Lunasly

5+ Year Member
May 17, 2010
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Hey guys,

In regards to force diagrams, I was asked to state which diagram represented the forces. The digram was of a ball on a flat, frictionless surface. It has a horizontal speed to the right of 3 m/s. The forces on this diagram are downward gravity, upward normal force. Now I picked the diagram that has those two, but also a force in the horizontal direction (to the right) because the ball was moving at 3 m/s. However, this is not the case and the answer was just a diagram with gravity and normal force.

The question asked: Which of the following represents the best force diagram at times after t = 0s?

Is it because it said AFTER t = 0 the we don't include a horizontal force? That makes sense to me if that is the case. Is it exactly at t=0 that there would be a horizontal force because it is at that extract point in time that we would need to apply a force in order for the ball to move at 3 m/s?

There is no friction.
 

Instant Noodles

5+ Year Member
Mar 27, 2010
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Does it say if anything happens at t=0?

If not, then no horizontal force.

Zero acceleration = zero net force and zero net torque
 
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Lunasly

Lunasly

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May 17, 2010
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So it's only when something occurs at t=0 then a force would be included. However, since they said AFTER t=0, we say there is no force because at that point we are moving at a constant velocity.
 

Propinquity

7+ Year Member
Apr 27, 2012
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Yup, yup. At t=0, there is a force applied, so they put in "after t=0" to cover their asses.
 
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Lunasly

Lunasly

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Okay another questions. I understand that after t=0, in this case, anyway, we are moving at a constant velocity and thus we don't have the force being applied.

However, had the question said we were speeding up, then we must still have a force present and so we would need to add that tangental arrow.

Is this correct? I am doubting myself because I have heard the the tangental force is the centrifugal force which is not a real force, but rather the tendency for an object to move along its tangent. So if its speeding up, does that mean we add an arrow saying that a force exists along the tangent or do we not because the centrifugal force is not a real force?
 

Propinquity

7+ Year Member
Apr 27, 2012
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Pre-Medical
If it is changing speeds, a force is acting on it.

Centrifugal forces have nothing to do with this problem, as it is not moving in a circle. Centrifugal forces are made from the perception that if the frame of reference only included the object and nothing else, it would appear as if a force was pushing it outward. In reality, when the frame of reference is expanded, it is actually the centripetal force acting towards the radius that acts on the object.
 
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Lunasly

Lunasly

5+ Year Member
May 17, 2010
798
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If it is changing speeds, a force is acting on it.

Centrifugal forces have nothing to do with this problem, as it is not moving in a circle. Centrifugal forces are made from the perception that if the frame of reference only included the object and nothing else, it would appear as if a force was pushing it outward. In reality, when the frame of reference is expanded, it is actually the centripetal force acting towards the radius that acts on the object.
Oh my apologies. It seems like I skipped to another question. Similar concept, but the object was moving in a circle.

So in regards to this questions where an object moves in a circle, a force diagram would include the centripetal force inward, but it would also include a force in the direction of the tangent because the object has a tendency to move straight but the centripetal force is pulling it in. So the force moving along the tangent – is that called the centrifugal force?