Speed And Acceleration

Classical Mechanics Level 1

A particle that is thrown vertically upwards stops momentarily at the highest point of motion. What is the acceleration of the particle at the highest point of motion?

Assume the size of the particle to be negligible.

Much more than the acceleration due to gravity

g

. Almost equal to the acceleration due to gravity

g

. Much less than the acceleration due to gravity

g

. Zero

5 solutions

Bryan Gonzalez
Mar 8, 2016

Anywhere on the projectile path, the vertical acceleration is always pointing down at the same magnitude. On earth, an object in the air whether it was just thrown, at the top of the projectile, or about the land always has a vertical acceleration of -9.8m/s^2.

That's not entirely true; if the particle rises to a significant height, the acceleration due to gravity will decrease as it's significantly farther from Earth's center of mass.

Andrew Leung - 5 years, 3 months ago

Thanks. I've edited the options accordingly.

Calvin Lin Staff - 5 years, 3 months ago

Well for basic not so high altitude projectiles always have -9.8m/s^2, if it involved some type of air resistance, or the altitude is getting really high, its much safer to say the acceleration is much different than if it were near ground-level. It probably decreases by very further decimal places every feet higher.

Bryan Gonzalez - 5 years, 3 months ago

Surely when the projectile 'stops momentarily' there is a net force of zero acting on it and therefore by F=ma there is also a net acceleration of zero?

Jon Wilson - 4 years, 6 months ago

Stops momentarily means that velocity is 0. It does not mean that acceleration is 0. We do not yet know what the acceleration is like. In fact, if we consider that it was moving upwards just before, and will be moving downwards just after, this tells us that the object is still undergoing acceleration!

So, while velocity is 0 (and hence kinetic energy is 0), that does not imply that force (which depends on acceleration) must be 0.

Calvin Lin Staff - 4 years, 6 months ago

Daniel Miranda
Jan 30, 2017

The fact that the particle is not moving when it reaches the highest point in its travel, while there's an opposing accelerating force applied to it, tells you - if size and mass is negligible - that the force (acceleration in this case) applied by the particle must be almost equal to the external force (constant gravity acceleration) applied to the particle so that it can overcome the "attraction" and stays unmoved.

Zaid Iqbal
Jan 26, 2017

When an object is thrown vertically upward. Acceleration due to gravity will produced (-ve) at peak position it will stop but ''g'' will remain constant then body will reverse its direction while ''g'' will remain constant

This isn't correct. The velocity will be zero momentarily, but, as the velocity at the point is changing (from positive to negative if we take upwards as positive), there MUST be an acceleration. The mistake many make is that zero velocity = zero acceleration. If this were the case, the particle would stay permanently at rest (in this case in mid air!).

Steve Edwards - 4 years, 4 months ago

I mention there will be a constant acceleration

Zaid Iqbal - 4 years, 4 months ago

Hemanth Kumar
Jan 26, 2017

It can be considered as acceleration of body placed at rest on earth which is always pointing towards the centre of earth.

Nathan Sproul
Dec 9, 2016

This problem needs to have a couple words replaced to be truly legitimate. Objects with sufficient mass and density when thrown upwards will (under normal weather conditions) have an acceleration virtually equal to the acceleration due to gravity, but a particle with negligible size will not. The word "negligible" means "so small or unimportant as to be not worth considering". The significant forces (those forces contributing significantly to an object's net acceleration) working on a particle, such as an electron, are not the same as the significant forces working on a more massive object. I recommend rewording the question as follows:

"An object is thrown vertically upwards and stops momentarily at the highest point of motion. What is the acceleration of the object at the highest point of motion?

Assume acceleration due to air resistance to be negligible."

Speed And Acceleration

5 solutions

0 pending reports