Force and Motion
Distance
and
Displacement
Distance
is the measurement from one point to another
8Th Grade
Reference Point
A stationary point of comparison
used to measure the distance an object has moved.
Motion
Displacement problems
Example: Suppose you walked
200 meters
East
to the park from your house, what would be your
displacement?
S8P3 Students will investigate relationships between force, mass and the motion of objects.
A. Determine the relationship between speed, velocity and acceleration.
B. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.
Examples:
The Earth rotates on it axis ~1,038 mi/hr (NASA.org)
The Earth revolves around the sun ~18.6 mi/hr
One billion billion air molecules collide with 1 inch of our skin per second
Everything in the universe is moving!
nasa.org
How do we know an object is moving?
Motion
is defined as an object having a change in position.
Relative Motion

An objects change in position relative to the reference point
" is just a way people will say different things about the same motion of the object"
Demonstration!
Displacement
: is the direction and the distance in an objects position from its starting and ending points.
Distance may refer to a physical length, a period of time, or an estimation.
Example 2: Suppose Dale Jr. drives the Daytona 500.
Leaves from the start/finish
line,
completes the race and crosses
the same start/ finish line.
What would be his
displacement?
Speed
: distance an object travels in a unit of time
During a typical trip to school, your car will undergo a series of changes in its speed.
Average speed
: divide the total distance by the total time taken to complete a change in an object position.
Instantaneous speed
: Speed of an object at a given instant in time.
s=d/t
s= speed
d=distance
t=time
Used to compare the speeds of two or more objects.
Time is plotted on the Xaxis.
Speed or velocity is plotted on the Yaxis
A straight horizontal line on a speedtime graph means the speed is
constant.
This means the speed is
not changing
over time.
Distance vs. Time Graph
Velocity
is an object’s speed in a particular direction. If the direction changes, the velocity will also change.
Velocity always includes
speed
and
direction
.
VELOCITY
It changes position
How do we know if changes position?
If it moves relative to its reference point.
Average Speed
You will need a pencil and your cell phone
you may download a QR Reader if you do not have one.
ATTENTION!
v=d/t
"v" is the velocity
"d" is the distance
"t" is the time
Velocity always includes speed and direction.
Acceleration
Acceleration
is an increase in speed. It is positive acceleration.
Forms of acceleration:
objects start moving
objects move faster
objects move slower
objects stop moving.
objects change direction
ANY TIME YOU HAVE A CHANGE IN VELOCITY OR SPEED, YOU WILL HAVE CHANGE IN ACCELERATION.
Units
m/s^2
SOLVING EQUATIONS
Calculate the acceleration of a bus whose speed changes from 6 m/s to 12 m/s over period of 3 s.
How to solve the problem:
1) Write down what you know.
2) Write down what you need to know (the question) and the unit.
4) Draw a diagram or picture
3) Look for a formula that is relevant
4) Use a procedure
5)Check your answers! "plug and chug"
As
SpeedTime Graph
When the line
rises
,
the object is
speeding up.
When the
line falls,
the object is
slowing down.
Distance v. Time
The Distance v. Time graph is used to compare speed of objects.
Compare the
Student A
with
Student B
:
Student A
(average speed of during the 1st second)
Speed=distance/time
speed= 1 m/s
s= 1 m/s
Student B
speed= d/t
s= 0.5 m/ 1 s
=.5m/s
Which one is steeper?
*
A steeper line on the distancetime graph represents a greater speed.
A horizontal line on the distancetime graph means that no change in position occurs. In that case the speed of the object is 0
Unit = m/s^2
Collisions
Collision occurs when an objects collide with one another. Collision can change speed of an object or direction of an object or both.
Changes in motion of colliding objects depend on their masses and their velocities before the collision. The more massive an object is the harder it is to change its motion.
The tendency of an object to resist a change its motion is called inertia. Inertia is a scalar (based on mass: only changes of mass changes.
Inertia example:
MOMENTUM
Conservation of momentum
: the total momentum of a group of
objects remains constant unless outside forces act forces act on the group.
Only an outside force can change the momentum of the group of objects. Friction will cause billiard balls to slow down as they roll on the table, so the momentum decreases.
What is force? A push or a pull.
For example, gravity pulls on us in a downward direction.
** **
An object at rest stays at rest
and an object in motion
stays in motion
in the same straight line of direction forever
unless acted upon by an unbalanced force
****
FORCE!!
Force can cause objects to accelerate!!
Types of collision
Types of collision
Sometimes objects will bounce off or stick together
Using Momentum Conservation, it can be used to predict the velocity of objects after they collide.
we have t o assume that the total momentum of colliding objects doesn't change.
How to predict velocity using conservation of momentum (example problem)
Example: Imagine wearing skates and someone throws a backpack at you.
The Law of Conservation of Momentum can be used to find your velocity after you catch the backpack.
Finding momentum after collision: two step problem
Supposed a 2kg backpack initially has a velocity of 5 m/east. Your mass is 48 kg, and initially you are at rest.
Then the initial momentum can be found like this:
p=mv
total momentum= total momentum of backpack + your momentum
total momentum =2kg x m/s east + 48 kg x0 m/s
=10 kg m/s east
After the collision the total remains the same, and only one object is moving. Its mass is the sum of your mass and the mass of the backpack. you can use the equation to find the final velocity:
p= m x v
Total momentum=(mass of backpack +mass) x velocity
10 kg m/s east= (50kg ) x velocity
0.2 m/s east=velocity
WHAT HAPPENS WHEN MASSES COLLIDE?
**Important slide**
Small marble hits a more massive marble that is at rest: the larger marble moves in the same direction that the small marble was initially moving.
Large marble hits a small marble that is at rest, then both marbles move in the same direction. The less massive marble always moves faster than the more massive one.
If two objects of the same mass moving at the same speed collide headon, they will rebound and move with the same speed in the opposite direction. The total momentum is zero before and after collision.
*momentum:
A measure of how difficult it is to stop a moving object; equals the product of mass and velocity, (how much motion something has).
momentum (based on mass and velocity) : can increase or decrease by changing velocity.
p=mv
Momentum (in kg m/s) =mass (in kg) X velocity (in m/s) p=momentum, m is mass, and v is velocity
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