Motion and Speed


An object is in motion if it changes position
relative to a reference point
Stationary objects make good reference points

Whether or not
an object is in
motion depends
on the reference
point you choose.

Distance is the total length of the actual path
between two points. Displacement is the length
and direction of a straight line between starting
and ending points.
What is the total distance this
person traveled (in blocks)?
7 Blocks
What is the total displacement of
this person?
5 Blocks Northeast


Quantities that have both a magnitude and a
direction
Example: Displacement

If you know the distance an object travels in a
certain amount of time, you can calculate the
speed of the object.

The speed of most moving objects is not
constant

Rate at which object is moving at a given
instant in time



Speed in a given direction
Velocity is a vector because it has both
magnitude and direction
Changes in velocity may be due to changes is
speed, changes in direction, or both

You can use distance-versus-time graphs to
interpret motion.
1. Is a moving bus a good reference point from which to
measure your position?
a. No, because it is often late.
b. No, because it is not a stationary object.
c. Yes, because it is very large.
d. Yes, because it can travel very far.
1. Is a moving bus a good reference point from which to
measure your position?
a. No, because it is often late.
b. No, because it is not a stationary object.
c. Yes, because it is very large.
d. Yes, because it can travel very far.
2. To describe a friend’s position with respect to you,
you need to know
a. Your friend’s distance from you.
b. The direction your friend is facing.
c. Your friend’s distance and direction from
you.
d. Your friend’s distance from a nearby object.
2. To describe a friend’s position with respect to you,
you need to know
a. Your friend’s distance from you.
b. The direction your friend is facing.
c. Your friend’s distance and direction from
you.
d. Your friend’s distance from a nearby object.
3. Two cars traveling in the same direction pass you at
exactly the same time. The car that is going faster
a. moves farther in the same amount of time.
b. has more mass.
c. has the louder engine.
d. has less momentum.
3. Two cars traveling in the same direction pass you at
exactly the same time. The car that is going faster
a. moves farther in the same amount of time.
b. has more mass.
c. has the louder engine.
d. has less momentum.
4. To describe an object’s motion, you need to know
its
a. position.
b. change in position.
c. distance.
d. change in position over time.
4. To describe an object’s motion, you need to know
its
a. position.
b. change in position.
c. distance.
d. change in position over time.