And I understand that mass B has tension and weight acting on it. The tension on A is apparently pulling it in the positive direction, while the tension on B is pulling it in the negative direction. Do those opposite tensions cancel one another, causing force of gravity to become the net force? There must be some sort of cancellation in play, I figure, because I've been told that when all the forces are summed, you just end up with force of gravity propelling the system.
How does the relationship between the force of gravity on mass B and the tension in the rope play into this? Isn't the tension caused by that force of gravity? Doesn't that mean that if tensions cancel, the force of gravity's effect is canceled as well? Does the pulley affect tensions? For example, we know that there's a positive tension affecting mass A. Is there still a positive tension in existence on the other side of the pulley, or just the negative tension that's acting on B?
Might there be some sort of effect whereby two sets of opposite tensions, one set on each side of the pulley, cancel each other out? If you pick any and all points on the rope, would there be two opposing tensions at every one of those points? How might differences in mass between object A and object B which, sorry if the diagram was misleading in the sizes, can have any mass play into the tension? You can think about the rope as a lot of tiny masses connected together by springs; this is a cheap approximation for how tension works on the atomic level, where the springs are stretching chemical bonds and the masses are atoms.
This is a good approximation as long as the total mass of the rope is much smaller than the masses of the blocks. The constraint here is that the rope is taut, which means that it can't be scrunching up or stretching out; that translates to the constraint that the accelerations of the two blocks are equal in magnitude. This equation determines the tension. No, the tension isn't equal to the weight of block B, it's whatever is necessary to satisfy the above constraint.
In this case, the tension is actually quite small compared to the weight of block B, because you only need a little tension to make block A have the same acceleration.
In fact, as block B gets infinitely heavy, you can show that the tension doesn't go to infinity -- instead, it becomes the weight of block A! It's neat to try to prove this, and see how it works.
This is a little tricky to word. The tensions of the two tiny springs attached to each atom approximately cancel out, as shown above. But that doesn't mean that the tension is zero -- all of those springs are still stretched. Since the pulley is frictionless, it doesn't have any effect except that it 'turns around' the tension. That's not your fault, the question is just worded badly.
There are lots of forces involved in this problem acting on different things: gravity on both blocks, normal on one block, and normal from the pulley on the rope.
It's not very clear what "the" net force even means. Actually, your understanding seems quite good, and much of what you wrote goes at least in the right direction. In particular you have done well on the crucial part of getting your free-body diagrams right. Let's take this one step at a time, starting with your first question which happens to be a good question to answer first:.
In the case of a man pulling a block with a rope, the rope experiences a tension in one direction from the pull of the man, and a tension in the other direction from the reactive force of the block:. The dynamics of a single rope used to transmit force is clearly quite simple: the rope just transmits an applied force.
When pulleys are used in addition to ropes, however, more complicated situations can arise. In a dynamical sense, pulleys simply act to change the direction of the rope; they do not change the magnitude of the forces on the rope. Just as we assumed the ropes to be massless, we will similarly assume that the pulleys we work with are massless and frictionless, unless told otherwise. Notice the forces T and -T: even when used in addition to a pulley, the rope must still experience two equal and opposite tension forces.
Yes No. Log in Social login does not work in incognito and private browsers. Please log in with your username or email to continue. No account yet? Create an account. Edit this Article. We use cookies to make wikiHow great. By using our site, you agree to our cookie policy. Cookie Settings. Learn why people trust wikiHow. Download Article Explore this Article methods. Related Articles. Article Summary. Method 1. Define the forces on either end of the strand.
The tension in a given strand of string or rope is a result of the forces pulling on the rope from either end.
Assuming the rope is stretched tightly, any change in acceleration or mass in objects the rope is supporting will cause a change in tension in the rope. Don't forget the constant acceleration due to gravity - even if a system is at rest, its components are subject to this force. As an example, let's consider a system where a weight hangs from a wooden beam via a single rope see picture.
Neither the weight nor the rope are moving - the entire system is at rest. Because of this, we know that, for the weight to be held in equilibrium, the tension force must equal the force of gravity on the weight. Account for acceleration after defining the forces. Gravity isn't the only force that can affect the tension in a rope - so can any force related to acceleration of an object the rope is attached to. Account for rotational acceleration. An object being rotated around a central point via a rope like a pendulum exerts strain on the rope caused by centripetal force.
Centripetal force is the added tension force the rope exerts by "pulling" inward to keep an object moving in its arc and not in a straight line. The faster the object is moving, the greater the centripetal force.
Remember also that the force of gravity is constantly acting on the object in a downward direction. So, if an object is being spun or swung vertically, total tension is greatest at the bottom of the arc for a pendulum, this is called the equilibrium point when the object is moving fastest and least at the top of the arc when it is moving slowest. We'll say that our rope is 1. If we want to calculate tension at the bottom of the arc when it's highest, we would first recognize that the tension due to gravity at this point is the same as when the weight was held motionless - 98 Newtons.
Understand that tension due to gravity changes throughout a swinging object's arc. As noted above, both the direction and magnitude of centripetal force change as an object swings. However, though the force of gravity remains constant, the tension resulting from gravity also changes. When a swinging object isn't at the bottom of its arc its equilibrium point , gravity is pulling directly downward, but tension is pulling up at an angle.
Because of this, tension only has to counteract part of the force due to gravity, rather than its entirety. If a cable or rope is massless, then it perfectly transmits the force from one end to another end. For example, if a man pulls the massless rope with a force of 30 N then the block will also experience the force of 30 N only. An important property of the massless rope should be that the total force on the rope must be zero at all times. The situation mentioned above is not physically possible and consequently, the massless rope can never experience the net force.
Thus, all the massless rope will experience the two equal and opposite tension forces. Tension and Pulleys:. The dynamics of a single rope is quite simple and easy as it transmits the applied force. But when pulleys are used instead of ropes then the complications arise. In the dynamical sense, the pulleys act to change the direction of the rope and they do not change the magnitude of the forces on the rope.
The diagram which is given above represents a small block on the left and it is lifted by the larger block on the right. Notice the forces T and -T in the figure. Even when the pulleys are used the rope must experience the two equal and opposite tension forces. In the figure above the rope actually experiences the two forces in the same direction, making the situation impossible. The presence of the pulley changes the situation to make it physically sustainable.
When rope and pulley are taking into existence it is useful to define a direction not in terms of up and down but in terms of the shape of the rope. In the above situation, we can define the positive direction on the rope as pointing up on the left side and pointing down on the right side of the pulley.
When the direction is defined in the way mentioned above the rope does actually experience the two equal and opposite force.
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