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

• B
• rudransh verma
In summary, the body continues to move with a constant velocity up until the upward force is stopped. Beyond this point, the body moves with the kinetic energy it gained from the cannon, which can be degraded by gravity, air drag, and impact.
rudransh verma
Gold Member
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 ?

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.

russ_watters
Dr.D said:
when it ends,
What ends ?

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

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.

Lnewqban said:
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.

Last edited:
berkeman
Dr.D said:
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.

rudransh verma said:
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.

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

According to Newton's first law of motion, an object at rest will remain at rest and an object in motion will continue in motion at a constant velocity unless acted upon by an external force. This means that if a body is at rest, it will not accelerate upward when a force is applied because there is no initial velocity to change. If the body is already in motion, it will continue moving at a constant velocity in the same direction unless a force is applied in the opposite direction.

2. What is the role of gravity in the upward acceleration of a body?

Gravity is a fundamental force that pulls objects towards each other. In the case of a body accelerating upward, gravity is acting in the opposite direction, pulling the body down towards the Earth. This counteracts the force applied to the body and prevents it from accelerating upward.

3. Can a body ever accelerate upward when a force is applied?

Yes, a body can accelerate upward when a force is applied if the force is greater than the force of gravity pulling the body down. This is seen in situations such as a rocket launch, where the force of the rocket's engines is greater than the force of gravity, allowing the rocket to accelerate upward.

4. How does the mass of a body affect its acceleration when a force is applied?

According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass. This means that the greater the mass of a body, the more force is needed to accelerate it. Therefore, a body with a larger mass will have a slower acceleration when a force is applied compared to a body with a smaller mass.

5. Is air resistance a factor in the upward acceleration of a body?

Yes, air resistance can play a role in the upward acceleration of a body. Air resistance is a force that opposes the motion of an object through the air. When a body is accelerating upward, it is moving through the air and therefore experiences air resistance. This resistance can decrease the overall acceleration of the body, especially at higher speeds.

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