# Measuring motion

Clocks and meter sticks are all that's needed to characterize motion.

### Estimate...

Estimate the distance from GC's 8th street entrance (near dining hall) to the RFC by two methods...

• Take a raw guess at the distance, in miles.
• Now, take a guess at how long it takes to walk (in minutes), and use the typical speed of walking which is 3 or 4 miles per hour to work out how far it is.

map

### Speed

Describing the motion of a moving body...

average speed = distance travelled / time to travel
$s = d / t$

Was their speed always the same at every moment of the trip?

How could you measure your "instantaneous" speed?

A bicycle speedometer (nowadays called a cyclocomputer) uses a magnet attached to a spoke, and a sensor which "feels" the magnet pass.

### Velocity

velocity = speed and direction of travel

Jane passes Fred, both of them moving North on 8th Street.

• Do they have the same speed?
• Do they have the same velocity?

Jane and Fred pass in opposite directions, both going 15 km / hour.

• Do they have the same speed?
• Do they have the same velocity

Jane and Fred are both going North on 8th St at 15 km / hour. But Jane has just crossed College Avenue, and Fred has already passed Plymouth Avenue.

• Do they have the same speed?
• Do they have the same velocity

## Acceleration

Acceleration: any change in an object's velocity.

### Acceleration?

See: HHSTT rocket sled video.

### Acceleration?

Orbit of the Hubble Space Telescope around the earth.

### Acceleration?

See: Crash test video.

### Acceleration?

A car...

1. Moving along a straight, level road at 120 km/hr?
2. Moving along a straight, level road and slowing from 120 km/hr to 70 km/hr?
3. Rounding a curve at a steady 50 km/hr?
4. Moving uphill along a straight incline at 50 km/hr?
5. Rounding a hill top at 50 km/hr?
6. ### Acceleration!

These things all mean "Acceleration" in Physics-speak:

• speeding up
• slowing down
• turning

What 3 acceleration devices can a car driver control?

## Inertia revisited

We said...

A body that is subject to no external influences* will keep moving, if it was moving to begin with, in a straight line with unchanging speed. Or, if it was at rest, such a body will stay at rest.

But now we can say

A body subject to no external influences must maintain an unchanging velocity.

## Acceleration while falling

How do we know objects speed up as they fall?

acceleration = change in speed (m/s) / time interval (s)

On the surface of earth all objects are accelerated by the same amount: 9.8 m/s/s

### Suggested Exercises

Conceptual Exercises in Chapter 3: 8, 11, 15, 16, 19, 27.
Problems in Chapter 3: 3, 5