Physics Chapter 3

"She moves at a constant speed in a constant direction." Rephrase the same sentence in fewer words.

"She moves at a constant velocity."

Ignoring wind resistance, how far does a ball, dropped from rest, travel after 3 seconds?

(45 m) The final velocity is 30 m/s, but the distance is not 30 m. The average velocity is (1/2)gt = 15 m/s, so the distance it travels is 15 m/s × 3 s = 45 m.

A person throws a ball upward with an initial speed of 15 m/s, and then a second (identical ball) with an initial speed of 30 m/s. How much higher up is the maximum height of the second ball, compared to that of the first ball?

(The second ball's maximum height is more than twice as high.) It takes twice as long for the second ball to reach its maximum height, but remember that it also has a higher average velocity. In fact, it gets four times higher than the first ball.

The steeper the slope of the incline

, the greater the acceleration of the ball.

Distance traveled=

1/2(acceleration × time × time)

Vector quantity

A quantity that has both magnitude and direction.

Scalar quantity

A quantity that has only a magnitude, not a direction.

If a car moves with an average speed of 60 km/h for an hour, it will travel a distance of 60 km. A. How far would it travel if it moved at this rate for 4 h? B. For 10 h?

A. Distance = 60 km/h × 4 h = 240 km B. Distance = 60 km/h × 10 h = 600 km

Suppose we are driving and, in 1 second, we steadily increase our velocity from 30 kilometers per hour to 35 kilometers per hour, and then to 40 kilometers per hour in the next second, to 45 in the next second, and so on. We change our velocity by 5 kilometers per hour each second. This change in velocity is what we mean by acceleration.

Acceleration = change of velocity/time interval = 5 km/h/1s = 5 km/h⋅s

acceleration due to gravity (g)

Acceleration of a freely falling object. Its value near Earth's surface is about 9.8 m/s each second.

Velocity

An object's speed and direction of motion.

In addition to the speedometer on the dashboard of every car is an odometer, which records the distance traveled. If the initial reading is set at zero at the beginning of a trip and the reading is 40 km one-half hour later, what has been your average speed?

Average speed= total distance covered/ time interval= 40km/0.5h= 80km/h

The speedometer of a car moving to the east reads 100 km/h. The car passes another car that is moving to the west at 100 km/h. Do both cars have the same speed? Do they have the same velocity?

Both cars have the same speed, but they have opposite velocities because they are moving in opposite directions.

Which undergoes greater acceleration: an airplane that goes from 1000 km/h to 1005 km/h in 10 seconds or a skateboard that goes from zero to 5 km/h in 1 second?

Both gain 5 km/h, but the skateboard does so in one-tenth the time. (The skateboard therefore has the greater acceleration—in fact, ten times greater. A little figuring will show that the acceleration of the airplane is 0.5 km/h·s, whereas the acceleration of the slower-moving skateboard is 5 km/h·s. Velocity and acceleration are very different concepts. Distinguishing between them is very important.)

Speed

How fast an object moves; the distance traveled per unit of time. speed= distance/time

Free fall

Motion under the influence of gravity only.

Would it be possible to attain this average speed and never go faster than 80 km/h?

No, not if the trip starts from rest. There are times in which the instantaneous speeds are less than 80 km/h, so the driver must drive at speeds higher than 80 km/h during one or more time intervals in order to average 80 km/h. In practice, average speeds are usually much lower than highest instantaneous speeds.

linear motion

Straight-line motion, as opposed to circular, angular, or rotational motion.

How does the acceleration of a ball rolling down an inclined plane with an angle of incline of 30° compare to that of a ball rolling down an inclined plane with an angle of incline of 15°?

The acceleration for the 30° ramp is higher. Since the incline is steeper, the acceleration is higher.

A particular car can go from rest to 90 km/h in 10 s. What is its acceleration?

The acceleration is 9 km/h·s. Strictly speaking, this would be its average acceleration because there may have been some variation in its rate of picking up speed.

During a certain period of time, the speedometer of a car reads a constant 60 km/h. Does this indicate a constant speed? A constant velocity?

The constant speedometer reading indicates a constant speed but not a constant velocity, because the car may not be moving along a straightline path, in which case it is accelerating.

Acceleration

The rate at which velocity changes with time; the change in velocity may be in magnitude (speed), or direction, or both. Physics Flashcards Acceleration = change of velocity/ time interval

Instantaneous speed

The speed at any instant.

Average speed

The total distance traveled divided by the time of travel. Average speed =total distance covered/time interval

What is the acceleration of a race car that whizzes past you at a constant velocity of 400 km/h?

Zero, because its velocity doesn't change.

What is the average speed of a cheetah that sprints 100 meters in 4 seconds? If it sprints 50 m in 2 s?

average speed= distance covered/time interval= 100 meters/ 4 seconds 50 meters/2 seconds=25 m/s

CG

center of gravity

The relationship between time up or down and vertical height is given by

d = 1/2gt^2

rate

how fast something happens or how much something changes per unit of time; a change in a quantity divided by the time it takes for the change to occur.

As a ball rolls down an inclined plane (say with an angle of incline of 30°), the ball's acceleration

is constant in time (Since the inclination of the plane doesn't change with position, the component of gravity pulling the ball down along the incline doesn't change, so the acceleration is constant.)

When a ball thrown upward reaches its highest point, its acceleration

is directed downwards. (Although the velocity is zero, the acceleration is still directed downwards. The acceleration due to gravity is always directed downward (with a magnitude of 9.8 m/s2), regardless of the object's velocity.)

A person drops a ball from rest, and the ball accelerates downward at a rate of -9.8 m/s2. If the person throws the ball upward, after the person lets go of the ball, its acceleration

is still -9.8 m/s2. (The acceleration due to gravity is always -9.8 m/s/s, regardless of the ball's velocity.)

When a ball thrown upward reaches its highest point, its velocity

is zero. (Since the ball has reached its highest point, it can't be moving upwards anymore.)

Ignoring wind resistance, how fast is a ball moving three seconds after being dropped from rest?

roughly 30 m/s (The velocity of a free-falling object is given by acceleration due to gravity times the time interval. So for this case v = 10 m/s2 × 3 s = 30 m/s.)

Speed is a

scalar quantity

If we know the vertical height d, we can rearrange this expression to read

t = √2d/g

Velocity is a

vector quantity

When you sit on a chair

your speed is zero relative to Earth but 30 km/s relative to the Sun.