What is Force?2.2 p. 32-38

Mr. Richter

Agenda

Warm-Up

Intro to Acceleration (to the hallway!)

What is Acceleration?

Notes: Definition of Acceleration Units of Acceleration (What Acceleration really is) Calculating Acceleration

Review HW Problems

Graphs of Acceleration (time Permitting)

Agenda Day 2 and 3

Tomorrow

What is Force?

Newton’s Second Law (N2L)

Force and energy.

Tuesday

Acceleration Lab

Objectives: We Will Be Able To…

Define and calculate acceleration.

Explain the relationship between force, mass, and acceleration.

Determine mass, acceleration or force given two of the quantities.

Warm-Up:

To the right is a Velocity vs. Time (VvT) graph of an object that starts at the origin.

1. Describe the motion of the object in one complete sentence.

2. Sketch the Position vs. Time graph you think describes the object

Speed

Time

Let’s go to the Hallway!

Warm Up

Velocity vs. Time Position vs. Time

Speed

Time Time

Posi

tion

Acceleration

What is Acceleration?

What comes to mind when I say the word acceleration?

Discuss at your tables for 1 minute and then we will discuss as a class.

Definition of Acceleration

Your book: Acceleration is the rate at which your speed increases.

Real definition: Acceleration is the rate of change of an object’s motion.

What’s the difference? Both have indicate that a change in speed is acceleration. The real definition implies that a change of direction is

also a form of acceleration, even if speed does not change.

Units of Acceleration

Acceleration is the rate of change of speed relative to time, so the calculation looks something like:

This means the units of acceleration are any unit of speed divided by the unit of time it takes to change that speed.

Can we think of some possible units of acceleration?

Units of Acceleration

The most common unit of acceleration we will use in this class is m/s2 (“meters per second squared”)

If acceleration is the rate of change in velocity [m/s] relative to time [s]…

The units for acceleration are:

But we really mean “meters per second per second”, or a change in meters per second (velocity) every second (time). Therefore, 50 m/s2 really means a 50 m/s change in velocity

every second.

Units of Acceleration: Practice30 km/h/sec

An increase of

30 km/h after 1 second

60 km/h after 2 seconds

90 km/h after 3 seconds

300 km/h after 10 seconds

Put it in a complete sentence.

15 m/s2 (= 15 m/s/s)

An increase of

15 m/s after 1 second

30 m/s after 2 seconds

45 m/s after 3 seconds

150 m/s after 10 seconds

Put it in a complete sentence.

Acceleration Graphics (p. 32)

Calculating Acceleration

Calculating Acceleration

Speed is how fast position changes, or the change in position divided by time.

Acceleration is how fast speed changes, therefore:

What if acceleration is negative? Slowing down as opposed to speeding up. Sometimes called deceleration.

Let’s Review Your Homework Problems

Remember to use the 5-step approach.

1. Looking for?

2. Given.

3. Formula

4. Solve

5. Check

Homework

p. 55 Section 2.2 #3-6

What is Force?

Force

Acceleration is the change in an object’s motion.

To change an object’s motion, a force is needed.

A force is any push or pull, or any action that has the ability to change an object’s motion. It can either change the speed of an object. Or change an object’s direction.

Force, Mass and Acceleration

Reminder: mass is the amount of matter (atoms) in an object.

The amount of force required to accelerate an object depends on: the amount of mass an object has the rate of acceleration the object reaches

The is the basis of Newton’s Second Law

Newton’s Second Law (N2L)

Isaac Newton discovered that the amount of force required to move an object is proportional to the amount of mass the object has, and the amount of acceleration of the object.

In other words:

F = ma force = mass*acceleration

Your book:

Newtons: Units of Force

Force = mass*acceleration = [kg]*[m/s2]

We’re too lazy to write this, so we use Newtons to abbreviate.

1 Newton [N]= 1 kg*m/s2

Newtons are very small units of force.

Mr. Richter has a mass of about 95 kg. It takes 95 N of force for Mr. Richter to accelerate at 1 m/s2

Mass resists acceleration Force cause acceleration

Net Force

The net force is another way of saying the sum of all of the forces acting on an object.

If the forces are in the same direction, they are added together. If they are in opposite directions, they are subtracted.

Net Force and Acceleration

If there is a net force acting on an object ,there is also an acceleration.

Similarly, if there is no net force acting on an object, then there is no acceleration. NOTE: this is not the same as no speed. An object can still be moving even if it is not acceleration.

Constant speed means no acceleration which means no net force.

Calculations with N2L: Practice

A car has a mass of 1,000 kg. If a net force of 2,000 N is exerted on the car, what is its acceleration?

2 m/s2

Force and Energy

Force is the action through which energy moves. Or:

Forces are created every time there is a difference in energy between a high-energy situation and a low-energy situation.

For an object to move from a high energy situation to a low energy situation, there must be a force acting on the object. Think, what force moves a car from the

top of a hill to the bottom?

Wrap-Up: Did we meet our objectives?