# speed

## calculating distance or time from speed

If you know the average speed of a moving object, you can calculate the distance it will travel in a given period of time or the time it will take to travel a given distance. To calculate distance from speed and time, use this version of the average speed formula given above: distance = speed time For example, if a car travels at an average speed of 60 km/h for 5 hours, then the distance it travels is: distance = 60 km/h 5 h = 300 km To calculate time from speed and distance, use this version of the formula: time = distance speed Q: If you walk 6 km at an average speed of 3 km/h, how much time does it take? A: Use the formula for time as follows: distance speed 6 km = 3 km/h =2h time =

## introducing speed

How fast or slow something moves is its speed. Speed determines how far something travels in a given amount of time. The SI unit for speed is meters per second (m/s). Speed may be constant, but often it varies from moment to moment.

## instantaneous speed

When you travel by car, you usually dont move at a constant speed. Instead you go faster or slower depending on speed limits, traffic lights, the number of vehicles on the road, and other factors. For example, you might travel 65 miles per hour on a highway but only 20 miles per hour on a city street (see the pictures in the Figure 1.1.) You might come to a complete stop at traffic lights, slow down as you turn corners, and speed up to pass other cars. Therefore, your speed at any given instant, or your instantaneous speed, may be very different than your speed at other times. Instantaneous speed is much more difficult to calculate than average speed. Cars race by in a blur of motion on an open highway but crawl at a snails pace when they hit city traffic.

## average speed

Even if speed varies during the course of a trip, its easy to calculate the average speed by using this formula: speed = distance time For example, assume you go on a car trip with your family. The total distance you travel is 120 miles, and it takes 3 hours to travel that far. The average speed for the trip is: 120 mi 3h = 40 mi/h speed = Q: Terri rode her bike very slowly to the top of a big hill. Then she coasted back down the hill at a much faster speed. The distance from the bottom to the top of the hill is 3 kilometers. It took Terri 41 hour to make the round trip. What was her average speed for the entire trip? (Hint: The round-trip distance is 6 km.) A: Terris speed can be calculated as follows: 6 km 0.25 h = 24 km/h speed =

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

speed determines how far something moves in a given amount of time.

``````-->  a. true

b. false
``````

the si unit for speed is

``````a) cm/s.

-->  b) m/s.

c) m/h.

d) km/h.
``````

if you ride your bike 20 miles and it takes you 120 minutes, what is your average speed?

``````a) 6 m/h

-->  b) 10 m/h

c) 20 m/h

d) 60 m/h
``````

if you know your average speed and how long you have been traveling, then you can calculate your distance with the formula

``````a) distance = speed/time.

-->  b) distance = speed x time.

c) distance = time/speed.

d) none of the above
``````

to calculate the amount of time it takes to travel a given distance at a certain speed, you would use the formula

``````-->  a) time = distance/speed.

b) time = distance x speed.

c) time = speed/distance.

d) none of the above
``````

## diagram questions

No diagram questions associated with this lesson