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# weight decreasing?

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1. Hello

This graph shows the downward force from a bird before take off. My question is why it decreases at the beginning? i know it's for a very short time and probably due to body movements affecting measuring instrument, but I want to see your opinion.

2. maybe it squats before pushing off...

3. It pushes down for momentum. So that when it takes off, it has more force to overcome gravity and doesn't just go fall.
4. (Original post by Joinedup)
maybe it squats before pushing off...

haha yes. I expected something like that, but do you know why lifting one leg would change the weight? (i know it doesn't)
5. (Original post by Daniel Atieh)
haha yes. I expected something like that, but do you know why lifting one leg would change the weight? (i know it doesn't)
well if it just lifted a leg without any other movements it'd mean it was moving it's centre of mass upward.
6. The weight is fixed, it is the force on a fixed mass due to a fixed gravitational field strength. What we're looking at here is the reaction force applied by the bird on the ground (or, by Newton's 3rd, by the ground on the bird), i.e. what you'd see on some scales.

The reason the reaction force is changing is because the bird is squatting/launching, which are both dynamic processes, and because they are dynamic, it means the forces acting on the bird aren't necessarily balanced. This means its weight is not necessarily equal to the reaction force.

For example, when the bird starts squatting, it's accelerating downwards which means the resultant force on the bird must be acting downwards, which means the weight is bigger than the reaction force. Since the weight is fixed, this can only happen if the reaction force falls. That is shown by the initial dip on your graph.
7. (Original post by mik1a)
The weight is fixed, it is the force on a fixed mass due to a fixed gravitational field strength. What we're looking at here is the reaction force applied by the bird on the ground (or, by Newton's 3rd, by the ground on the bird), i.e. what you'd see on some scales.

The reason the reaction force is changing is because the bird is squatting/launching, which are both dynamic processes, and because they are dynamic, it means the forces acting on the bird aren't necessarily balanced. This means its weight is not necessarily equal to the reaction force.

For example, when the bird starts squatting, it's accelerating downwards which means the resultant force on the bird must be acting downwards, which means the weight is bigger than the reaction force. Since the weight is fixed, this can only happen if the reaction force falls. That is shown by the initial dip on your graph.
Thanks a bunch really
8. (Original post by Joinedup)
well if it just lifted a leg without any other movements it'd mean it was moving it's centre of mass upward.
Thank you

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