Section 2.2 Acceleration and Newton’s Second Law

Review What was Newton’s First Law of Motion? What is Inertia?

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

The First Law Newton said that a force is required to change the motion of an object, but how does a force change an objects motion? Answer: Acceleration

Acceleration Acceleration is the rate at which speed increases. It is the change in an objects speed (velocity) over time.

Time to Think Can you do it??? Which has a greater acceleration? A car that accelerates from 0 mph to 40 mph in 10 s Or A car that accelerates from 0 mph to 60 mph in 10 s

Time to Think Which skier has a greater acceleration? Skier on the bunny hill Or Skier on a double black diamond WHY?

We Can Calculate Acceleration Remember, acceleration is the change in speed over time SI units of acceleration are m/s 2

Try it! 1. If a car accelerates from 0 m/s to 40 m/s in 10 s, what is its acceleration?

Try it! 1. If a car accelerates from 0 m/s to 40 m/s in 10 s, what is its acceleration? a = v = 40m/s = 40m x 1 = 4m/s 2 t 10s s s t 10s s s

What about this one? A car is traveling 40 m/s and accelerates to 60 m/s in 10 s, what is its acceleration? REMEMBER: Acceleration is determined by the change in velocity over time

How to Calculate it

Let’s do it! A car is traveling 40 m/s and accelerates to 60 m/s in 10 s, what is its acceleration? a = v 2 – v 1 = 60m/s – 40m/s = 20m/s =2m/s 2 t 10s 10s t 10s 10s

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

You try it Webster is running to his car after school. His speed is 7m/s, but he gets tired quickly and slows down to 4m/s over 3s. What is Webster’s acceleration? a = v 2 – v 1 = 4m/s – 7 m/s = -3m/s = -1m/s 2 t 3s 3s t 3s 3s

What’s the dilly with the negative? A negative acceleration means an object slows down; positive means it speeds up. Positive acceleration = acceleration Negative acceleration = deceleration

Back to Newton He discovered a relationship between the force applied on an object, the mass of the object, and the resulting acceleration. It was called: Newton's Second Law of Motion (probably because it was his second law? IDK)

Newton’s Second Law

To use Newton’s second law properly, keep the following important ideas in mind. They are a good guideline for how to apply the second law to physics problems. 1. The net force is what causes acceleration. 2. If there is no acceleration, the net force must be zero. 3. If there is acceleration, there must also be a net force. 4. The force unit of newtons is based on kilograms, meters, and seconds.

Ways to write the 2 nd Law ► a = F/m ► m = F/a ► F = ma

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

Test Answer: 1.5 m/sec2

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

Test Answer: 40 kg·m/sec2 or 40 N

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

Test Answer: 15 kg

Vocabulary ► Acceleration ► Deceleration ► Newton’s Second Law

2.2 Section Review 1. List three units in which acceleration can be measured. 2. According to Newton’s second law, what causes acceleration? What resists acceleration?

2.2 Section Review 3. An 8,000 kg helicopter’s speed increases from 5 m/sec to 25 m/sec in 5 seconds. Calculate its acceleration and the net force acting on it. 4. Define the term “net force.”