Physics 670 Speed, Velocity, and Acceleration Notes

Name: ______Section:_____Date:____/____/____

670

Notes: Acceleration

Review:

Speed:

• Speed is the rate at which an object changes position (moves)

• Speed tells us how ______an object is moving.

• Symbol: s

• Example: Sally runs at a speed of 5 m/s.

Velocity:

• Velocity is similar to speed

• Velocity is ______in a given ______

• Symbol: v

• Example: A car moves with a velocity of 20 m/s North.

• To change velocity, you could

1. ______

1. ______

1. ______

Force:

• A force is a ______or a ______.

• When all of the forces on an object are ______, the object is in equilibrium. Its motion remains the ______.

• When the forces on an object are ______, the motion of the object ______.

New:

Acceleration:

• The rate of change in velocity is known as acceleration.
• Basically, if your velocity is changing, you are accelerating.
• Symbol: a
• Example: A car accelerates from 20 m/s to 25 m/s in 5 seconds.
• To accelerate, you could

1. ______

1. ______

1. ______

Speeding Up:

• What do people mean when they say “speed up?”

• Examples of things which are speeding up:
• a car speeds up when it leaves a stop sign
• a car speeds up when the traffic light turns green
• a basketball speeds up when it falls
• a runner speeds up at the beginning of a race
• a child speeds up when she is needs to get away from another child during a game of tag
• a plane speeds up when it is getting ready to take off
• a hockey puck which was at rest speeds up when it is hit with a stick
• a soccer ball speeds up when it rolls down a hill
• a football speeds up while someone is throwing it (while it is still in the person’s hand)
• a roller coaster speeds up when it is going down a hill

• LOOK through a magazine and find a picture of something that is speeding up.
• What did you find a picture of?

Slowing Down:

• What do people mean when they say “slow down?”

• Examples of things which are slowing down:
• A car slows down when it approaches a stop sign.
• A football slows down when it is moving upward.
• A runner slows down at the end of a race.
• A diver slows down when he enters the water.
• A baseball slows down when someone catches it.
• A golf ball slows down as it rolls across the grass.
• A hockey player slows down when she collides with another player.

• LOOK through a magazine and find a picture of something that is slowing down.
• What did you find a picture of?

Change Direction:

• What do people mean when they say “change direction?”

• Examples of things which are changing direction:
• A car turning a corner.
• A football moving in a curved path.
• A baseball player running around the bases
• A basketball player who isn’t moving in a straight line.
• A runner running around a curved track.
• A baseball that is being hit by a bat.
• A boxer’s fist when throwing a left hook.
• A tennis ball when it being hit by a racket.

• LOOK through a magazine and find a picture of something that is changing direction.
• What did you find a picture of?

What causes acceleration?

• Imagine the soccer ball was sitting on the floor, and you wanted it to move across the floor.

• Is causing the ball to start to move an acceleration? ______

Explain:

• What would you do to make the ball start moving?

• If you push (or pull) something, you are applying a ______to it.

• Forces can make things ______.

Acceleration Units:

• The units for acceleration look kind of weird, but they aren’t that bad once you get to know them.

• Typical Units: [meters/second/second] or [m/s/s] or [m/s2]

• The number part of acceleration tells you how much the ______changes every second.

• For example, an acceleration of 5 m/s/s tells you that the speed is increasing by 5 m/s every second that the object is accelerating.

Using Accelerations:

• Fill in the following chart for an object which is speeding up with an acceleration of 2 m/s/s. Assume the object starts from rest.

• Fill in the following chart for an object that is speeding up with an acceleration of 5 m/s/s. Assume the object starts from rest.

• Fill in the following chart for an object that is speeding up with an acceleration of 10 m/s/s. Assume the object starts from rest.

• Fill in the following chart for an object that is speeding up with an acceleration of 10 m/s/s. Assume the object starts with a speed of 10 m/s.

• Fill in the following chart for an object that is slowing down with an acceleration of 5 m/s/s, until it stops. Assume the object starts with a speed of 30 m/s.

How long does it take the object to stop? ______seconds

• The following chart shows speed information for each second. Answer the questions that follow:

1. What is the object’s initial speed? ______

1. What was the object’s speed after 1 second? ______

1. How much did the speed change in the first second? ______

1. How much did the speed change between 1 and 2 s? ______

1. How much did the speed change between 9 and 10s? ______

1. What is the object’s acceleration? ______

Is acceleration a vector or a scalar?

• Remember: Direction matters for vector quantities and doesn’t matter for scalars.

• Acceleration is a vector, so direction matters for acceleration.

• Forces cause accelerations. To determine the direction of the acceleration, figure out what direction the total force is acting (the direction of the total force and the direction of the acceleration are the same).

• For example, the box is at rest, so the man applies a force to start it moving:

He is pushing the box to the right (he is applying a force to the right)

The box accelerates to the right

The box moves to the right.

We could use labeled arrows to show the direction of the acceleration and the direction of motion (THIS IS NOT A FREE-BODY DIAGRAM)

• Do the direction of motion and the direction of acceleration always have to be the same?
• Imagine a bicycle (which is moving to the right) needs to stop:

1. What is the direction of motion? ______

1. In what direction would you push the bike to make it stop? ______

1. What is the direction of the bike’s acceleration as it stops? ______

1. Draw and label 2 arrows on the bike: one showing the direction of motion and the other showing the direction of acceleration.

For you to try:

1. A cat who is standing still starts to move forward to play with a toy. Draw and label 2 arrows: one showing the direction of motion and the other showing the direction of acceleration.

1. Upon reaching the toy, the cat stops. Draw and label 2 arrows: one showing the direction of motion and the other showing the direction of acceleration.

1. The stopped cat then starts moving backwards from a stop. Draw and label 2 arrows: one showing the direction of motion and the other showing the direction of acceleration.

1. The cat backs up at a constant speed in a straight line. Draw and label 2 arrows: one showing the direction of motion and the other showing the direction of acceleration.

1. If the cat was moving backwards at a constant speed of 4 m/s and had an acceleration of -1 m/s2 [= -1 per second]:
2. How fast would the cat be going after 1 second?______

1. How fast would the cat be going after 2 seconds? ______

1. As you watch the cat speed up forwards (from rest), she has a speed of 2 m/s after 1 second. Two seconds later, she is moving at 4 m/s. Three seconds later, she is moving at 6 m/s.
2. What is the cat’s acceleration? ______

1. If the cat kept accelerating at this rate, how fast would she be going after 6 seconds?

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