Download reading – motion and forces review – innovation lab

READING – MOTION AND FORCES REVIEW – INNOVATION LAB
Motion - Speed, Direction & Position
Position, direction, and speed are some of the important factors that describe an object's motion.
Some important factors when describing an object's motion include:

Position is the direction and distance the object is from a point of reference.

Direction is the straight-line path from a reference point to the object's position. Direction is very closely related to
position.

Motion is the change in an object's position over time relative to a reference point. An object in motion always
changes its position and may change its direction.
Position & Reference Points
An object's position must be given relative to something
else. The place from which its position is measured is
called a reference point. For example, look at the scene
of the koala, rabbit, tree, and thylacine to the right.
The rabbit's position cannot be given as "1.7 meters."
However, the rabbit could be described as "1.7 meters
from the tree." In that case, the reference point is the
tree. The rabbit's position in that case would be 1.7
meters to the right of the tree. If the koala is used as the
reference point, the rabbit could be "two meters from
the koala." Adding direction (such as down and to the right) allows the rabbit's position to be given exactly.
Speed
An object's speed is the distance it travels divided by the time it takes to travel that distance. Objects can move at
different speeds. Faster objects move farther than slower objects if both objects move for the same amount of time.
For example, if a car traveled 200 kilometers in 2 hours, the car's speed would be
Direction
Direction is the path along which something is moving. Direction can be described using words, such as
 up or down

left or right

north, south, east, or west
Velocity
Velocity describes both the speed of an object and the direction in which it is traveling. The speed of the car shown
above might be 100 km/h, but to describe its velocity, you'd need to know the direction in which the car is
traveling. For example, the car's velocity could be 100 km/h due west.
When an object is traveling at constant velocity, this means it is traveling with a constant speed and direction. A
change in object's velocity can be caused by a change in its speed, a change in its direction, or both.
Frame of Reference
A frame of reference is an object or point from which movement is determined.
There are many ways in which you can think about the frame of reference. You can think of it as a
starting point like a point or place on a map (e.g. your house), the origin on a graph, or a standard like the equator
(shown below).
Image courtesy of NASA/Goddard Space Flight Center Scientific Visualization Studio
You can also think of the frame of reference as your location or position. For example,
imagine that you are at the train station.
If you are standing in the terminal next to a moving train, the terminal is your frame of
reference because you are standing in the terminal. In this case, you perceive the train
to be moving while the terminal is staying still. If you are standing on the train,
however, your frame of reference is the train, and the terminal appears to be moving
while the train is staying still.
Distance vs. Time Graphs
Motion is a key characteristic of all matter that can be observed, described, measured, and graphed.
As an object moves, its position changes through time. Since there is a relationship between position and time, it can
be shown on a graph. A straight line on a distance vs time graph represents on object moving at a constant speed. If a
graph shows a bend or curve, then the object's motion has changed. The motion of an object can only be changed by
forces.
Showing Motion on Graphs
Plotting motion on graphs can show many things. In the graphs below, distance from the reference point increases as
you move up the vertical axis.Time increases as you move to the right along the horizontal axis.
Graph I shows an object moving away from a reference point at a constant speed. The change in distance is the same
for each increment of time—1 mile every 6 minutes.
Graph II shows an object moving at the same constant speed as shown in Graph I, but the object is moving TOWARD
the reference point (distance is decreasing with time).
Graph III shows the same constant speed as in Graph I, but at the 18-minute mark, the object stops abruptly. As time
continues, the distance from the reference point does not change because the object is motionless.
Graph IV shows an object moving away from a reference point at a constant speed before immediately moving back
toward the reference point at the same speed.
Force & Motion
The motion of an object can be changed by an unbalanced force. The way that the movement changes depends on the
strength of the force pushing or pulling and the mass of the object. Forces have both a direction and a magnitude (size).
Force & Mass
A force is a push or a pull that acts on an object and may change its
motion. Forces are needed to start, stop, or change the direction of an
object's motion. The more force that is applied to an object, the greater
the change in the object's motion. In other words, the harder an object is
pushed, the faster it will move.
But, if the same amount of force is used to move two objects with
different masses, the object with less mass will move faster. For instance,
if the woman in the picture below pushed two different lawn mowers
using the same amount of force, the lawn mower with less mass would move faster.
Friction
Friction is a force that opposes motion. If an object is already
moving, friction can slow it down or make it stop. For example,
when a box is slid across the ground, it will only travel a certain
distance before it stops. Friction is the force that brings it to a
stop. Objects on rough surfaces require more force to
move.Objects on smooth surfaces require less force.
Gravity
Gravity is the force of attraction that exists between any two objects that have
mass. Gravity keeps the planets orbiting around the Sun, and it pulls objects on Earth
toward the Earth's center. For example, gravity causes fruit on a tree to fall to the
ground. Gravity depends on the mass of the objects and the distance between them. The
gravity we feel from Earth is very strong because we are close to the Earth and the mass
of the Earth is very large.
Net Force
A net force is the total unbalanced force acting on an object. A net force has a certain strength, or magnitude, and a
direction. If the net force is not zero, then it will cause the object to speed up, slow down or change direction.
Balanced forces have equal magnitudes (sizes) but opposite directions.
When the forces on an object are balanced, the object does not accelerate because the net force is zero. When this
happens, the object can either be moving at a constant velocity or at rest. The apple on the desk below is pulled
downward by the force of gravity. This force equals the weight of the apple.The table simultaneously exerts
a normal force that is equal in magnitude but opposite in direction pushing up on the apple. Since these forces are
balanced, the apple does not accelerate in either direction. And since the apple was not in motion to begin with, it
remains at rest.
Unbalanced forces have unequal magnitudes and/or directions. Unbalanced forces result in net forces that are not
zero. An unbalanced force will change an object's speed and/or direction.
When forces are unbalanced, the change in movement will take place in the direction of the net force.
Newton's 1st Law of Motion
Newton's first law of motion is also known as the law of inertia. It states that an object in motion stays in motion and an
object at rest remains at rest unless acted on by an unbalanced outside force.
Newton’s first law of motion states that an object’s motion will not
change unless an unbalanced force acts on the object. If the object is
at rest, it will stay at rest. If the object is in motion, it will stay in
motion and its velocity will remain the same. In other words, neither
the direction nor the speed of the object will change as long as the
net force acting on it is zero.
Look at the pool balls in Figure to the right. When a pool player
pushes the pool stick against the white ball, the white ball is set into
motion. Once the white ball is rolling, it rolls all the way across the
table and stops moving only after it crashes into the cluster of colored
balls. Then, the force of the collision starts the colored balls moving.
Some may roll until they bounce off the raised sides of the table.
Some may fall down into the holes at the edges of the table. None of these motions will occur, however, unless that
initial push of the pool stick is applied. As long as the net force on the balls is zero, they will remain at rest.
Pool balls remain at rest until an unbalanced force is applied to them. After they are in motion, they stay in motion until
another force opposes their motion.
Inertia
Newton’s first law of motion is also called the law of inertia. Inertia is the tendency of an object to resist a change in its
motion. If an object is already at rest, inertia will keep it at rest. If the object is already moving, inertia will keep it
moving.
Think about what happens when you are riding in a car that stops suddenly. Your body moves forward on the seat. Why?
The brakes stop the car but not your body, so your body keeps moving forward because of inertia. That’s why it’s
important to always wear a seat belt. Inertia also explains the amusement park ride in Figure above. The car keeps
changing direction, but the riders keep moving in the same direction as before. They slide to the opposite side of the car
as a result.
Inertia and Mass
The inertia of an object depends on its mass. Objects with
greater mass also have greater inertia. Think how hard it
would be to push a big box full of books, like the one
in Figure below. Then think how easy it would be to push the
box if it was empty. The full box is harder to move because it
has greater mass and therefore greater inertia.
The tendency of an object to resist a change in its motion
depends on its mass. Which box has greater inertia?
Overcoming Inertia
To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object. Until the
soccer player kicks the ball in Figure below, the ball remains motionless on the ground. However, when the ball is kicked,
the force on it is suddenly unbalanced. The ball starts moving across the field because its inertia has been overcome.
Force must be applied to overcome the inertia of a soccer ball at rest.
Once objects start moving, inertia keeps them moving without any
additional force being applied. In fact, they won’t stop moving unless
another unbalanced force opposes their motion. What if the rolling soccer
ball is not kicked by another player or stopped by a fence or other object?
Will it just keep rolling forever? It would if another unbalanced force did
not oppose its motion. Friction — in this case rolling friction with the
ground — will oppose the motion of the rolling soccer ball. As a result, the ball will eventually come to rest. Friction
opposes the motion of all moving objects, so, like the soccer ball, all moving objects eventually come to a stop even if no
other forces oppose their motion.
Acceleration, Force, and Mass
Newton determined that two factors affect the acceleration of an object: the net force acting on the object and the
object’s mass. The relationships between these two factors and motion make up Newton’s second law of motion. This
law states that the acceleration of an object equals the net force acting on the object divided by the object’s mass. This
can be represented by the equation:
Force = mass x acceleration
Direct and Inverse Relationships
Newton’s second law shows that there is a direct relationship between force and acceleration. The greater the force that
is applied to an object of a given mass, the more the object will accelerate. For example, doubling the force on the
object doubles its acceleration. The relationship between mass and acceleration, on the other hand, is an inverse
relationship. The greater the mass of an object, the less it will accelerate when a given force is applied. For example,
doubling the mass of an object results in only half as much acceleration for the same amount of force.
Consider the example of a batter, like the boy in Figure below. The harder he hits the ball, the greater will be its
acceleration. It will travel faster and farther if he hits it with more force. What if the batter hits a baseball and a softball
with the same amount of force? The softball will accelerate less than the baseball because the softball has greater mass.
As a result, it won’t travel as fast or as far as the baseball.
Hitting a baseball with greater force gives it greater acceleration. Hitting a
softball with the same amount of force results in less acceleration. Can you
explain why?
This empty trunk has a mass of 10
kilograms. The weights also have a mass
of 10 kilograms. If the weights are
placed in the trunk, what will be its
mass? How will this affect its
acceleration?
Acceleration and Weight
Newton’s second law of motion explains the weight of objects. Weight is a measure of the force of gravity pulling on an
object of a given mass. It’s the force (F) in the acceleration equation that was introduced above:
Force =mass × acceleration
The acceleration due to gravity of an object equals 9.8 m/s2, so if you know the mass of an object, you can calculate its
weight as:
weight = mass × 9.8 m/s2
As this equation shows, weight is directly related to mass. As an object’s mass increases, so does its weight. For example,
if mass doubles, weight doubles as well. You can learn more about weight and acceleration at this
Helpful Hints
The equation for calculating weight (F=m×a) works only when the correct units of measurement are used.

Mass must be in kilograms (kg).

Acceleration must be in m/s2.

Weight (F) is expressed in kg·m/s2 or in newtons (N).
Action and Reaction
Newton’s third law of motion states that every action has an equal and opposite reaction. This means that forces
always act in pairs. First an action occurs, such as the skateboarders pushing together. Then a reaction occurs that is
equal in strength to the action but in the opposite direction. In the case of the skateboarders, they move apart, and the
distance they move depends on how hard they first pushed together. You can see other examples of actions and
reactions in Figure below.
Each example shown here includes an action and reaction.