# If resultant force is acting upwards, does this mean the object is moving upwards?

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#1
If resultant force is acting upwards, does this mean the object is moving upwards?
0
2 months ago
#2
If isaac newton saw this, he would headbutt you.

In answer to your question, not necessarily. If the resultant for it moving upwards in means the object is ACCELERATING upwards, not moving upwards
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2 months ago
#3
yes
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#4
(Original post by tej3141)
If isaac newton saw this, he would headbutt you.

In answer to your question, not necessarily. If the resultant for it moving upwards in means the object is ACCELERATING upwards, not moving upwards
Could you explain this situation to me?

How is it that when a parachutist falls (after opening her parachute), the resultant force is upwards even though the parachutist is falling?
In this situation is air resistance decreasing? If so, why?
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2 months ago
#5
(Original post by t7nu9-)
Could you explain this situation to me?

How is it that when a parachutist falls (after opening her parachute), the resultant force is upwards even though the parachutist is falling?
In this situation is air resistance decreasing? If so, why?
I'll answer the 2nd question first. Yes air resistance is decreasing and that is because air resistance is dependant on velocity. So the faster something falls, the larger the air resistance is. And as the parachutist is slowing down (as a result of opening the parachute),the air resistance is decreasing.
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2 months ago
#6
(Original post by t7nu9-)
If resultant force is acting upwards, does this mean the object is moving upwards?
Not necessarily.

You can be moving in a particular direction and the resultant force acting on you is in the opposite direction.
In this case the force will cause you to decelerate and eventually stop.
If the force continues to act on you, you will of course then start to move in the other direction.

In the case of a parachute, when you first jump you are initially accelerating downwards, as initially the force on you is just gravitational.
As you get faster and the parachute opens, air resistance, an upwards force here, gets bigger and eventually is equal to the gravitational force.
At this point the resultant is zero and you fall downwards at a constant speed.
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2 months ago
#7
(Original post by t7nu9-)
If resultant force is acting upwards, does this mean the object is moving upwards?
Yes and no.
Just in case, you need a "more detailed explanation" with examples.

Let take a step back to see the relationship between velocity (or “motion”) and acceleration (or “change in motion”).
In essence, velocity describes the motion of an object at a given instant of time; acceleration describes how the object’s motion is changing with time.

This is best illustrated with a picture as shown below.

From the picture, it shows that:
The velocity and acceleration of an object have the same direction or same sign when the object is speeding up.
However, when the object is slowing down, the velocity and acceleration of the object do not have the same direction or same sign.

Hope you have a better understanding between velocity and acceleration.

Next, is the “relationship” between resultant force and motion. Although we are trying to explain the relationship between resultant force and motion, in essence, there is no direct relationship between resultant force and motion.
Resultant force causes the object to change in motion and this is deduced from Newton’s 2nd law (F= ma).
Put it in another way, a resultant force is needed to speed up or slow down the motion of an object.
F=ma also tells us that the acceleration vector and resultant force vector are always pointing in the same direction.
Let us now add the direction of the resultant force F in the picture above.

In conclusion, resultant force does not cause motion but resultant force causes a change in motion of an object, so the direction of resultant force and velocity do not necessarily point in the same direction.

Hope it makes sense.
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