Instantaneous Speed (OCR GCSE Physics A (Gateway Science Suite)): Revision Notes
5.6.9 Instantaneous Speed
Instantaneous Speed: The speed of an object at a particular instant. It's how fast the object is moving at that exact moment, rather than over an entire journey or time interval.
Practical Examples
- Driving a Car
- When you look at your car's speedometer, it shows you the instantaneous speed, which is how fast you're going at that exact moment.
- Running or Cycling
- If you're tracking your run or bike ride, a fitness app may show your speed at different moments. This speed at each moment is your instantaneous speed.
How It's Measured
- Speedometers: In cars, the speedometer shows the instantaneous speed by measuring how fast the wheels are turning at that exact moment.
- Radar Guns: Used by law enforcement to measure the speed of moving vehicles at a particular instant.
- Sensors and GPS: In various technologies, sensors or GPS devices can provide real-time speed data. The instantaneous speed of an object can be calculated if we know the distance travelled in a very short period of time. We might think about the speedometer on a car or most kinds of speed cameras.
Question:
A speed camera flashes twice at a 0.7-second interval. If a car travels 4.2m in this time, what is its instantaneous speed?
An object is travelling at a steady or constant speed when its instantaneous speed has the same value throughout its journey. For example, if a car is travelling at a constant speed the reading on the car's speedometer does not change. The speedometer reads the speed of the car at each moment in time throughout the car's journey. In cases like this, where the motion involves constant speed, the instantaneous speed of the object can be worked out using the relationship:
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