Download Getting to Know: Speed, Velocity, and Acceleration

Getting to Know: Speed, Velocity, and Acceleration
Our lives are full of motion. Motion is required to move our bodies out of bed every morning,
and motion is needed to take us to school. Understanding motion requires knowledge of
several concepts. Once you understand these concepts, you’ll have a better understanding of
motion and be able to perform calculations to help you describe an object’s motion.
What is motion and how can we describe it?
Motion can be described as the change in position over time. We can describe motion with three different concepts: speed, velocity, and acceleration. You may already have an understanding of some of these concepts, but to make sure, we’ll start with a review of Newton’s Laws of Motion.
Misconception 1: Aren’t speed and velocity the same thing? I hear people use the
two terms in the same way.
Speed and velocity are not the same, although many people do use the terms
interchangeably. Remember that speed refers to the distance an object travels over
time. Velocity also describes the distance an object travels over time, but it also
describes the direction in which that object has traveled.
What are Newton’s Laws of Motion?
Sir Isaac Newton lived from 1643 to 1727. His investigation of forces in the natural world led him to develop his laws of motion. Newton’s three laws of motion help explain the ways objects in our world move. His first law states that objects at rest will stay at rest until a force acts upon that object. Similarly, an object in motion will stay in motion until a force, like friction, acts upon that object. This law is
commonly called the law of inertia.
Newton’s Second Law of Motion states that
objects will accelerate when a force acts upon
a mass. In this law, we learn that there is a
relationship between the mass of an object and
the amount of force required to move it. Most
people have experienced this law in simple
ways in everyday life. Think about pushing a
truck: because the truck has a lot of mass, it
requires a lot of force to move it. One person
may not be able to push the truck. It may take
the force from several people before the truck
is able to move forward. If you then attempt
to push a toy truck, you’ll find that it requires
much less force because it has much less mass.
A lot of force is required to push a large car
because it has so much mass.
Concept: Speed, Velocity, and Acceleration
Getting to Know
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The second law can be written using the following formula: F= m•a. We say this equation as,
“Force equals mass times acceleration.”
Newton’s Third Law of Motion states that for every action there is an equal and opposite
reaction. We won’t use this law much in this concept, but it’s still a good idea to have a basic
understanding of it. To illustrate this law, imagine that you are sitting on a skateboard, about
to push a friend who is also sitting on a skateboard. When you push your friend, she will move
forward, but you will also move backwards. The backwards
motion is the equal and opposite reaction to the forward
motion of the other skateboard.
How are speed, velocity, and acceleration
related?
Once you understand Newton’s laws of motion, you should begin to see and understand the relationships between speed, velocity, and acceleration. Speed is the distance an object travels in a given time. We measure speed by its magnitude, or its numerical value. You’re probably familiar with the way that we describe the speed of cars. For example, a car may travel at 50 kilometers per hour. Speed is considered a scalar measurement, because we only measure its magnitude, and we don’t measure its direction. Unlike speed, velocity measures the direction an object travels in addition to the speed it is traveling. Velocity is related to speed because velocity is a measure of speed, plus a measure of distance. Finally, acceleration is also related to velocity because it measures the change in velocity over time.
Velocity measures both
speed and direction.
Misconception 2: Is it true that if an object is accelerating, it is speeding up?
It isn’t always the case that accelerating objects are speeding up. Acceleration can
describe an increase in speed, but it can also describe a change in direction. So, an
object that changes its direction of travel, but not the speed at which it travels, can
still be accelerating.
As you complete the lessons in this concept, you’ll learn more about the connections
between speed, velocity, and acceleration. You’ll learn about key equations that can help you
solve problems related to motion. As you do, remember that motion is a concept we describe
numerically, and operate on mathematically. The equations you use will always need units,
such as kilometers per hour, or meters per minute. Pay close attention to the units you use,
and make sure you keep the units the same in each equation: that might mean you have to
first convert minutes into hours, or hours into seconds. This simple tip will help you as you
continue on your journey to learn more about motion.
Concept: Speed, Velocity, and Acceleration
Getting to Know
www.discoveryeducation.com
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© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.