what is acceleration? my speedometer:. acceleration occurs when speed changes now we don’t want...
TRANSCRIPT
Acceleration occurs when speed changes
• Now we don’t want our cruise control on
• We want to use our gas pedal or brake to make our car go faster or slower.
Let’s start out with an new type of graph
This graph is called a velocity-time graph (also called a speed-time graph)
• Now we can show how the car’s speed changes
• We can get different shapes on the graph depending on how the speed changes
Think about this: Let’s say we are at a stopped light stopped. It turns green and we speed away. Let’s
say that we speed up 10 miles per hour per second. We can see this change with a speedometer.
• Our speedometer needle is going up
If my car is increasing its speed at 10 mph / second, I would see it say:
* 0 mph at 0 seconds (started my stopwatch)10 mph at 1 second20 mph at 2 seconds
30 mph at 3 seconds, etc.
This is the mathematical definition of acceleration
We can draw a line And we can get the slope of that line.
• This slope is the acceleration.
• It is defined as the slope of the line formed on a speed-time graph
• Our slope is this case is 10 mph/sec or we can say that every second my speed increases by 10 mph.
For this we can generate an equation.
• This equation is called our ‘how fast’ equation:• V = a(t), the velocity of the car is the
acceleration (a) x time.
• For example in my car ‘how fast’ is my car traveling at the end of 8 seconds?
• V = a x (t) = 10 x 8 or 80 mph
So we are getting faster
• This is obvious to you drivers, right? You can press on the gas and get the car to go faster and faster.
• We now know that the speed of the car can be found using the how fast equation
• Speed or Velocity, V = a x (t)
But what about ‘how far’ the car moves?
• It turns out we can develop an equation for this, too.
• This is the distance that the car moves
• On my fit and on your car, you have an odometer which can help us measure distances we drive
Odometer below the speedomter
We can determine the ‘how fast’ equation.
• In a real physics class, we would develop this equation; however, I will give it to you now.
• Distance = ½ x (a) x (t2)• Distance is one-half times the acceleration x
the square of time.• How far would my car go in 8 seconds?• Distance = ½ x 10 x 82 = 320 miles·sec/hour• (don’t worry about the units)
We can show our movement on a distance-time graph (also called a position-time graph). We have studied these already.
This shape is called a PARABOLA. We call this our getting faster parabola because
the car is getting faster
What does this parabola mean?
• My distance is INCREASING over successive time intervals.
• For example, if my car was traveling at 60 mph constant speed, every every hour I would travel 60 miles.
• Now my distance doesn’t stay constant, it increases.