angle between the

contact surface and horizontal)

The Normal Force

• Where is the normal force?

What is Friction?

-is a force resisting the relative motion of two surfaces in contact with each other. -is the parallel component of the force on an object that is in contact with a surface.-is a force that prevents motion from occurring.

-Types-Static-Kinetic-Fluid

What causes friction?• There are two factors which affect

friction between two surfaces:– Kind of surfaces in contact (rough or

smooth)

– Amount of force pressing the surfaces together.

The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.

From Glencoe Physical Science

Static and Kinetic Friction

Static Friction- Friction force that acts on objects that are not moving

Kinetic Friction - Friction force that opposes the direction of motion of an object as it slides over a surface. I better be

safe Ump!!

Static Friction

• Static Friction is the force that prevents the start of motion.

• With no movement, Fs = Fapplied

• Has a range of 0 – Fs,max

Kinetic Friction

• Once motion has begun, the static friction force is no longer a concern.

• Instead, kinetic friction is important. • Kinetic friction is resistance to movement of an

already moving object.

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

Frictional Forces

• The frictional force will vary depending on the surface interactions. – Ex. Carpet vs tile

• Friction can be calculated by using the coefficient of friction.

• Use the basic formula of: – Ff = μFn

• It can be rearranged to:– μk = (Fk/Fn) and μs = (Fs/Fn)

Frictional Forces

• Use the formula of: – Ff = μFn

– Fk = μkFn and Fs = μsFn

• A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

Force of Tension

• Tension commonly means the tendency of a rope to be pulled apart.

• Tension is typically the same everywhere in a rope because the rope is treated as massless. If it isn’t (and is vertical) you must account for the mass of the rope.

Finally, AIR RESISTANCE!!!

2.2.6 State the condition for translational equilibrium

• Translational equilibrium is equilibrium in a straight line.

• The net force acting on a free falling object at terminal velocity is zero, so it falls to the ground at constant speed.

Terminal Velocity

Force of Gravity

Air Resistance

T.V. is the constant velocity of a falling object when the force of gravity equals the force of air resistance

The velocity at which a free falling object ceases to accelerate.

Spring Force• We will dive deep into

spring force when we get to elastic energy.

• Until then, play safely!!

Electric Forces

- forces that act between charged objects or particles such as protons and electrons

Magnetic Forces

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

Nuclear Forces

- Nuclear forces act over very short distances and are what hold the particles of an atomic nucleus together.

• Use the formula of: Fk = μkFn and Fs = μsFn

• A hockey puck has a coefficient of kinetic friction of μk = .10. If the puck feels a normal force (FN) of 5 N, what is the frictional force that acts on the puck? What is the mass of the puck?

• A desk has a mass of 71.25 kilograms. If the coefficient of static friction between the desk and the floor is 1.14, what force must be used to move the desk from rest?

Quick Review (Net Force)• There is a 15kg book sliding down a desk

that has an incline of 25º. If there is a frictional force of 20N, what is the acceleration of the book?

• Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. There is a frictional force of 11N, a normal force of 18N and gravitational force of 22N. What is the net force?

On your own!! (Net Force)• A student is designing a support to keep a tree upright. Two

wires have been attached to the tree and placed at right angles to each other. One wire exerts a force of 30N on the tree; the other wire exerts a 40N force. Determine where the third wire should be placed and how much force is should have so that the net force acting on the tree is zero.

• A stationary, flying kite is acted on by a force of 9.8N downward. The wind exerts a force of 45N at an angle of 50° above the horizontal. Find the force that the string exerts on the kite.

Putting it all together!!• A student attaches a rope to a 20kg box

of books. He pulls with a force of 90N at an angle of 30° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.50. Find the acceleration of the box.

• Slow down, take a deep breath and lets take this one step at a time.

Putting it all together!!

• Now lets try pg 141 Practice F, # 1-4• Don’t get bogged down with #2

What is momentum?• The momentum of an object is equal to

the product of its mass and its velocity. The unit for momentum is kg m/s

How some things react without forces…