Chapter 8 Motion and Forces

Motion- a change in position against a
stationary background
a. Speed
1. Describes how fast an object moves
2. 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.
 Refers
to objects that cover equal
amounts of distance in equal amounts of
 Ex.
A race car that has a constant speed
of 96 m/s travels at a distance of 96m
every second
Time always goes on the x-axis
Distance is always on the y-axis
Moving fast
Moving slow
At rest
 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
1. Depends on an objects velocity and its mass
2. P = momentum
3. 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.
2000 kg • m/s
500 kg •m/s
1. Law of conservation of momentum : total
momentum of two cars before a collision
equals the momentum after the collision
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 : A change in an object’s
There are 2 ways an object can
1. Change its speed
2. Change its direction
a = final velocity –initial velocity
Acceleration Practice 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 Time
Solving 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?
Constant Velocity
Speeding up
Slowing down

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