9.1 Describing Acceleration An object travelling with uniform motion has equal displacements in equal time intervals. 1.8m 1.8m 1.8m 1.8m (  d) 0s 1s 2s 3s 4s (  t) Not all objects exhibit uniform motion. It is important to be able to analyze situations where the motion is not uniform. This is what we will be studying in Chapter 9. As she slides, the velocity of the baseball player is continually changing, therefore her motion is non-uniform.

9.1 Describing Acceleration An object travelling with non-uniform motion will:  have different displacements during equal time intervals 1.8m 2.6m 1.3m 0.7m (  d) 0s 1s 2s 3s 4s (  t)  take different amounts of time to travel equal displacements 1.3m 1.3m 1.3m 1.3m (  d) 0s 1.2s 2.3s 3.1s 3.9s (  t)  have a continuously changing velocity

Positive and Negative Changes in Velocity A change in velocity ( ) occurs when the speed and/or direction of motion of an object changes. A change in velocity can be calculated by:  If the change in velocity is the same sign (+, -) as the initial velocity, the speed of the object is increasing.  If the change in velocity is the opposite sign of the initial velocity, the speed of the object is decreasing.  If the change in velocity is zero, the object is travelling with uniform motion. If forward is designated positive, this dragster’s change in velocity is positive. If forward is designated positive, this landing shuttle has a negative change in velocity.

ASSIGNMENT 1.Complete the activity on the last page of your notes package. 2.When finished, read the following pages:  P.380  P.383  P Complete the Reading Check on p.384.

Acceleration Acceleration ( a ) is the rate of change in velocity.  This change in velocity can be due to a change in: SPEED and/or DIRECTION. In this case, the runners are changing speed (not direction) as they cross the finish line, so their velocity is changing and they are accelerating. In this case, the cars of the ferris wheel are changing direction (not speed) as they move, so their velocity is changing and they are accelerating.

Acceleration Two objects with the same change in velocity can have different accelerations.  This is because acceleration describes the rate at which the change in velocity occurs. Suppose both of these vehicles, starting from rest, speed up to 60 km/h. They will have the same change in velocity, but, since the dragster can get to 60 km/h faster than the old car, the dragster will have a greater acceleration.

Positive and Negative Acceleration Acceleration is a vector and needs a direction. The direction of the acceleration is the same as the direction of the change in velocity. Predicting the sign of the acceleration: 1.A positive (+) acceleration will occur if an object is speeding up in the positive direction. Example: This car is speeding up. If we designate the forward direction as positive (+), then the change in velocity is positive (+). Therefore the acceleration is positive (+).

Positive and Negative Acceleration Predicting the sign of the acceleration: 2.A negative (-) acceleration will occur if an object is slowing down in the positive direction. Example: NOTE:  The acceleration experienced by an object slowing down is sometimes called deceleration. This car is slowing down. If we designate the forward direction as positive (+), then the change in velocity is negative (-). Therefore the acceleration is negative (-).

Predicting the sign of the acceleration: 3.A negative (-) acceleration will occur if the object is speeding up in the negative direction. Example: A car backs up. It starts at 1 m/s [backward], then speeds up to 4 m/s [backward]. Step 1: Let’s designate the backward direction as negative (-). Step 2: Now let’s write the velocities with the correct signs: Step 3: Now we can calculate the change in velocity using the formula:  Since the change in velocity is negative (-), the acceleration is also negative (-). Positive and Negative Acceleration

Predicting the sign of the acceleration: 4.A positive (+) acceleration will occur if the object is slowing down in the negative direction. Example: A car is backing up, but slowing down. Step 1: Let’s designate the backward direction as negative (-). Step 2: Now let’s write the velocities with the correct signs: Step 3: Now we can calculate the change in velocity using the formula:  Since the change in velocity is positive (+), the acceleration is also positive(+). Positive and Negative Acceleration

SUMMARY OF THE SIGN OF THE ACCELERATION: A positive acceleration means one of these:  Object is speeding up in the positive direction  Object is slowing down in the negative direction A negative acceleration means one of these:  Object is speeding up in the negative direction  Object is slowing down in the positive direction * Recall: The positive direction is generally assigned to mean up, right, forwards, north, and east.

Assignment: Homework: 1.Reading Check p Workbook 9.1 (p )