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cat.MOVIf a car runs down a slope, and then a steeper slope but from the same height, will it be going faster when it gets to the bottom?
If a car runs down a slope, and then a steeper slope but from the same height, will it be going faster when it gets to the bottom?
What do you think? Do you have a logical reason for your decision or is it just a 'feeling'?
Can we investigate?
I've set up a slope and I'm going to video the car and then analyse it with an app called 'Video Physics' by a company called Vernier.
Sometimes you need a bit of patience with practical set-ups, as you can see.
Experiment-4-Bricks-Diag.movHere's the car being released down a slope propped up by four bricks.
Here's the car being released down a slope propped up by four bricks.
Experiment-6-Bricks-Diag.movAnd here's the same car running down the same ramp but propped up by 6 bricks.
And here's the same car running down the same ramp but propped up by 6 bricks.
To get the same starting height the car has to be released from further along the ramp.
With the steeper ramp we'd expect it to go faster, but it travels a shorter distance.
How do the two speeds compare when the car gets to the bottom?
These are the points added by the Video Physics app to the 4-brick experiment
...and these are the points for the 6-brick experiment
The data is plotted automatically and shows the distance travelled (blue dots) and the speed (pink dots) of the car.
The maximum speed of the car is 1.78 m/s (highlighted in yellow)
This one looks a bit more complicated because the video ran on for a bit longer as the car travelled across the flat ground, so just look at the first part of the graph (up to about 0.8 seconds)
The maximum speed reached by the car is 1.87 m/s
What next? It's important to remember that physics deals with real things and with real measurements. We can't expect experiments like this to give us perfect answers. These two numbers look very similar, but how close are they?
So we look at how much they differ as a percentage: The difference between them is (1.87 - 1.78) = 0.09 m/s
0.09 as a percentage of one of them is 0.09/1.87 x 100% = about 5%
That's pretty close, so we might tentatively say that it looks like the car reaches the bottom of the slope with the same speed each time.
Is there a reason why this might be true?
Think about how much energy the car had at the top of the ramp. What type of energy is it? Do you know a formula for calculating it?
What type of energy does it have when it reaches the bottom? Do you know a formula for calculating that too?
So if the car started off at roughly 0.3 m above the ground, and the first type of energy is transformed into the second type, can you calculate what speed you would expect it to have at the bottom? (Perhaps surprisingly, you don't need to know the mass of the car, as it cancels out).
How close is your answer to the values we got from the videos? Can we explain that?
What's the overall conclusion here?
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