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 5h comment How does parallel plate lose energy when deflecting charged particles What does you fire an electron inside the plates mean? Do you shoot an electron on the plate? Or is it just suddenly moving around inside the plate? 1d revised Walking and friction added 368 characters in body 1d answered Walking and friction 1d comment Find the magnitude of the resultant force which these engines exert on the rocket? A drawing would be appreciated to make the situation clear. For example, how does it make sense to talk about North and where is the y-axis? 1d comment Free body diagram for two masses on inclined plane with frictions @Gianolepo The green set of arrows is correct, so you should remove the purple ones. Friction is always pointing in the direction that will prevent relative motion. So in this case, while the bottom box is moving upwards (because of the pull), the weight tries to pull the top box downwards. To prevent the boxes from moving in different directions, friction must pull upwards on the top box to keep it following the other box, and friction pulls downwards on the bottom box to try to brake it to not move away from the top box. 2d revised Free body diagram for two masses on inclined plane with frictions deleted 1 character in body 2d answered Free body diagram for two masses on inclined plane with frictions 2d comment Free body diagram for two masses on inclined plane with frictions Another thing, about the wording: Friction is not due to a force; it is due to the tendency of (relative) motion. There can namely be friction without any other forces being involved - for example when a box slides over an asphalt road and slows down because of friction. 2d comment Free body diagram for two masses on inclined plane with frictions What are $F(A)$ and $F(B)$? And why are there three arrows on the top box? Could you tell exactly what each force is, and which box they act on? 2d comment Free body diagram with forces of friction Opposes relative motion! Just stating that friction opposes motion is a cause of confusion in examples like the given, where the surface (the truck) itself moves. 2d comment Free body diagram with forces of friction +1 As you say, the statement true for any kind of friction is: "Friction opposes relative motion". So, when the truck accelerates, friction will try to keep the box on the truck. So, friction tries to give the box the same acceleration. So, friction naturally MUST pull the same way as the truck accelerates in order to apply this acceleration. And this is true for both kinetic and static friction. Feb 3 comment Negative friction force, positive normal force @tristanslater You would never get it with a minus. The formula is as it is, empirically derived and containing non-negative variables. You seem to wish to determine the direction even when the formula only gives magnitude. Direction is not included in this formula Feb 3 comment Negative friction force, positive normal force You've made a slight text mistake. If positive is left and you move right, then friction will be positive (pointing to the left) and the force keeping you move is negative (to the right, in the motion direction) Feb 3 answered Negative friction force, positive normal force Feb 3 comment A possible proof that time traveling to the past with the aid of a wormhole is impossible? Do you have an argument to back up the idea that if time travel in a time machine will be possible, one can not travel farther back than to when the time machine was created? Feb 2 comment m1v(initial) +m2v (initial) = m1v (final) +m2v(final) Apart from the homework, which should not be solved by this forum, what is the physics question? Feb 2 comment A possible proof that time traveling to the past with the aid of a wormhole is impossible? "If time travel is possible, where are the tourists from the future?" - Stephen Hawking Jan 30 comment Does inelastic collision mean the colliding particles have to necessarily stick? @dmckee Thanks for the correction. Jan 30 comment Does inelastic collision mean the colliding particles have to necessarily stick? Kinetic energy is not about direction of velocity but only about the speed. For a perfectly inelastic collision all this initial kinetic energy is "spent" and transformed in the collision. Jan 30 comment Why is that a question can be answered with several theories? @SufyanNaeem Flat Earth theory... Well... If you make a theory from your observations, then it isn't settled that this theory holds for other areas that you have not yet investigated! Earth might look flat from our eyes' perspective and from "local" measurements, so all models and theories can very well work WITHIN this range. Later on the Earth was found to be round... So a new model was established - it still works in the local area, as the large curvature appears flat over shorter distances, but also it correctly models the whole Earth. And has never since been experimentally disproven...