Unit 01 “Forces and the Laws of

Motion”

Test Review

What Do I Need To Know?• Content– Net Force, equilibrium, force diagrams, Newton’s Laws

• Constructed Responses– Identify which Law and answer questions with A.E.S.

• Lab Skills– No graphing this time … because we need to practice

this skill first!

• Problem SolvingFNet=F1+F2+F3+…

F=maFw=mg

ForceA force is a Push or a Pull

A NET force can change the motion of an object.

Net force: All forces on an object.Unit: Netwons (N) Kgm/s2

There are 3 ways a Net Force can affect the motion of an object.

A Net Force ACCELERATES an object… which means it can make an object…

Speed UpSlow Down

Change Direction

Force Diagramor

Free-Body Diagram

• A picture of the forces on an object.– Box represents object.– Arrows represent forces.• Different sizes to showdifferent amounts of force

A cow has two forces on it… 34N East, 50N West

F= 34NF= 50N

Force Diagram

Different size arrows –Accelerating!

Speed up, slow down or change direction!

F = 68 NF = 68 N

Equilibrium

The forces on an object add to zero.

Fnet = 0

The forces on the object are “balanced”

An object in equilibrium can:•Not move (motionless)•Move with a constant velocity

Same size arrows – Equilibrium-

Constant Velocity

OrAt Rest!

1. Define Force, Net Force and Equilibrium.2. List 3 ways a Net Force can change motion:3. Which force diagram shows an object in

equilibrium?

4. An object in Equilibrium can have 2 motions:5. What is the net force on a box moving

with a constant velocity of 60m/s? 6. Draw a force diagram

a) for a book pushed forward at a constant velocity in the presence of friction

b) for a book accelerating forward in the presence of friction.

1. Define Force:

Define Net Force:

Define Equilibrium:

Push or a Pull

The sum of all the forces on an object.

The forces on the object add to zero. FNet=0The forces on the object are balanced.

2. List 3 ways a Net Force can change motion:

3. Which force diagram shows an object in equilibrium?

4. An object in Equilibrium can have 2 types of motion:

• Motionless (not moving)• Moving with a Constant

Velocity

(not speeding up, not slowing down, not changing direction)5. What is the net force on a box

moving with a constant velocity of 60m/s? constant velocity FNet = 0

Speed Up Slow Down

Change Direction

6. FN (Normal Force)

Fw (gravity)Fw (gravity)

Different sized arrows show NET FORCE – accelerating!

Same sized arrows show EQUILIBRIUM– at rest or constant velocity!

FN (Normal Force)

FpushFpushFfriction Ffriction

1. What is Newton’s 1st Law of Motion? List at least one example of the 1st Law shown in class.

2. What is Newton’s 2nd Law of Motion? List at least one example of the 2ndLaw shown in class.

3. What is Newton’s 3rd Law of Motion? List at least one example of the 3rd Law shown in class.

4. What happens when a cart with a big mass and a cart with a small mass are pushed with the same force? Which Law does this demonstrate?

5. What happens to a person without a seatbelt who is in a moving car if the car suddenly stops? Which Law does this demonstrate?

6. What happens when a balloon filled with air is released? Which Law does this demonstrate?

1. What is Newton’s 1st Law of Motion?List at least one example of the 1st Law shown in

class.

“Inertia”“An object at rest will stay at rest

andAn object in constant velocity motion will stay in constant velocity motion

Unless acted on by an unbalanced force.”

Examples:Tablecloth trick Hoop/NutsBoy on Skateboard Guy on BusCar Accidents (Seatbelts) Ball in Truck

2. What is Newton’s 2nd Law of Motion ?List at least one example of the 2nd Law shown in

class. F=ma

“The acceleration of an object is directly proportional to the force applied to object.

The acceleration of an object is inversely proportional to the object’s mass”

Examples:•Skateboarders•Basketball and Tennis Ball•Heavy and Light Carts•Earth and Object

3. What is Newton’s 3rd Law of Motion?List at least one example of the 3rd Law shown in

class.

Action/Reaction“For every force applied to an object, the object

applies an equal and opposite force back.”

Examples:Balloon Rocket Walking“Pushing off table” Newton’s CradlePunching A Face

4. What happens when a cart with a big mass and a cart with a small mass are pushed with the same force?

The objects pushed off each other

With equal and opposite forces

And both moved back, the one with less mass faster.

The cart with more mass accelerates less,

the cart with less mass accelerates more!

Newton’s 2nd Law states: acceleration and mass are inversely proportionalMore mass, less acceleration. Less mass, more acceleration.

5. What happens if a person is moving in a car and then the car

suddenly stops?

When the car suddenly stops the person continues moving.

According to Newton’s 1st Law an object in motion will stay in motion unless an outside force is applied.

There was a force on the car, so the car stops. But there is no force on the person, so they continued moving.

6. What happens when a balloon filled with air is

released?

When released…

The balloon moved forward

And the air moved backwards.

The balloon pushed the air out (action)

So the air pushed the balloon too (reaction)

Newton’s 3rd Law states: for every force put on an object [the action], that objects puts an equal and opposite force back [the reaction].

Action-Reaction Pairs

The air molecules push the flag.

The Earth pulls a ball down.

Action Reaction

With equal and opposite force!!!!

The chair pushes me backwards.

The flag pushes the air molecules

The ball pulls the Earth up.

I push the chair forward.

A.E.S.Look at the data table below. What factor

affects the acceleration of an object?Object Force

(F) NMass (m)

kgAcceleration (a)

m/s2

Round Ball 100 50 2Box 100 15 7

Rubber Ducky 100 4 25

Look at the data table below. What factor affects the acceleration of an object?

• The mass of the object affects it’s acceleration.• According to Newton’s 2nd Law mass and

acceleration are inversely proportional.• The object with greater mass of 50kg had the

smallest acceleration of 2m/s2. While the object with the smallest mass of 4kg had the greatest acceleration 25m/s2.

A.E.S.A girl at a roller rink was trying to skate so she “pushed herself off the table” to begin moving backwards. Why is it wrong to say “she pushed

herself off the table”?

A girl at a roller rink was trying to skate so she “pushed herself off the table” to begin moving backwards. Why is it wrong to say “she pushed herself off the table”?• It is wrong to say “she pushed herself off the table”

because she pushed the table, not herself.• According to Newton’s 3rd Law, for every force

applied to an object the object applies an equal and opposite force back.

• In this case the girl pushed the table and the table pushed her back. This is why she moved.

A.E.S.A truck is driving with passengers in the bed of the truck. When the truck stops quickly do the

people “fly out” because they get pushed forward?

A truck is driving with passengers in the bed of the truck. When the truck stops quickly do the people “fly out” because they get pushed forward?• No, the people do not get pushed forward.• According to Newton’s 1st law, an object in constant

velocity motion will stay in constant velocity motion unless acted on by an outside force.

• The truck and the people were moving but only the truck had a force on it, so it stopped. There was no force on the people so they CONTINUED to move forward … they were not pushed forward.

Write the Equations for…

• Net Force

• Force

• Weight

Fnet = F1+F2+ …

Fw= mg

F = ma

Complete the following problems in your groups using the 5-Problem Solving Steps

1. What is the weight of a 60kg woman on Earth?

2. What is the net force on a box which has a mass of 20kg and an acceleration of 5m/s2.

3. Find the net force on a rope if it is pulled to the right with a force of 75N, pulled to the left with a force of 55N and left with a force of 30N.

4. What is the acceleration of a 200kg car pushed with a force of 600N?

5. What is the mass of a toy car if it is pushed forward with a force of 78N forward and friction applies a backwards force of 18N resulting in an acceleration of 2m/s2?

Answer: 100N

Answer: -10N

Answer: 588N

Answer: 3m/s2

Answer: 30kg

Solve using the Problem Solving Steps.

What is the weight of a 60kg woman on Earth?

Fw = ?m = 60kgg = 9.8m/s2

Fw = mg

Fw = (60kg)(9.8m/s2)

Fw = 588 kgm/s2

Fw = 588N

Solve the problem using the “Problem Solving Steps”.

What is the net force on a box which has a mass of 20kg and an acceleration of 5m/s2. F = ma

F = (20kg) x ( 5m/s2)F = 100 kgm/s2

F = 100 N

F = ?m = 20kga = 5m/s2

Solve using the Problem Solving Steps.

Find the net force on a rope if it is pulled to the

right with a force of 75N, pulled to the left with

a force of 55N and left with a force of 30N.

FNet=F1+F2+ F3

FNet= 75N + (-55N) + (-30N)

FNet= 75N + (-85N)

FNet= -10N

Fnet = ?F1 = 75NF2 = -55NF3 = -30N

Solving using the Problem Solving Steps.

What is the acceleration of a 200kg carpushed with a force of 600N?

F = ma600 N = (200kg) x (a)600 kgm/s2 = (200 kg)(a) 200kg 200kg 3m/s2 = a

a = ?m = 200kgF = 600N

What is the acceleration of a 30kg toy car if it is pushed forward with a force of 78N forward and friction applies a backwards force of 18N?

Solve using the Problem Solving Steps.

FNet=F1+F2

FNet= 78N + (-18N)FNet= 60N

Fnet = ?F1 = 78NF2 = -18Na = ?m = 30kg

F = ma60 N = (30kg) x (a)60 kgm/s2 = (30kg)(a) 30kg 30kg

2m/s2 = a