U3: Force Newton’s 1st and 2nd Laws

4 Common Types of Forces Push Pull Torque Tension/Compression

Identify the Forces

What is Force? We will study forces that can come either as a push a pull, or a tension. Measured by the Newton, Because anytime we accelerate 𝑚 𝑠 2 a mass (kg), we are applying a force. 𝑁=𝑘𝑔 𝑚 𝑠 2

Mass (kg) 1kg is the mass of 1L of water. 1kg = 2.2lbs

Newton’s 3 laws of motion Only THREE?! Newton’s 3 laws of motion

1st Law

Newton’s 1st Law of Motion An object at rest or an object moving with a constant velocity will continue behaving in the same way unless acted on by an unbalanced force. What’s an unbalanced force?

Balanced Forces Balanced forces mean no movement. Think of two people who kick a soccer ball at the same time in opposite directions. The ball won’t move! Why? The force were balanced!

Balanced vs. Unbalanced Balanced Force Unbalanced Force Two oppositely faced forces are exactly the same. Will “cancel” each other out. The net force will equal 0N. Two oppositely faced forces are different. Will not “cancel” each other out. The net force will equal 5N to the right. 5N 5N 10N 5N

Net Force? A net force is the overall force that acts on the object we are focusing on. We can find this net force by adding them together and keeping positive (+) and negative (-) direction in mind.

Balanced vs. Unbalanced Balanced Net Force Unbalanced Net Force 𝐹 = 𝐹 𝑛𝑒𝑡 𝐹 =5𝑁+(−5𝑁) 𝐹 =0𝑁  therefore: The net force will equal 0N. 𝐹 𝑛𝑒𝑡 = 0N 5N 5N 𝐹 = 𝐹 𝑛𝑒𝑡 𝐹 =10𝑁+(−5𝑁) 𝐹 =5𝑁  therefore: The net force will equal 5N. 𝐹 𝑛𝑒𝑡 = 5N 10N 5N

1st Law Definition: At rest will stay at rest: In motion will stay in motion: If there is no unbalanced force acting upon it. If there is no unbalanced force acting upon it.

1. A ball is put into the end of the tube indicated by the arrow 1. A ball is put into the end of the tube indicated by the arrow. The ball is then shot out of the other end of the tube at high speed. Pick the path the ball will follow after it exits the tube. Note – you are looking down on these tubes, they are not vertical.

1st Law and Inertia Inertia: The tendency for an object to stay at rest or stay in motion. Why it takes lots of force to… Start making a car move Slow down a moving car

2. Which object has the most inertia? Baseball Golf ball Bowling Ball Football

Challenge: Determine which jar is filled. B Things you can’t do to the jar: lift tap the side open drop Things you can do: Anything else.

Story time

Ryan rode on the freeway more than anything else Ryan rode on the freeway more than anything else. Was he correct to never fill his tank beyond ½? No. More mass -> more inertia -> less easy for air resistance and friction to slow down his car.

What scenario(s) exist where it would be better to have the less massive car?

2nd Law

2nd Law Definition: The relationship between an object’s mass and its acceleration is given by: 𝑎 = 𝐹 𝑚 If we use what we learned about net force, the 2nd Law becomes: 𝑎 𝑛𝑒𝑡 = 𝐹 𝑛𝑒𝑡 𝑚

Force of gravity! (weight) Gravity pulls downward with an acceleration of: 𝑎 𝐺 =9.8 𝑚 𝑠 2 The force of gravity will be proportional to the mass being pulled down. Heavier mass, more force Lighter mass, less force Therefore: 𝐹 𝐺 =𝑚 𝑎 𝐺

Mass is what weight is in the metric system (kilograms) What is mass vs. weight? Mass is what weight is in the metric system (kilograms) Mass is a measure of how much stuff there is, which is like weight. Both of the above options

What is mass and weight? Weight is the force of gravity pulling down on it. Mass is a measure of how much matter there is. Link

Inertia The tendency for an object to stay at rest or stay in motion. Why it takes lots of force to… Start making a car move Slow down a moving car

Inertia is based on… Mass Weight A combination of the two

5N 5N It will increase (car go faster!) Your car’s engine puts force on the car to move forward. Wind resistance of the same magnitude acting against your car pushes back on it. What will happen to your car’s velocity? It will increase (car go faster!) It will decrease (car slow down) It will stay the same Impossible to tell from the given information. 5N 5N

It is some positive value of Newtons. If the car accelerates to the right, what can we conclude about the net force on the car? It is 0N. It is some positive value of Newtons. It is some negative value of Newtons. It is equal to the force of the engine minus the force of wind resistance.

If gravity pulls down on a middle-weight ball vs. a heavy-weight ball, They will accelerate at the same rate, because the forces are the same. They will accelerate at different rates, because one of them experiences more force. They will accelerate at the same rate because the forces are different (greater on very heavy ball). Link

3 Incorrect Laws of Motion Link

A truck hits a small car, which experiences more force? They experience equal force. The car experiences more force. The truck experiences more force.

3rd Law

Newton’s 3rd Law of Motion For every force, there is an equal and opposite force.

For every force, there is an equal and opposite force. What does this mean? Think of the car and the truck. Which has more inertia? Truck: More inertia Takes more force to accelerate Force is a cause of… …Acceleration. (effect) Car: Less inertia Takes less force to accelerate Force is a cause of… …Acceleration. (effect)

To the Whiteboard! Truck: Car: Experiences force of car on truck = 90,000N mT = 3,000kg Experiences force of truck on car = 90,000N mC = 500 kg To the Whiteboard!

The cannonball because it has more acceleration. Let’s analyze a cannon and ball. Which experiences more force when firing, the cannon or the cannonball? The cannonball because it has more acceleration. The cannon because it has more mass. Both experience the same force.

CP #13 𝒎 𝑪 𝑭 𝐄𝐂 = 𝒂 𝑪 𝐹 𝐸𝐶 𝐹 𝐸𝐵 𝒂 𝑩 𝑭 𝐄𝐁 = 𝒎 𝑩

Free Body Diagraming (FBD) Diagram forces on an object Pretend the object is a point Show all push/pull forces acting on the object Usually multiple forces

Example Drawing and FBD: 𝐹 𝑆𝑅 𝐹 𝑆𝑅 𝐹 𝐺𝑅 𝐹 𝐺𝑅

Force of Gravity and Mass Find the mass of the object or Find the Force of the Earth on the object

Force of Gravity and Mass Mass of the Rock 𝐹 𝐺 𝐹 𝐺 = 𝑚 𝑅 𝑎 𝐺 Acceleration due to Gravity

Let’s Draw a FBD for each object DURING the firing of the gun. Rifle Bullet Boom! FBD: FBD: 𝐹 𝐸𝐵 𝐹 𝐸𝑅

Yes, it accelerates just as much as I do because the forces are equal. If you step off a ledge, you accelerate noticeably toward the Earth because of the gravitational interaction between you and earth. Does the Earth accelerate toward you as well? Yes, it accelerates just as much as I do because the forces are equal. Yes, it accelerates less than I do because the forces are equal. Yes, it accelerates more than I do because the forces are equal. No, it does not accelerate at all, the equal and opposite force exists elsewhere.

FBD of Earth and Jumper FBD Earth: FBD Jumper: FBD Air: Force of…. … person’s gravity on earth. Force of Air Resistance Force of Earth’s Gravity on person FBD Air:

Yes, it accelerates just as much as I do because the forces are equal. If you step off a ledge, you accelerate noticeably toward the Earth because of the gravitational interaction between you and earth. Does the Earth accelerate toward you as well? Yes, it accelerates just as much as I do because the forces are equal. Yes, it accelerates less than I do because the forces are equal. Yes, it accelerates more than I do because the forces are equal. No, it does not accelerate at all, the equal and opposite force exists elsewhere.

What is the reaction force to an action force of 1000N exerted by the earth on an orbiting communications satellite? A 1000N Surface Force on the ground A 1000N Tension Force on the Earth A 1000N gravitational force on the Earth

The bicycle The truck The same for each If a bicycle and a massive truck have a head-on collision, upon which vehicle is the impact force greater? The bicycle The truck The same for each

Which vehicle undergoes the greater change in motion? The bicycle The truck The same change in motion Acceleration…

No. I’m not exactly sure why. Since the force that acts on a bullet when a gun is fired is equal and opposite to the force that acts on the gun, does this imply a zero net force and therefore the impossibility of an accelerating bullet? Yes. No. I know exactly why. No. I’m not exactly sure why.

Estimate # of people on Earth. Estimate average mass of a person. If we got all people together in one place on the planet and made everyone jump, we can figure out with what acceleration the Earth will move. Estimate # of people on Earth. Estimate average mass of a person. Estimate the mass of the Earth. Determine what force each person could jump up with. Use that force to figure out the total force applied on the Earth.

Forces Review Concept Problems (CP)

CP #4 Constant 𝑣 No change in 𝑣 No acceleration ( 𝑎 = 0 𝑚 𝑠 2 ) Based on 𝐹 𝑛𝑒𝑡 = 𝑚 𝑎 , the net force must also be 0, so forces must be balanced.

CP #5 Net force is all the forces on JUST ONE OBJECT added together.

CP #6 25N 𝒂 15N 𝑵𝒐 𝒂 𝒂 25N 25N 25N 𝐹 𝑛𝑒𝑡 = −25𝑁 𝐹 𝑛𝑒𝑡 = −10𝑁 𝐹 𝑛𝑒𝑡 =0𝑁

𝑷𝒓𝒆𝒕𝒆𝒏𝒅 𝒄𝒂𝒓𝒕 𝒉𝒂𝒔 𝒏𝒐 𝒎𝒂𝒔𝒔. CP #7 𝑷𝒓𝒆𝒕𝒆𝒏𝒅 𝒄𝒂𝒓𝒕 𝒉𝒂𝒔 𝒏𝒐 𝒎𝒂𝒔𝒔. 𝒎 𝑭 = 𝑭 = 𝒎 𝒂 𝒂

CP #8 𝟐𝒌𝒈 𝟏𝒌𝒈 𝐹 𝐸𝑅 =−10𝑁 𝐹 𝐸𝑅 =−20𝑁

CP #13 𝒎 𝑪 𝑭 𝐄𝐂 = 𝒂 𝑪 𝐹 𝐸𝐶 𝐹 𝐸𝐵 𝒂 𝑩 𝑭 𝐄𝐁 = 𝒎 𝑩

CP #9 𝐹 𝑁𝐻 𝐹 𝐻𝑁

CP #10 𝐹 𝐻𝐹 𝐹 𝐹𝐻

CP #11 𝐹 𝐻𝑊 𝐹 𝑊𝐻

CP #12 𝐹 𝑆𝐻 𝐹 𝐻𝐸

CP #14 𝐹 𝐸𝐻 𝐹 𝐻𝐸

CP #15 𝐹 𝑆𝐻 𝐹 𝐻𝐸