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Name: bbo
Status: other
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Location: South Africa
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Question:
I have read a few books on a negatively accelerating moving object. If the forward moving object is accelerating negatively, many books claim that the dragging/ frictional force is greater than the forward thrust. If this is true, then the net force acting on the object should point backward and must sure cause the object to move backward but in fact the object is slowing down but still move forward. Why?



Replies:
Hi BBO

If we just consider the forces acting on an object as either forward or reverse having values of positive and negative, respectively, the sum of the forces is the total force acting on the object. Therefore, if a backward force is greater than the forward force, the net force will be in the backward direction. Now, if an object is already moving in the forward direction and a steady backward force is applied, the object will begin to slow down, eventually stop and then start moving backward at an ever-increasing speed. The acceleration that the object experiences is directly related to how much force is applied. Consider tossing a ball straight up into the air. As the ball leaves your hand, the force exerted by gravity acts on the ball and causes it to slow down. Eventually it stops at some point and begins to fall back to the ground. Gravity is providing a downward force that is acting on the moving object. Hope this helps!

Bob Froehlich


B-bo,

A net force acting "backwards" on an object moving "forward" will cause the object to slow down, but it can still move "forward".

Perhaps the best example is an airplane. During take-off, thrust ("forward force") is greater than drag ("backward force") and the plane accelerates forward. This is a net forward force on the plane and everyone in the plane feels it (and that is why you are seated and seat-belted during take off).

When the plane reaches a "cruising state", all four forces (thrust, drag, lift, and gravity) are balanced, so there is no net force on the plane. The plane is still moving forward relative to the ground (at around 400 to 500 mph usually, depending on the airplane and the wind), but the net force is zero. People can get up and walk around the cabin if the turbulence is not too bad.

When the pilot gets ready to land, he/she reduces the thrust from the engines and the net force is "backwards" (negative) causing the plane to slow down (but still move forward). This is an example where the net force on the plane is negative, but the plane still moves forward.

Usually, when the plane lands, the pilot also applies a reverse thrust from the plane's engines to slow the plane down on the runway. What would happen if the pilot did not stop applying this reverse thrust? The plane would slow down to a stop, and then begin to accelerate backwards (again, another example of a negative net force being applied, to an object that moves forward, then stops, then moves backwards). Although the plane's velocity would change sign in this case (from positive (forward) to negative (backward)), the net force and the net acceleration would be negative (backward) during the entire time that the reverse thrust is being applied.

My physics students used to really struggle with this one. I would talk about throwing a baseball up in the air. After the ball is released from my hand, the only big force acting on the ball is gravity (I am neglecting air resistance for the sake of simplicity). So the net force on the baseball is always down (toward the center of the Earth). Does that mean the baseball is always MOVING down? No, it moves up about half the time (during which the velocity is positive (upward) and decreasing) and down about half the time (when the velocity is negative (downward) and increasing -- until it hits the ground). So the ball can be moving up (but slowing down) while a negative (downward/backward) net force is applied to the ball. If it helps, remember F=ma. The force of gravity is always acting on the ball (even when it changes direction at the top of its trajectory), so "a" (acceleration) in F=ma can never be zero.

I hope this helps!

Regards,

Todd Clark


An object will accelerate in the direction of the net force acting on it. So if an object moves in a direction we call "forward," a net force in the opposite direction ("backward") will cause it to accelerate in the opposite direction (backward).

How can this be? How can an object be moving forward but accelerating backward?

Simple. If it is moving forward and accelerating forward, that means that it is speeding up. If it is moving forward but not accelerating (its acceleration is zero) then its speed is constant. If it is moving forward but accelerating backward, then it is slowing down.

If the net backward force acts on the object for a long enough time, the object will slow down, stop, and begin moving backwards, faster and faster. You see this whenever an object is thrown into the air. Its initial velocity is upward, but the net force on it (mostly from gravity) is downward. So the object moves upward more and more slowly, until at the top of its path it stops and begins moving downward faster and faster.

Richard Barrans


Bbo,

Acceleration is NOT how an object is moving. Acceleration is how an object's motion is CHANGING:

A real life example is a very heavy cart moving forward. You push backward on the cart, providing a backward force on the cart. The cart does of course push forward on you, but that forward force is on only you. The cart feels the backward push and the backward acceleration. If you keep pushing backward, if you do not fall down or give up, the cart slows to a stop and then starts moving backward. If you keep pushing, the backward-moving cart then speeds up.

Acceleration in the same direction as velocity speeds up an object. Acceleration directed opposite of velocity slows down an object. Acceleration on an object with zero velocity makes the object START to move in the direction of the acceleration.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


If you keep pushing backward, negatively, long enough, than sure, the object will stop, and go the other way. But this has to be an outside force. friction can make you stop ok, but it will not push you back the other way, it depends on you moving in the first place to even show up.

An easy example is throwing a ball up in the air. It is moving upward. But it is getting a negative acceleration from gravity. It slows its vertical climb, then stops, then turns around and comes back down. The force has to work over a time to counter the inertia of the object already moving upward. It will win, but not instantly, it has to slow the rising ball. As a hint see that the upward momentum, mass * velocity, has the same units as force * time.

kg * m/sec = kg m /sec/sec * sec

change in upward momentum = force * length of time force acts.

You start rising with a certain speed, and it takes time for the gravity force to drop this to zero, then turn it negative.

Steve Ross



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