Newton's Second Law Rocket Problem

[mailmas]

Homework Statement

A person whose mass is 20 kg needs to accelerate vertically from the surface of the Earth at 5.0m/s^2
and is trying to pick which rocket he should strap to his back. How much thrust
does he need if each rocket has a mass of 30 kg?

Homework Equations

F=ma

The Attempt at a Solution

F = m*a
F = 50*5 = 250N
I don't understand why the answer is 740N
no friction, no air resistance, only gravity and the normal force

 

[mjc123]

Is there a force he has to overcome before he can lift off?

 

[mailmas]

Is there a force he has to overcome before he can lift off?

I assumed he was on the surface of the Earth.

 

[mjc123]

He is. What forces are acting on him?

 

[mailmas]

He is. What forces are acting on him?

I'm pretty sure there is no friction and air resistance. Just his weight and the Normal Force from the surface that are acting on him. Also his mass is 20kg not his weight my bad.

 

[mjc123]

What would he have to do to be just touching the surface of the Earth in such a way that the normal force is zero?

 

[mailmas]

What would he have to do to be just touching the surface of the Earth in such a way that the normal force is zero?

Would he have to be weightless?

 

[mjc123]

He isn't weightless. His weight is still acting on him. What would he have to do to appear effectively weightless?

 

[mailmas]

He isn't weightless. His weight is still acting on him. What would he have to do to appear effectively weightless?

He would have to have a force in the opposite direction with the same magnitude as his weight so that Fnety = 0?

 

[mailmas]

He isn't weightless. His weight is still acting on him. What would he have to do to appear effectively weightless?

In order for him to accelerate upwards 5m/s^2 he would have to first overcome the acceleration from Earth.

 

[mjc123]

Yes

 

[mailmas]

Yes

Thanks :)

 

[mjc123]

The normal force equals the apparent weight, so it doesn't help you get off the ground. If you applied an upward force mg/2, for example, his apparent weight would be mg/2, so the normal force would be mg/2. The total upward force would still be mg, balancing his weight, so he wouldn't lift off. You have to exceed mg to get an upward acceleration. Your force equation should be F - mg = ma, hence F = m(a + g)

 

[mailmas]

The normal force equals the apparent weight, so it doesn't help you get off the ground. If you applied an upward force mg/2, for example, his apparent weight would be mg/2, so the normal force would be mg/2. The total upward force would still be mg, balancing his weight, so he wouldn't lift off. You have to exceed mg to get an upward acceleration. Your force equation should be F - mg = ma, hence F = m(a + g)

Thank you for your help!