B Why doesn't a body accelerate upward when a force is applied?

[rudransh verma]

When we apply a force to throw a body upward why doesn’t it accelerate in upward direction. I think the answer is continuous force of gravity slowing it down. So it is de accelerating from the moment it’s released. But recently we have applied a force so it should accelerate ?

 

[Dr.D]

The key is in your word "recently." That means the upward force was previously applied, but has now ended and when it ends, that when the downward acceleration begins.

 

[rudransh verma]

when it ends,

What ends ?

 

[Dr.D]

When the net upward force ends, the upward acceleration ends. The upward motion ceases when the upward velocity goes to zero.

 

[Lnewqban]

A vertical cannon applies force to a projectile while it is inside the barrel.
The velocity of the projectile goes from zero to a maximum value when leaving the barrel.
That is the only time during which the expanding gases of the propellant transfer energy to the projectile.

Beyond that point or moment, the projectile flies uniquely with the kinetic energy that it has gained from the cannon.
That energy can be degraded by gravity, air drag and impact.

The mass of the projectile needs a constantly applied force in order to increase or decrease its velocity.

That is why rockets and airplanes and helicopters keep burning fuel in engines that push their masses, either to accelerate them or to keep them flying at constant speeds.

 

[rudransh verma]

A vertical cannon applies force to a projectile while it is inside the barrel.
The velocity of the projectile goes from zero to a maximum value when leaving the barrel.
That is the only time during which the expanding gases of the propellant transfer energy to the projectile.

Beyond that point or moment, the projectile flies uniquely with the kinetic energy that it has gained from the cannon.
That energy can be degraded by gravity, air drag and impact.

The mass of the projectile needs a constantly applied force in order to increase or decrease its velocity.

That is why rockets and airplanes and helicopters keep burning fuel in engines that push their masses, either to accelerate them or to keep them flying at constant speeds.

Got it! The body with initial acceleration moves up with constant velocity when released but soon it experiences de acceleration and slows down.
So if there were no gravity the body would move forever with constant velocity.

 

[rudransh verma]

When the net upward force ends, the upward acceleration ends. The upward motion ceases when the upward velocity goes to zero.

I don’t think there is any upward force and acceleration. Only downward force and acceleration.

 

[Lnewqban]

Got it! The body with initial acceleration moves up with constant velocity when released but soon it experiences de acceleration and slows down.
So if there were no gravity the body would move forever with constant velocity.

You got it!
While inside the gun, the projectile complies with the Newton's second law: there is a mass and a net upwards force (gases pressure acting up and weight acting down); therefore, it must be an acceleration (the projectile is gaining momentum).

In the first law, an object will not change its motion unless a force acts on it.
Once the projectile is released into an ideal world with no gravity, the mass remains, but the force disappears; threfore, nothing is accelerating the projectile, which keeps flying at constant velocity.

In the real world, the downwards force of weight (mg) persists after the upwards force of the expanding gases ceases beyond the muzzle; therefore, the projectile complies again with the second law and shows a negative acceleration coming to a stop at certain height, just to free fall down.