AnswerHigh positive acceleration becauseyou’re speeding up quickly.
Example:You’re driving down the highway when your car runs out of gas and you slowly roll to a stop on the side of the road.
AnswerLow negative acceleration becauseyou’re slowing down slowly.
Example:• You are at cruising speed on the highway. You no longer feel any pressure against the seat or are lurching forward.• What is your acceleration?
AnswerZero, because you are at constant velocityThat velocity can be fast, slow, or notmoving at all.
Challenge Question: You throw a basketball up in the air. It rises to a certain height and then falls back to earth. Describe its accelerationHint: There are multiple steps
AnswerFirst, high positive acceleration because it speeds upquickly as it leaves your hand.
AnswerThen, as it rises, low negative acceleration, because itbegins to slow down slowly because of gravity.
AnswerAt the top of the arch, it has zero acceleration, because itbriefly stops moving upward and has constant velocity.
AnswerAs it begins to fall again, low positive acceleration,because it speeds up slowly.
AnswerFinally, when you catch it, high negative accelerationbecause it slows down very quickly in your hand.
How Do I Graph the Motion of a Particle? 30 Car 1 Car 2 Car 3 25 20 15 10 5 0 0 1 2 3 4 5 6 7
30 Car 1 Car 2 Car 325201510 5 0 0 1 2 3 4 5 6 7 Car 1 is moving with a velocity of 1 m / sec There is zero acceleration
30 Car 1 Car 2 Car 32520151050 0 1 2 3 4 5 6 7 Car 2 has constant velocity throughout at 1 m /sec Its acceleration remains zero
30 Car 1 Car 2 Car 325201510 5 0 0 1 2 3 4 5 6 7 Car 3 has a changing velocity throughout, finishing at 7 m / sec Acceleration is 1 m / sec2
Quick Review• Velocity: speed of motion in a direction• Acceleration: change in velocity What causes an object’s velocity to change?
• If you’re riding your bike and want to speed up, what do you do?• If you’re driving a car and want to slow down, what do you do? • If you’re playing soccer and want to move the ball, what do you do?
Nothing will change speed unless acted upon by a force.If it’s sitting still, it will keepsitting still.If it’s moving fast, it willkeep moving fast.
Only a force can change the velocity of an object.• That force can act to accelerate it NSF North Mississippi GK-8
Only a force can change the velocity of an object.• That force can act to decelerate it
Then why do things slow down?• Flying balls slow down and eventually stop because of the force of gravity.• Cars and bikes eventually slow down and stop because of the force of friction.
Then why do things slow down?• In space with no friction and no gravity, objects will keep going at the same speed forever without a force to slow them down or speed them up.