acceleration, when jumping???

Question

i understand that is acceleration is the change in velocity over time. where i am getting confused is with deceleration vs. acceleration in a negative motion...how do you tell them apart on a graph?
for example, say i am going to jump in the air. i realize that from the moment i leave the ground, my acceleration will be equal to my gravity...but, what happens with my acceleration as i bend my knees down and then on my upward movement before i leave the floor...any help will be appreciated...i can't seem to get my head around this.

i'm talking about the acceleration of my movement from standing still to bending my knees to being up in the air to landing...if a motion detecotor was above my head, the distance would be changing as soon as i start to bend my knees...that is where i am having the most difficulty (since the motion detector is above me, the distance increases as i bend my knees...)

Answer 1

Your leg muscles provide a variable force that is stronger than the gravitational attraction (your weight). The difference turns into a net upward acceleration, giving your body an upward velocity. After that, as you've noted, gravity is the only significant remaining force.

For instance, if you weigh 100 pounds and your legs can provide 200 pounds of thrust, then there's a net upward force of 100 pounds for some short time.

Does that help?

Answer 2

What part of your body are you analyzing? From the perspective of your feet, there is no acceleration until your feet actually leave the ground.

At this point, your acceleration will instantaneously reach some maximum value and you will start to decelerate as gravity pulls you back down.

At your maximum height, your velocity goes to zero and you begin to accelerate toward Earth at ~9.8 m/sec².

When you hit the Earth, you instantaneously decelerate to 0 velocity and hit with a momentum equal to your mass times the velocity at impact.

Your velocity at any point is going to be the slope of your displacement vs. time curve. Your acceleration is going to be the slope of you velocity vs. time curve.