Science High School Reviewer Physical Science: Forces and Motion

Motion

Motion is a change in an object’s position compared to a fixed object. If you ride in a car, your position changes compared to a tree or a telephone pole.

Distance is the length between two places. Using distance and direction can help you identify the position of something. An object’s position is its location compared to other things.

When you ride in a car, the trees and buildings appear to you to move backward. This movement is apparent motion, when things appear to an observer to be moving but are not actually changing position. You use apparent motion to determine what direction you are moving in and to find out how fast you are really moving.

Speed, velocity, and acceleration

When you describe how fast

something is moving, you are describing

its speed. Speed is how fast an object’s

position changes with time at any

given moment. Speed is calculated by

dividing the distance traveled by the

time taken to travel. A car with a

speed of 80 kilometers (48 miles) per

hour is going faster than a car traveling

at 70 kilometers (42 miles) per hour.

If you know both the speed of an

object and the direction in which it is

moving, then you know the object’s

velocity. Velocity is a description of a

moving object’s speed and direction.

When you know the velocity and

present position of an object, then you

should be able to predict where it will

be located after a certain amount of

time.

Acceleration

If an object travels in a straight

line at a steady speed, its velocity is

constant. Any change in the speed

or direction of an object causes its

velocity to change. Acceleration is

a change in the velocity of an object

over time. Like velocity, acceleration

also has both size and direction. If the

velocity of the moving object increases

with time, then the acceleration is in

the direction of the velocity. If the

velocity of the moving object decreases

with time, then the acceleration is in

the direction opposite the velocity.

Many people think of acceleration

as an object’s either speeding up or

slowing down. However, an object can

accelerate while maintaining the same

speed. For example, if a car moved at

a constant speed and turned a corner

without changing its speed, the change

in direction would be a change in the

velocity of the car. This means that the

car is actually accelerating.

People who ride on a merry-goround

also experience acceleration.

When the ride begins, the speed of the

merry-go-round increases from zero.

The riders undergo a steady change

in both speed and direction, which is

acceleration, as the ride moves. Once

the ride maintains a constant speed,

the riders continue to accelerate. This

is because they continuously change

direction, even though their speed is

constant. As the ride slows down, the

riders still experience acceleration as

the speed decreases and eventually

reaches zero once again.

Force

Any push or

pull on an object is a force. Forces can cause

a moving object to accelerate.

Forces can also change the shapes

of objects. When you mold clay,

your hands apply forces that squeeze

the clay into a different, permanent

shape.

There are different kinds of forces
that push and pull on objects. Buoyancy
is a force that pushes up on floating
objects in a fluid. Magnetic force attracts
or repels certain objects. Gravitational
force is an attraction that exists between
any two objects, such as the gravity that
pulls objects toward Earth.

The metric unit that measures force

is the newton. The force of gravity on a

1-kilogram object is about 10 newtons

(2.2 pounds). A newton produces an

acceleration of 1 meter per second

squared when applied to a mass of

1 kilogram. A spring scale is used to

measure newtons. The spring stretches

when force is applied to it.

Friction

As the book is pulled across a table,
a force works against this movement.
Friction is a force that opposes the
motion of an object. Friction occurs
when two or more objects come into
contact. In order to move the book
across a table, you must pull on it with
a force larger than the force of friction.

There are many types of friction.

For example, the force between the

surfaces of two solid objects which

keeps the objects from moving is called

static friction. Static friction keeps a

book from starting to move across a

table as a breeze blows through an open

window. Sliding friction is the force that

opposes the sliding of an object over a

surface. You feel sliding friction as you

move a book across a table. The force of

sliding friction is less than that of static

friction. Rolling friction is what opposes

the motion of a wheel turning along a

surface. The force of rolling friction is

less than that of sliding friction.

How do forces affect each other?

Net force is the sum of all

the forces that are acting on an object.

Balanced Forces

When the net forces are equal in
strength and opposite in direction,
they are balanced forces. The motion
of an object remains unchanged when
forces are balanced. It does not matter
whether the object is motionless or
moving at a constant velocity.

Unbalanced Forces

Forces of unequal strength or forces
that are not opposite in direction
are called unbalanced forces.

Inertia

Newton’s Laws of Motion

Newton’s first law of motion states

that an object at rest tends to stay at

rest and that an object in motion will

remain in motion. In other words, an

object moving in a straight line at a

constant speed tends to keep moving

that way. According to this law, the way

to change an object’s velocity is to

apply an unbalanced net force to it.

inertia, which

is the tendency of an object to keep

moving at the same speed and in the

same direction.

These trials demonstrate Newton’s second law of motion: acceleration depends on the object’s mass and the amount of net force applied to it. Newton’ssecond law can be written as a formula: a = F ÷ m.

An object’s acceleration (a) equals the

net force on the object (F) divided by

its mass (m). If the force increases,

then the acceleration also increases.

However, if the mass increases, then

the acceleration decreases. Newton’s

first law shows that a net force is

needed in order for an object to

accelerate. Newton’s second law

shows how much acceleration this

net force will cause.

Momentum

The combination of the mass and
the speed of an object is momentum.
A baseball has more momentum than
a tennis ball traveling at the same
speed because the baseball has more
mass. A tennis ball can have more
momentum than a baseball, if the
tennis ball’s speed is great enough.
Momentum is useful for studying
the motion of colliding objects. Total
momentum does not change when
objects collide. Scientists call this
principle conservation of momentum.

Each
of these situations involves action-reaction
forces. The downward force
a diver puts on a board is an action
force. The upward push that propels
the diver into the air is a reaction force.
Action-reaction forces are described
in Newton’s third law of motion: for
every action force, there is an equal and
opposite reaction force.

Newton’s Third Law of Motion

Each
of these situations involves action-reaction
forces. The downward force
a diver puts on a board is an action
force. The upward push that propels
the diver into the air is a reaction force.
Action-reaction forces are described
in Newton’s third law of motion: for
every action force, there is an equal and
opposite reaction force.

Mass, Weight, and Gravity

Mass and weight are different
properties. Mass is the amount of
matter in an object, and weight is the
force of gravity pulling down on an
object. Objects with more mass have
more weight. Although Earth and the
Moon both have gravity, Earth exerts a
far greater gravitational force than the
Moon, because Earth has much more
mass. The distance between objects
also affects the force of gravity. As the
distance between objects increases, the
force of gravity decreases.

Newton accurately concluded that
gravity can occur everywhere, not just
on Earth. He summarized this idea in
Newton’s law of universal gravitation.
According to this law, the planets, the
stars, and all particles of matter exert
gravitational force.

Work and Energy

Scientists define work as what is necessary for a force to move an object through a distance.

When is work done?

Calculating Work

Work is equal to the force of a push
or pull multiplied by the distance the
object is moved. The force must act
in the same direction as the motion. If
the force is expressed in newtons and
the distance is expressed in meters, the
units for the work done are newtonmeters
(Nm), also called joules (J).

How does energy change form?

If work is to be accomplished,
energy is needed. Energy is the ability
to do work. Like work, energy is
measured in joules. Stretched rubber
bands or wound-up springs store
energy because of their elasticity. A
rock on the edge of a cliff stores energy
because of its position and the force
of gravity.

Potential energy is energy
that is stored. However, not all energy
remains stored. A moving object also
has energy, because it has mass and
speed. The energy of motion is called
kinetic energy. Objects with greater
masses or higher speeds have greater
kinetic energy. All energy is either
potential or kinetic.

Energy transformations

Energy often changes from one
form to the other. When a roller coaster
first moves over the top of a
hill, it travels slowly.

Most of its energy is potential energy.
A roller coaster speeds up as it starts
down the hill. At the bottom, the roller
coaster is at its highest speed. Most
of the potential energy is changed to
kinetic energy. However, this change
in energy is not the end of the ride.

Thermal energy is the heat energy
in an object. Friction between the track
and the roller-coaster cars changes
some of the total energy to heat. This
thermal energy can warm the track,
but it cannot run the roller coaster.
Because some energy is wasted, the
roller coaster would not be able to
reach the top of the next hill if it were
as high as the first one. This explains
why the first hill is always the tallest.

Conservation of Energy

Energy does change from one
form to another, but energy cannot
be created or destroyed. Scientists call
this the law of conservation of energy.

Forms of Energy
Form Example of Source
nuclear the Sun, radioactive material
chemical food
electrical a generator, a battery
light the Sun, an electric lamp
mechanical moving parts in a machine
sound vibrations of a stereo speaker
thermal hot water in a radiator

What is power?

The amount of work
done per unit of time, or work divided
by time, is called power. The elevator
moves you to the top floor much faster,
so the elevator has more power than
you do.

If work is expressed in joules and
time is expressed in seconds, then
power is expressed in joules per second
(J/s). One joule per second is also
known as a watt (W). The watt is the
standard unit of power.

Calculating power

Suppose you moved a wagon 5
meters. The task required 400 joules of
work and took 20 seconds. What is the
power for this task? You can calculate
power by using the following formula:
power = work ÷ time
power = 400 J ÷ 20 s
power = 20 J/s = 20 W

Light-bulb labels indicate the power
the device uses per second. The more
power a light bulb has, the brighter the
light bulb is. Household bulbs has about
100 watts or less of power. Kilowatts
(kW) measure large amounts of power.
The prefix kilo- means “one thousand.”
A kilowatt is 1,000 watts of power.
Horsepower (hp) is a measurement
of power as well. More than 200 years
ago, James Watt invented this unit to
compare the power of a machine to the
work done by a horse. A horsepower
equals 746 watts. The watt as a unit of
power is named in honor of James Watt.