Mass and Weight
Physical Science Home Page
Speed or Velocity?
The speed of an object is a scalar quantity, which means that it has size only.
The velocity of an object is a vector quantity, which means that it has size and direction.
Look at these diagrams :
A and B represent the routes taken by two cars travelling at 60 km/h. Since car A is constantly changing its direction, we should refer to its speed. Since car B is travelling in one direction, we should refer to its velocity.
N.B Speed and velocity are measured in the same units.
This is an important topic when you consider the number of cars on the roads at the present, and the increase in numbers forecast for the future.
Car Stopping Distances
Several factors affect the stopping distances of a car. These are mainly:
The time needed to stop a car = thinking time + reaction time + braking time
Car Crumple Zone
This zone, is usually, just at the front of a car. Its purpose is to absorb the
energy of an impact, say, a car hitting a wall. The crumple zone, crumples, and so in doing so the car
takes longer to come to a stop, which is better for the passengers inside.
Speed, Acceleration and Car Safety
Speed is a measure of how fast an object is travelling.
The S.I unit for Speed is m/s or ms-1, but we can use cm/s or km/s
This refers to an increase in speed, or if the speed decreases, we talk about deceleration.
The S.I units for acceleration are m/s/s which can be written as m/s2 or ms-2
These refer to the acceleration of an object and are very similar to distance-time graphs : so you to need look at the axes of the graphs very carefully.
Speed and Acceleration
Since the area under a speed-time graph is:
on cancelling time in this equation, we get :
AREA under the graph = distance travelled
Practical Work : To find the average speed of a 'car' on a ramp
Apparatus : ramp, 'car', stopwatch, metre ruler.
Measure the distance d between A and B, which are the start and stop points on the ramp (make d about one metre).
When the front wheels of the 'car' are on mark A, let the car go (DO NOT PUSH IT)
Start timing and then stop the stopwatch when the front wheels pass mark B
Repeat this procedure at least twice more to find the average time taken to move distance d
Calculate the average speed for this distance:
Change the height of the ramp above the table.
Keeping the distance d the same, repeat the above procedure.
Is this average speed the same as before? Explain your answer.
Is the average speed, the same as the constant speed of the car? Explain your answer.
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