Section 2.2 Acceleration and Newton’s Second Law

Section 2.2Section 2.2Acceleration and Newton’s Acceleration and Newton’s

Second LawSecond Law

ReviewReviewWhat was Newton’s First Law of What was Newton’s First Law of Motion?Motion?

What is Inertia?What is Inertia?

ReviewReviewWhat was Newton’s First Law of What was Newton’s First Law of Motion?Motion?An object at rest will stay at rest and an An object at rest will stay at rest and an object in motion will continue in motion with object in motion will continue in motion with the same speed and direction UNLESS acted the same speed and direction UNLESS acted on by a force.on by a force.What is Inertia?What is Inertia?The property of an object that resists The property of an object that resists changes in its motion.changes in its motion.

The First LawThe First LawNewton said that a force is required to Newton said that a force is required to change the motion of an object, but change the motion of an object, but how does a force change an objects how does a force change an objects motion?motion?

Answer: AccelerationAnswer: Acceleration

AccelerationAccelerationAcceleration is the rate at which speed Acceleration is the rate at which speed increases.increases.

It is the It is the changechange in an objects speed in an objects speed (velocity) over time.(velocity) over time.

Time to ThinkTime to ThinkCan you do it???Can you do it???

Which has a greater acceleration?Which has a greater acceleration?A car that accelerates from 0 mph A car that accelerates from 0 mph

to 40 to 40 mph in 10 smph in 10 sOrOr

A car that accelerates from 0 mph A car that accelerates from 0 mph to 60 to 60 mph in 10 smph in 10 s

Time to ThinkTime to ThinkWhich skier has a greater acceleration?Which skier has a greater acceleration?Skier on the bunny hill Skier on the bunny hill

OrOrSkier on a double black diamondSkier on a double black diamond

WHY?WHY?

We Can Calculate We Can Calculate AccelerationAcceleration

Remember, acceleration is the change in Remember, acceleration is the change in speed over timespeed over time

SI units of acceleration are m/sSI units of acceleration are m/s22

Try it!Try it!1.1. If a car accelerates from 0 m/s to 40 If a car accelerates from 0 m/s to 40

m/s in 10 s, what is its acceleration?m/s in 10 s, what is its acceleration?

Try it!Try it!1.1. If a car accelerates from 0 m/s to 40 If a car accelerates from 0 m/s to 40

m/s in 10 s, what is its acceleration?m/s in 10 s, what is its acceleration?

a = a = vv = = 40m/s40m/s = = 40m40m x x 11 = 4m/s = 4m/s22

t 10s s st 10s s s

What about this one?What about this one?A car is traveling 40 m/s and A car is traveling 40 m/s and accelerates to 60 m/s in 10 s, what is accelerates to 60 m/s in 10 s, what is its acceleration?its acceleration?

REMEMBER: Acceleration is REMEMBER: Acceleration is determined determined by the by the changechange in in velocity velocity over timeover time

How to Calculate itHow to Calculate it

Let’s do it!Let’s do it!A car is traveling 40 m/s and A car is traveling 40 m/s and accelerates to 60 m/s in 10 s, what is accelerates to 60 m/s in 10 s, what is its acceleration?its acceleration?

a = a = vv22 – v – v11 = = 60m/s – 40m/s60m/s – 40m/s = = 20m/s20m/s =2m/s=2m/s22

t 10s 10st 10s 10s

You try itYou try itWebster is running to his car after Webster is running to his car after school. His speed is 7m/s, but he gets school. His speed is 7m/s, but he gets tired quickly and slows down to 4m/s tired quickly and slows down to 4m/s over 3s. What is Webster’s over 3s. What is Webster’s acceleration?acceleration?

You try itYou try itWebster is running to his car after Webster is running to his car after school. His speed is 7m/s, but he gets school. His speed is 7m/s, but he gets tired quickly and slows down to 4m/s tired quickly and slows down to 4m/s over 3s. What is Webster’s over 3s. What is Webster’s acceleration?acceleration?

a = a = vv22 – v – v11 = = 4m/s – 7 m/s4m/s – 7 m/s = = -3m/s-3m/s = - = -1m/s1m/s22

t 3s 3st 3s 3s

What’s the dilly with the What’s the dilly with the negative?negative?

A negative acceleration means an A negative acceleration means an object slows down; positive means it object slows down; positive means it speeds up.speeds up.

Positive acceleration = accelerationPositive acceleration = accelerationNegative acceleration = decelerationNegative acceleration = deceleration

Back to NewtonBack to NewtonHe discovered a relationship between He discovered a relationship between the force applied on an object, the the force applied on an object, the mass of the object, and the resulting mass of the object, and the resulting acceleration.acceleration.

It was called:It was called:Newton's Second Law of Motion Newton's Second Law of Motion

(probably because it was his second law? IDK)(probably because it was his second law? IDK)

Newton’s Second LawNewton’s Second Law

To use Newton’s second law properly, keep To use Newton’s second law properly, keep the following important ideas in mind. They the following important ideas in mind. They are a good guideline for how to apply the are a good guideline for how to apply the second law to physics problems.second law to physics problems.

1. The 1. The net net force is what causes acceleration.force is what causes acceleration.2. If there is 2. If there is no no acceleration, the net force acceleration, the net force

must must be be zero.zero.3. If there 3. If there is is acceleration, there acceleration, there must must also be also be

a net a net force.force.4. The force unit of newtons is based on 4. The force unit of newtons is based on

kilograms, kilograms, meters, and seconds.meters, and seconds.

Ways to write the 2Ways to write the 2ndnd Law Law►a = F/ma = F/m►m = F/am = F/a►F = maF = ma

TestTest1. What is the acceleration of a 1,500-1. What is the acceleration of a 1,500-kilogram car if a net force of 1,000 N is kilogram car if a net force of 1,000 N is exerted on it?exerted on it?

TestTest1. What is the acceleration of a 1,500-1. What is the acceleration of a 1,500-kilogram car if a net force of 1,000 N is kilogram car if a net force of 1,000 N is exerted on it? exerted on it?

Answer: Answer: 1.5 m/sec21.5 m/sec2

TestTest2. As you coast down the hill on your 2. As you coast down the hill on your bicycle, you accelerate at 0.5 m/sec2. bicycle, you accelerate at 0.5 m/sec2. If the total mass of your body and the If the total mass of your body and the bicycle is 80 kg, with what force is bicycle is 80 kg, with what force is gravity pulling you down the hill?gravity pulling you down the hill?

TestTest2. As you coast down the hill on your 2. As you coast down the hill on your bicycle, you accelerate at 0.5 m/sec2. bicycle, you accelerate at 0.5 m/sec2. If the total mass of your body and the If the total mass of your body and the bicycle is 80 kg, with what force is bicycle is 80 kg, with what force is gravity pulling you down the hill? gravity pulling you down the hill?

Answer: Answer: 40 kg·m/sec2 or 40 N40 kg·m/sec2 or 40 N

TestTest3. You push a grocery car with a force 3. You push a grocery car with a force of 30 N and it accelerates at 2 m/sec2. of 30 N and it accelerates at 2 m/sec2. What is its mass?What is its mass?

TestTest3. You push a grocery car with a force 3. You push a grocery car with a force of 30 N and it accelerates at 2 m/sec2. of 30 N and it accelerates at 2 m/sec2. What is its mass? What is its mass?

Answer: Answer: 15 kg15 kg

VocabularyVocabulary►AccelerationAcceleration►DecelerationDeceleration►Newton’s Second LawNewton’s Second Law

2.2 Section Review2.2 Section Review1. List three units in which 1. List three units in which acceleration can be measured.acceleration can be measured.

2. According to Newton’s second law, 2. According to Newton’s second law, what causes acceleration? What what causes acceleration? What resists acceleration?resists acceleration?

2.2 Section Review2.2 Section Review3. An 8,000 kg helicopter’s speed 3. An 8,000 kg helicopter’s speed increases from 5 m/sec to 25 m/sec in increases from 5 m/sec to 25 m/sec in 5 seconds. Calculate its acceleration 5 seconds. Calculate its acceleration and the net force acting on it.and the net force acting on it.

4. Define the term “net force.”4. Define the term “net force.”