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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 :

speed or velocity

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.

 

 

Car Safety

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 driver

  • The condition of the car

  • The condition of the driving surface

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.

 

PHYSICAL SCIENCE

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Mechanics

Speed, Acceleration and Car Safety

 

Speed

Speed is a measure of how fast an object is travelling.

Equation 1

We say:

Equation 2

The S.I unit for Speed is m/s or ms-1, but we can use cm/s or km/s

 

Acceleration

This refers to an increase in speed, or if the speed decreases, we talk about deceleration.

Equation 3

The S.I units for acceleration are m/s/s which can be written as m/s2 or ms-2

 

Speed-Time graphs

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-time graph

Speed and Acceleration

Since the area under a speed-time graph is:

area speed-time graph

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.

Method

  1. Measure the distance d between A and B, which are the start and stop points on the ramp (make d about one metre).

  2. When the front wheels of the 'car' are on mark A, let the car go (DO NOT PUSH IT)

  3. Start timing and then stop the stopwatch when the front wheels pass mark B

  4. Repeat this procedure at least twice more to find the average time taken to move distance d

  5. 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.

 

Questions

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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