Alberto Ghizzi Panizza is easily the best macro photographer in Italy and quite likely the world, having featured in many websites and magazines across the globe, like the National Geographic, as well as winning innumerable photography awards. An example of his great creativity, patience and skills can be seen below in one of Alberto’s iconic signature macro photographs of droplets refracting light reflected by objects places behind them. Did he take this one from space? 😉
As well as an amazing macro expert, Alberto is a superb nature photographer and he was kind enough to allow us to use two photos he took recently of a bird diving to fish in a river.
So, from the photo below we could ask students to draw a free body diagram of the forces acting on the bird, assuming it has reached vertical terminal velocity.
We think the only two forces acting are the weight of the bird, obviously pointing vertically down (towards the centre of the earth) and the air resistance along the same direction of the motion of the bird, but in opposite direction. This question is quite relevant to the new GCSE specifications in England, as we will see below, because it allows students to resolve forces into their components to find a resultant force. How do you think your students would draw this free body diagram?
So, if the bird is in vertical free fall, hence reached vertical terminal velocity, how would you resolve these forces to find the resultant force on the diving bird?
By resolving the air resistance on the bird into its vertical component we can show that this force balances the weight of the bird, and this is necessary for the bird to move with constant vertical velocity. But the direction of motion is not just along the vertical and the bird has some initial horizontal velocity (to the right in this case), so the horizontal component of the air resistance will decelerate the bird’s horizontal motion.
Therefore, the resultant force on the bird is the horizontal component of the air resistance which will cause the bird to move with constant vertical velocity downwards and decreasing horizontal velocity to the right, as seen in the resultant force diagram below.
A perhaps easier scenario is the next stage if the bird needed to opens its wings to break its fall before it reaches the water, maybe because it notices a danger. What will the free body diagram forces look like now?
More students should be able to draw the forces on the bird correctly here and have something similar to the diagram below.