Honors Physics - Chapter 3: Lesson 1

Chapter 3: Lesson 1

Acceleration

Chapter 3: Lesson 1 - Acceleration

Review:

Vectors: Remember vectors show magnitude (size) and direction. We can represent vectors with arrows. Position, velocity, and acceleration are all vector quantitites.

Motion Diagrams: Remember from Chapter 2, Lesson 4, a motion diagram is a series of images showing the positions of a moving object at equal time intervals. It is an easy way for physicists to quickly draw what is happening. Notice on the bottom picture they have translated the stick figure drawings to dots. We can see in the bottom left picture that the person is speeding up because the dots are getting father apart. The bottom right picture shows the person slowing down because the dots are getting closer together.

Changing Velocity:

You can feel a difference between uniform and nonuniform motion. When you move in nonuniform motion, you feel pushed or pulled. In contrast, when you are in uniform motion and your eyes are closed, you feel as though you are not moving at all.

Have you ever fallen asleep in the car? When the driver is driving at a constant speed, you may not even feel like you are moving (uniform motion.) If the driver slams on the brakes, you would feel that and wake up (nonuniform motion!)

Acceleration:

Acceleration is the rate at which the velocity is changing. Remember from chapter 2, that a rate is a quantity devided by time.

Here is the equation for average acceleration. Remember that the triangle means "change in." It is a math symbol for an english word. It tells you that you have to take the final quantity minus the initial quantity.

We abbreviate acceleration with a lowercase a. The unit for acceleration is m/s2.

A uniform, or constant acceleration is when the acceleration does not change in time.

Why is the unit m/s2? Look at the graphic above. Acceleration is the change in velocity divided by the time interval. Change is velocity is d/t. Changing all of those abbreviations to units, we have m/s in the numerator and s in the denominator. If we then do some simple algebra, we have m in the numerator and two seconds in the denominator, s2!

Something with an acceleartion of 1 m/s2 means that its speed changes by 1 m/s during every 1 second interval.

Look at the .gif above. The blue arrow shows the velocity and the green arrow respresents the acceleration of the ball as it rolls down the ramp.

Notice when the ball is rolling down the first part of the ramp. The velocity arrow gets larger as it picks up speed, but the length of the accleration arrow stays the same. Your velocity can increase and the acceleartion can stay the same. This just means that the rate that the velocity is increasing by each second is the same.

When the ball reaches the first and the second flat parts, the acceleration vector switches direction and the velocity vector gets smaller. It is telling us that the object is slowing down.

Look at the curved part of the track. We will study circular motion in chapter 6. Notice that the acceleration always points towards the center of the circle and the velocity is always perpendicular to the acceleration vector as it goes around the circle.

Check Your Understanding:

On the three questions below, think about the question, then click on the down arrow when you have your answer to check to see if you are correct. Use the animation to answer question number three.

If an object begins at rest and experiences an acceleration of 2.5 m/s2 at the end of the first second, it will be traveling at 2.5 m/s.

At the end of the second second, its velocity would be _____ m/s.

5.0 - since another second has passed, you take the speed after the first second, 2.5 m/s and then add an additional 2.5 m/s.

2. At the end of the third second, the object will be traveling at _____ m/s.

7.5 - since another second has passed, you take the speed after the second second, 5.0 m/s and then add an additional 2.5 m/s.

3. Find the car(s) that is/are accelerating?

All of the cars are accelerating! They all started from rest, meaning that their velocity is zero. When you are driving and at a stop light, you have to move your foot from the brake to the accelerator (gas pedal) in order to get the car to move. You are CHANGING your speed from zero in order to move. You are accelerating.

You have finished Lesson 1!

Be sure to head over to google classroom and fill out the exit pass.

Report abuse