Chapter 8Motion and Forces

MotionI. Motion- a change in position

against a stationary backgrounda. Speed

1. Describes how fast an object moves2. Calculating speed : distance traveled /

time it took to travel that distance 3. Calculated speed is always an average or

instantaneous speed.4. Measured in m/s, or km/hr, or mi/hr, etc.

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Constant Speed Refers to objects that cover equal

amounts of distance in equal amounts of time.

Ex. A race car that has a constant speed of 96 m/s travels at a distance of 96m every second

Distance – Time Graphs

1. Time always goes on the x-axis

2. Distance is always on the y-axis

3. Examples……

Moving fast Moving slow

At rest

Velocity Same as speed but includes direction

-Ex. 45 mi/hr south

Does not have to be a compass direction• Ex. 23 m/s towards third base

Calculated the same way as speed

Momentumo Momentum

1. Depends on an objects velocity and its mass

2. P = momentum3. P= mass x velocity• Momentum Equation

   The SI unit for mass is kg. The SI unit for velocity is m/s.

The SI unit for momentum is kg x m/s.

500 kg •m/s east

2000 kg • m/s west

1. Law of conservation of momentum : total momentum of two cars before a collision equals the momentum after the collision

PRACTICE PROBLEMS :  

1.  Calculate the momentum of a 0.15 kg ball that is moving toward home plate at a velocity of 40m/s.

2.  Which has greater momentum, a 2.0kg hockey puck moving east at 2.5m/s or a 1.3kg hockey puck moving south at 3.0m/s?

3.  A track athlete throws a 2kg discus into a field with a velocity of 21m/s.  What is the momentum of the discus?

4.  Calculate the momentum of a 700g ball that is rolling down a ramp at 4.6m/s.

5.  A cannon fires a 40.5kg shell toward a target and the shell moves with a velocity of 120m/s.  Calculate the shell's momentum.

Feeling pretty good about your abilities?  

Acceleration Acceleration : A change in an

object’s velocity. There are 2 ways an object can

accelerate…1. Change its speed2. Change its direction

Equation: a = final velocity –initial

velocity time

Acceleration Practice ProblemsPRACTICE PROBLEMS:1.   As a shuttle bus comes to a normal stop, it slows from 9.00m/s to 0.00m/s in 5.00s.  Find the average acceleration of the bus.     2.    During a race, a sprinter increases from 5.0 m/s to 7.5 m/s over a period of 1.25s.  What is the sprinter’s average acceleration during this period?

3.   A baby sitter pushing a stroller starts from rest and accelerates at a rate of  0.500m/s2.  What is the velocity of the stroller after it has traveled for 4.75 minutes?   

• Practice Problems: Solving for TimeSolving for Time:

Marisa’s car accelerates at an average rate of 2.6m/s2.  Calculate how long it takes her car to accelerate from 24.6m/s to 26.8m/s.

If a rocket undergoes a constant total acceleration of 6.25m/s2, so that its speed increases from rest to about 750m/s, how long will it take for the rocket to reach 750m/s.  

A dog runs with an initial speed of 1.5m/s on a waxed floor.  It slides to a stop with an acceleration of -0.35m/s2.  How long does it take for the dog to come to a stop?

 

Velocity – Time Graphs

Constant Velocity Speeding up

Slowing down