Ch. 18 Motion and Momentum

• What is motion?• Acceleration• Momentum

What is Motion…– A change in position has occurred.

• We visually see it move• or relative motion is observed

– Motion that has occurred relative to a reference point– Eyes closed, but position changed

• Other senses detect motion– Smell, hear, taste, touch.

Distance vs. Displacement

Distance– Length of a route of movement

Displacement– Distance and direction between the starting point

and the ending point– “As the crow flys”

What is the distance traveled and what is the displacement?

2.5 meters

10 meters

5 meters

Speed

The measurement of motion.Average Speed– The total distance traveled divided by the total time it

takes to travel that distance– SI units (standard international) = meters/second

Speed Calculations

s = d/t– A poodle runs beside a car for a distance of 30

meters. It takes the poodle 6 seconds to catch the tire and bite a hole in it. How fast was the car going?

• s = d/t• s = 30 meters / 6 seconds• s = 5 m/s

PracticeYour school bus travels a total of 20 miles on your trip to school. It takes you 1 hour to get to school. What is your average speed?Use the three steps in calculating a physics problem.Don’t forget the units.1. s = d/t2. s= 20 mi. / 1 hr.3. s = 20 mi/hr

Instantaneous Speed

The speed of an object at one instant in time.– Radar gun catching a speeding motorist

Graphing Motion

Distance– On the vertical (Y) axis

Time– On the horizontal (X) axis

Which student is fastest?

• Student A traveled faster than student B.

Distance-Time Graphs and Speed

What is Motion?What is Motion?

11

• The line representing the motion of student A is steeper than the line for student B.

• Student A traveled the same distance in less time

HomeworkGraph the turtle’s motion if.– Walks 1 mi/hr for the 1st hour– Drinks water for the 2nd hour– Walks 2 mi/hr for the 3rd hour

Distance (mi)

Time (hr.)

Velocity

The speed of an object and the direction of travel– The car was going 100 mph. Speed or Velocity?

• Speed– The car was going 100 mph north toward Altoona.

• Velocity

Velocity changes if…..– Speed changes– Direction changes

Velocity

What is the velocity in ft/s of an eagle that dives from 150 ft. in the air to catch a trout from the stream. It takes the eagle 5.0 seconds to catch lunch.V = d/t same as speed only w/ directionV = 150 ft./ 5.0 sec.V = 30 ft/sec toward the stream

Speed CalculationsWhat is the average speed a salmon swims if it travels 400 miles in 200 hours?

What is the avg. velocity of an Eastern Box Turtle that travels from the bottom of the mountain to the top, 550 yards, in a time of 11 days?

What is the avg. speed of a runaway bride that just realized her soon to be husband is ugly. She travels 3000 miles in 200 hours?

More practice.

Calculate the average speed of a football that is thrown 50 meters in 5.0 seconds.What is the average speed of Mr. Dutchcot on his way to lunch and back. He travels 600 feet in 120 seconds. What is his displacement when he returns to his room?Calculate the average velocity of an arrow that travels 200 meters in a time of 2.5 seconds toward a dragon.

Acceleration

What is acceleration?– The change in velocity divided by the time it takes to

change the velocity.

What is a change in velocity?– Change in speed (+ or -)

• Or– Change in direction.

Calculating Acceleration

a = (final velocity - initial velocity) time

a = ( vf-vi ) t

A rider on a four wheeler at the races goes from 0 m/s to 60 m/s in a time of 6.0 sec. What is the a of the racer?

– a = (vf – vi) t- a = 60 m/s - 0 m/s 6.0 sec- a = 10.0 m/s2 to the finish line

- The four wheeler speeds up 10 m/s every second.

Negative example.

A biker in the tour de ‘toona is racing toward the finish line. Suddenly Mrs. Hann rushes onto the course to save a stranded caterpillar. The biker must reduce his speed from 50 ft. / sec. to 5 ft. / sec. to avoid our favorite office lady. He only has 5.0 seconds to slow down. What is his a ?

Lookout Mrs. Hann!

a = velocity final – velocity initial time

a = (vf – vi

tVf

– 5 ft./sec.Vi

– 50 ft./sec.a = 5 f/s – 50 f/s

5.0 sa = - 9.0 ft. / sec. 2

When do objects accelerate?

Positively when a force is applied.Negatively when a force in the opposite direction is applied or when a force is no longer applied.Slip ‘n Slide Video– Positive – sliding down the hill– Negative – up the ramp– Negative – in the air till he reaches the peak– Positive – from the peak of the flight to the water– Negative – hitting the water

Football thrown?

Acceleration Graph

Change in S

peed (m/s)

Time (sec)

Slip and Slide….

Change in S

peed (m/s)

Time (sec)

Acceleration Lab PracticeCalculate the skateboarders acceleration.

• Distance between the lines is 2 meters.• It takes him 5 seconds to pass through the first lines.• It takes him 1 seconds to pass through the last lines.• His entire trip takes a time of 8 seconds. What is his a?

Acc. Lab part 23 m between each Sf and Si2.0 seconds for Si and 5.0 seconds for SfIt takes 5.0 sec. from Si to Sf

Si Sf

Homework

What is the a of a sprinter that accelerates from 0 m/s to 24 m/s in a time of 4.0 seconds?

An arrow leaves prince charming’s bow string traveling 300 feet/second and slows to a speed of 20 feet/second as it travels through the apple on top of the princess’s head. It takes the arrow 7.0 seconds to make the journey. What is a ?

Momentum

• Measure of how hard it is to stop a moving object. Watch the big kid at running back!Mass– The amount of matter that is in an object– SI units = Kg kilogram

Weight– Related to an object’s mass.– Pull of gravity downward.

Inertia

The tendency of an object to resist a change in its motion. The more mass an object has, the greater its inertia.– Resist movement

• Resist speeding up• Resist slowing down• Resist turning

Calculating Momentum

A measure of how hard it is to stop an object.– p = mass · velocity (p= m · v)

Depends on the object’s mass and velocity.– The greater the mass the harder to stop.– The greater the velocity the harder to stop.

Units– Mass · velocity– kg · m/s

Momentum problem.

What is the momentum of a 10 kg poodle traveling at 15 m/s north after a mailman.– p = m · v– p = 10 kg · 15 m/s– p = 150 kg· m/s to the mailman

Which would you rather …..

– STOP (momentum)• A compact car going 1 mile/hour or a large truck going 1

mile/hour. Why?– KICK (inertia)

• A bowling ball or a basketball. Why?– CATCH (momentum)

• A fastball thrown by a Jr. High pitcher or a fastball thrown by A.J. Burnett?

Calculate the momentum of…

…….a .5 kg arrow that travels toward a monster 10 point at 100 meters/second.

Law of Conservation of Momentum

The total momentum of objects that collide is the same after the collision as it is before the collision.Collision with objects sticking together. – STICKY p = m1v1 + m2v2

The same poodle latches on to the running mailman. What is their total p?

Poodle + Mailman

Poodle has a mass of 10 kg and is traveling 20 m/s.Mailman has a mass of 100kg and is traveling 15 m/s. p = m1v1 + m2v2

p = 10 kg (20 m/s) + 100 kg (15 m/s)p = 1700 kg m/s toward the bushes

Collisions when objects bounce apart.

When a collision occurs and the two objects bounce apart a new form of the equation is used.BOUNCEY p m1v1 = m2v2

Momentum of 1 = Momentum of 2 How much momentum will the bowling ball have left over? It has a mass of 25 kg and a velocity of 10 m/s.. The pins have an inertia of 200 kg m/s.

m1v1 = m2v2

25 kg(10 m/s) = 200 kg m/s250 kg m/s = 200 kg m/s50 kg m/s extra momentum

Classwork

What velocity does an 0.25 kg arrow have that hits the target with 500 kg m/s of momentum?What is the mass of a bullet that is traveling at 1000 m/s and knocks over a target that has 2000 kg m/s of inertia?What is the total momentum of a horse and Zorro when he jumps from a roof to the moving horse. The mass of the horse is 500 kg and its velocity is 10 m/s. The mass of Zorro is 150 kg and his velocity is 5 m/s?What is the final velocity of a 6 kg baseball hit from a tee? The mass of the bat is 25 kg and the velocity of the swing is 40 m/s.