Kinematics Question: Child on Trampoline Jump Height Calculation

[Gurvir]

Homework Statement

A child on a trampoline remains in the air for 1.5s after having jumped straight up. What height did the child reach?

Homework Equations

All kinematics equations
I was thinking of using Net Force but we don't have the mass and if you use it how could you find displacement.

The Attempt at a Solution

d = v_it + 1/2at²
v_i= 0
So:
d= 1/2at²
d = 1/2(9.81m/s²)(1.5s)²
d= 11m

The answer is suppose to be 2.8m

 

[Doc Al]

The time given is the total time in the air. So what's the time it took to reach the highest point?

 

[Gurvir]

The time given is the total time in the air. So what's the time it took to reach the highest point?

Half of that which is 0.75s, and wow. There it is, simplest question. Thanks man!

Omg I keep getting stuck on these questions, kinematics is so easy but I can't remember anything. Check back I will have a new question posted in 1min.

Homework Statement

A truck uniformly decelerates by 5.00m/s² to a velocity of 60.0m/s [W] in 30.0s. What was the truck's initial velocity before the driver stepped on the brakes?

Homework Equations

All kinematics equations
I'm looking for an equation but I don't have distance so I can't figure one out that will work. The closest formula I could maybe see working is
v_f² = v_i² + 2ad

The Attempt at a Solution

I'm not sure.

(EDIT)

Found it:

Solution

a = Δv / Δt
a = v_f - v_i / Δt

Rearrange it and you get this
v_i = 60m/s -at

Put in the variables
v_i = 60m/s - (-5.00m/s)(30.0s)
v_i = 60m/s + (5.00m/s)(30.0s)

Answer:
v_i = 210 m/s [W]

 

[Doc Al]

Looks good!

 

[Filip Larsen]

Based on the numbers you quote the result is correct, but then it sure is one fast truck. If I were you I would double check that the given numbers are correctly taken from the problem text.

 

[Doc Al]

Based on the numbers you quote the result is correct, but then it sure is one fast truck. If I were you I would double check that the given numbers are correctly taken from the problem text.

Yes, the numbers are comically unrealistic. Worth checking that you copied the problem correctly, but it's also not unusual for problems to use unrealistic data. (Unfortunately.)

 

[joshmdmd]

Yes, the numbers are comically unrealistic. Worth checking that you copied the problem correctly, but it's also not unusual for problems to use unrealistic data. (Unfortunately.)

i guess I am just lucky, my textbook uses realistic numbers, it even has a textblock on the side that says they're all realistic =]..

and yes, that's an insanely fast truck xD