12/12Newton’s 1 st law/inertia In each of the following situations, determine of the object will be at rest, speeding up, slowing down or going at a constant.

12/12 Newton’s 1st law/inertiaIn each of the following situations, determine of the object will be at rest, speeding up, slowing down or going at a constant speed. Copy diagrams.

12/02 Newton’s 1st Law/Inertia

• IQ: You have the following 3 forces:5N, 10N, 15N and a toy car. Using all three forces show:a. 2 situations where the forces are unbalanced.

Show net force and dir. Of motion of the car.b. 1 situation where the forces are balanced.Show net force and dir. of motion.

An object at rest willremain at rest, and an object in motion willremain at a constantvelocity (straight line, constant speed) unless acted on by an unbalanced outside force.

Also called law of Inertia• Inertia– tendency to keep moving or stay at rest.• A moving object doesn’t want to stop

moving• A nonmoving object doesn’t want to start

moving• Objects won’t start or stop unless

FORCED to• The Seatbelt Law

• Inertia & Mass– Mass is the amount of matter in

an object– The more MASS an object has,

the more INERTIA the object has.– Bigger objects are harder to start

& stop

1st Law = Inertia• An object’s resistance to ANY change in

motion (LAZINESS!!)• Mass is a measure of inertia–> mass = > inertia–< mass = < inertia**easier to stop a bike than a car**easier to move a mouse than an elephantFirst law in NFL

Common Example of First Law

• People commonly encounter the 1st law while sitting in a fast moving vehicle that comes to a sudden stop.

1st Law

• An outside force (the collision) stop the vehicle, but passengers continue to move long after the vehicle is stopped.

1st Law

• Why were the passengers thrown forward?

EXAMPLES

Newton's Laws of MotionNewton’s First Law: A body continues at rest or in motion in a straight line unless acted on by some force.  

Q: Why do we not observe this usually?

Q: Can you think of other examples (like the astronaut) of times when we do?

What about the ladder on top of the truck?

The ladder is in motion because the truck is in motion.

When the truck stops, the ladder stays in motion because of inertia.

The truck is stopped by the force of the car, but the ladder is not.

What force stops the ladder?

Gravity and friction– the two unbalanced forces that slow down and stop most objects on earth.

Inertia and gravity animation study jams

Friction & the 1st Law

A moving object wants to keep moving. The only reason it doesn’t on earth is because of friction—the force that causes objects to slow

down or stop.

• NASA video

OQ: What would this cat’s motion look like if there was no gravity or friction? Use the terms inertia, velocity, and force in your answer.

12/11 Newton’s 2nd Law

• IQ: Complete the following based on Newton’s first law:

A pushed cart will __________ in a _________at a ____________, unless an __________ such as _______acts on it.

12/3 Newton’s 2nd Law of MotionIQ: Explain how inertia is involved in the following cartoon:

Force = mass X acceleration

F = Maa = F/MM= F/a

2nd Law of Motion• Acceleration depends on the mass of the

object & the force applied to the object.• To cause a higher acc. you need a bigger

force.• To cause the same acc. on a higher mass

you need a bigger force• AKA: LAW OF ACCELERATION

VS.

Newton's Laws of Motion

Newton’s Second Law:               

F ma

• Force = mass X acceleration (F = ma)

INCREASE FORCE = INCREASE ACCELERATION

_____ F = ___A

DECREASE FORCE = DECREASE ACCELERATION

___F = ___A

Directly proportional

INCREASE MASS = DECREASE ACCELERATION

__ M = __ A

DECREASE MASS = INCREASE ACCELERATION

__ M = __ A

Inversely Proportional!

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

object comes from its mass and its acc.

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.

2nd Law• All falling objects have

the same acc. (due to gravity, 9.8 m/s2), ignoring air resistance.

• That means the one with more mass will have more force when it hits the ground.

• Which will have more force, the elephant or the hockey puck?

Second law in NFL

• Weight of an object (F)= mass(m) x acc. due to gravity (g)

• On Earth g= 9.8 m/s/s, rounded to 10 m/s/s

• Unit of weight: N• Unit of mass: kg• Unit of acc: m/s/s

P. 88- Do under IQ– If the acc. due to gravity were doubled

to 19.6 m/s/s, what would happen to your weight?

1) Calculate the force needed to accelerate a 5kg mass by 25 m/s/s.2)If a force of 100 N accelerates an object by 5 m/s/s, what is the object’s mass?3) How much will an object of mass 3 kg accelerate, given a force of 60 N?

• NASA videos

• OQ: Based on Newton’s 2nd law, what are 2 ways to make a bowling ball roll faster and farther?

12/5 Newton’s 3rd law

IQ: Which of the following is true when you punch a punching bag?a. The only force is applied by you to the bag.b. The bag applies a small force back on you.c. The bag applies an equal force back on you. d. The bag applies a bigger force back on you.

For every action,there is an equaland oppositereaction.

3rd Law of Motion• Every action force has an equal & opposite

reaction force• Action/reaction forces don’t work on same object,

so don’t cancel out. – If a/r are =, then net force would be 0 = no

motion, or nothing to make something stop!– Example: Walking, we push back on ground,

ground pushes us forward.– SWIMMING:

oAction force = hand pushing on waterEQUALS

oReaction force = water pushing on hand

3rd Law

• Forces come in pairs – action and reaction• If body A exerts a force on body B, the body B

exerts an equal and opposite force on body A

Equal but Opposite

• Action and reaction forces will be equal and opposite, the same force acting on a greater mass results in a smaller acceleration.

Newton's Laws of Motion

Newton’s Third Law: To every action, there is an equal and opposite reaction.

Person pushes back on skateboard to get off, skateboard moves back.

Newton’s 3rd Law Example

Another Example

• The engine pushesgases down.

• The rocket goes up.Third law NFLThird law NASASimpsons

• OQ: Based on the 3rd law draw action identify the action/reaction forces in the following examples:

Ex. Hitting your knee against the tableAction: knee hitting table, table hitting back knee with same forcea. An egg hitting the ground. b. Wheels of a moving cart.