<< Chapter < Page Chapter >> Page >
  • Explain the relationships between instantaneous velocity, average velocity, instantaneous speed, average speed, displacement, and time.
  • Calculate velocity and speed given initial position, initial time, final position, and final time.
  • Derive a graph of velocity vs. time given a graph of position vs. time.
  • Interpret a graph of velocity vs. time.
Snails leaving slime trails as they race each other along a flat surface.
The motion of these racing snails can be described by their speeds and their velocities. (credit: tobitasflickr, Flickr)

There is more to motion than distance and displacement. Questions such as, “How long does a foot race take?” and “What was the runner’s speed?” cannot be answered without an understanding of other concepts. In this section we add definitions of time, velocity, and speed to expand our description of motion.


As discussed in Physical Quantities and Units , the most fundamental physical quantities are defined by how they are measured. This is the case with time. Every measurement of time involves measuring a change in some physical quantity. It may be a number on a digital clock, a heartbeat, or the position of the Sun in the sky. In physics, the definition of time is simple— time    is change , or the interval over which change occurs. It is impossible to know that time has passed unless something changes.

The amount of time or change is calibrated by comparison with a standard. The SI unit for time is the second, abbreviated s. We might, for example, observe that a certain pendulum makes one full swing every 0.75 s. We could then use the pendulum to measure time by counting its swings or, of course, by connecting the pendulum to a clock mechanism that registers time on a dial. This allows us to not only measure the amount of time, but also to determine a sequence of events.

How does time relate to motion? We are usually interested in elapsed time for a particular motion, such as how long it takes an airplane passenger to get from his seat to the back of the plane. To find elapsed time, we note the time at the beginning and end of the motion and subtract the two. For example, a lecture may start at 11:00 A.M. and end at 11:50 A.M. , so that the elapsed time would be 50 min. Elapsed time Δ t is the difference between the ending time and beginning time,

Δ t = t f t 0 ,

where Δ t size 12{Δt} {} is the change in time or elapsed time, t f is the time at the end of the motion, and t 0 is the time at the beginning of the motion. (As usual, the delta symbol, Δ size 12{Δ} {} , means the change in the quantity that follows it.)

Life is simpler if the beginning time t 0 is taken to be zero, as when we use a stopwatch. If we were using a stopwatch, it would simply read zero at the start of the lecture and 50 min at the end. If t 0 = 0 , then Δ t = t f t .

In this text, for simplicity’s sake,

  • motion starts at time equal to zero ( t 0 = 0 ) size 12{ \( t rSub { size 8{0} } =0 \) } {}
  • the symbol t size 12{t} {} is used for elapsed time unless otherwise specified ( Δ t = t f t ) size 12{ \( Δt=t rSub { size 8{f} } equiv t \) } {}


Your notion of velocity is probably the same as its scientific definition. You know that if you have a large displacement in a small amount of time you have a large velocity, and that velocity has units of distance divided by time, such as miles per hour or kilometers per hour.

Questions & Answers

Wheatstone bridge problems
Sonali Reply
what is motion
Ali Reply
When a body changes its position with respect to its surroundings then it is said to be in motion
it is a change in position, direction, shape and arrangement of a body or an object which is initially at rest
Pls guys am having problem on these topics: latent heat of fusion, specific heat capacity and the sub topics under them.Pls who can help?
hamidat Reply
Thanks George,I appreciate.
this will lead you rightly of the formula to use
Most especially it is the calculatory aspects that is giving me issue, but with these new strength that you guys have given me,I will put in my best to understand it again.
you can bring up a question and let's see what we can do to it
I also have a problem with the calculation sincerely
the distance between two suasive crests of water wave traveling of 3.6ms1 is 0.45m calculate the frequency of the wave
Idris Reply
v=f×lemda where the velocity is given and lends also given so simply u can calculate the frequency
You are right my brother, make frequency the subject of formula and equate the values of velocity and lamda into the equation, that all.
lExplain what happens to the energy carried by light that it is dimmed by passing it through two crossed polarizing filters.
Christoper Reply
When light is reflected at Brewster's angle from a smooth surface, it is 100% polarizedparallel to the surface. Part of the light will be refracted into the surface.
What is specific heat capacity?
hamidat Reply
Specific heat capacity is the amount of heat required to raise the temperature of one (Kg) of a substance through one Kelvin
formula for measuring Joules
Rowshan Reply
I don't understand, do you mean the S.I unit of work and energy?
what are the effects of electric current
What limits the Magnification of an optical instrument?
Naeem Reply
Lithography is 2 micron
what is expression for energy possessed by water ripple
Prabesh Reply
what is hydrolic press
Mark Reply
An hydraulic press is a type of machine that is operated by different pressure of water on pistons.
what is dimensional unite of mah
Patrock Reply
i want jamb related question on this asap🙏
sharon Reply
What is Boyles law
Pascal Reply
it can simple defined as constant temperature
Boyles law states that the volume of a fixed amount of a gas is inversely proportional to the pressure acting on in provided that the temperature is constant.that is V=k(1/p) or V=k/p
what is motion
Mua Reply
getting notifications for a dictionary word, smh
Practice Key Terms 7

Get the best College physics course in your pocket!

Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?