Forces and Motion Show Floating and sinkingPhysics Narrative for 11-14 Buoyancy forces cause things to rise in fluidsHere are four situations where a non-zero resultant force predicts a change in motion:
In all cases the resultant force determines the change in motion: from rest to moving off in the direction determined by the sum of the forces. A volume of warmer air or warmer water can take the place of the balsa wood or the helium filled balloon. The buoyancy forces on it are greater than the gravity forces, so it starts to rise. Similarly a volume of colder fluid will sink, as the gravity force will be greater than the buoyancy force. These are convection currents. Are convection currents important? Yes! Convection streams dominate our global environment, appearing in ocean currents and the winds and driving both plate tectonics and the dynamo that produces the Earth's magnetic field. Convection currents: volumes of warmed fluid moving aroundIn the process of convection, volumes of already warmed liquids and gases move around. Convection works by convection currents. These are streams of liquids and gases which carry warm volumes from a hot place to cooler parts of the system. So, as with conduction, convection still relies on particles to shift the energy (SPT: Energy topic). The crucial difference from conduction is that in convection there is mass movement of the particles. How do convection streams get started, and what drives them? It's all a matter of floating and sinking. Energy shifted from the thermal store associated with the hot object (such as the radiator in our example) to the thermal store of a volume of particles warms that volume (the air above the radiator). This process is conduction: the pathway is heating by particles. The volume of air expands, but still contains the same number of particles (in other words its density is reduced). This air then rises, floating up through the cooler, denser air around it. As the air moves it takes a quantity of energy away with it. It's not a case of heat rising, but of the warmed air floating up through the surrounding cooler air. As the warm air rises, cool air moves in to take its place. The process is continuous, with a rising stream of air moving away from the hot radiator. This upward movement is a clear pointer to distinguish convection from conduction. Although the mechanism of convection has been described here in relation to the warm air rising from the radiator example, precisely the same processes occur when convection currents are set up in liquids. Recommended textbook solutions Fundamentals of Physics10th EditionDavid Halliday, Jearl Walker, Robert Resnick 8,522 solutions College Physics11th EditionChris Vuille, Raymond A. Serway 5,036 solutions Principles and Practice of Physics1st EditionEric Mazur 2,411 solutions Conceptual Physics11th EditionPaul G. Hewitt 3,117 solutions ForceA force is an interaction between objects that tends to produce acceleration of the objects. A force is a vector, with both a magnitude and a direction. The MKS unit of force is the newton (N). 1 N = 1 kg m / s2. Acceleration occurs when there is a net force on an object; no acceleration occurs when the net force (the sum of all the forces) is zero. An acceleration produces a change in velocity (magnitude and/or direction), so an unbalanced force will change the velocity of an object. Isaac Newton (1642-1727) studied forces and noticed three things in particular about them. These are important enough that we call them Newton's laws of motion. Newton's First Law
Newton's first law: an object at rest tends to remain at rest, and an object in motion tends to remain in motion with a constant velocity (constant speed and direction of motion), unless it is acted on by a nonzero net force. The net force is the sum of all the forces acting on an object. The tendency of an object to maintain its state of motion is known as inertia. Mass is a good measure of inertia; light objects are easy to move, but heavy objects are much harder to move, and it is much harder to change their motion once they start moving. Do Newton's laws apply all the time? As long as we're in a stationary reference frame, or even moving at constant velocity, Newton's law are valid. Such reference frames are called inertial reference frames. Newton's Laws are not enough to account for motion observed from non-inertial (accelerating) reference frames. Newton's Second LawWhat is the acceleration produced by applying a force to an object? Newton's second law states that the acceleration of an object is proportional to the net force and inversely proportional to the mass of the object. Newton's Second Law: ΣF = m a Newton's Third LawA force is an interaction between objects, and forces exist in equal-and-opposite pairs. These statements are summarized by: Newton's third law: when one object exerts a force on a second object, the second object exerts an equal-and-opposite force on the first object. "equal-and-opposite" is short for "equal in magnitude but opposite in direction". Although the forces between two objects are equal-and-opposite, the effect of the forces may or may not be similar - it depends on the relative masses of the objects. If we drop a 100 g (0.1 kg) ball, it experiences a downward acceleration of 9.8 m/s2, and a force of about 1 N, because it is attracted towards the Earth. The ball exerts an equal-and-opposite force on the Earth, so why doesn't the Earth accelerate upwards towards the ball? It does, but the mass of the Earth is so large (6.0 x 1024 kg) that the acceleration is much too small (about 1.67 x 10-25 m/s2) for us to notice. When objects have similar mass, the equal-and-opposite pairs of forces are much easier to see. Force of GravityWhenever two objects are touching, they usually exert forces on each other. The force of gravity, on the other hand, is an example of a force that exists between objects without them having to be in contact. Objects with mass exert forces on each other via the force of gravity. This force is proportional to the mass of the two interacting objects, and is inversely proportional to the square of the distance between them.
The factors G, M, and r are the same for all masses at the surface of the Earth. We roll those factors together into the constant g, which we call the acceleration due to gravity. At the Earth's surface the gravitational force exerted on an object of mass m by the Earth has a magnitude mg and is directed down. TensionWhenever we use a string or rope to exert a force on an object, we're creating tension in the rope that transmits the force we exert at one end of the rope to the object at the other end. This force is usually labeled T. We usually assume that the rope has no mass, and does not stretch. When we exert a certain force on our massless unstretchable rope, the rope exerts that same force on the object. Tension makes the rope feel like it's being pulled apart. One rule to remember - you can't push with a rope. The tension force always goes along a string or rope away from the object attached to it. The Normal ForceObjects in contact generally exert forces on one another. A book rests on a table: the book exerts a downward force on the table, and the table exerts an equal-and-opposite force up on the book. We call this the normal force - "normal" is the technical physics word for perpendicular. The normal force is perpendicular to the interface where the book meets the table. The normal force is one component of the contact force between objects, the other component being the frictional force. The normal force is usually symbolized by N. When the normal force is the only thing counteracting the force of gravity, the normal force is equal in magnitude to the force of gravity. This is not always true - always be careful to calculate the normal force by applying Newton's second law. Objects lose contact with one another when the normal force goes to zero. The normal force is the force that would be measured by a scale placed between the objects in contact. Your "apparent weight" (how heavy you feel) is directly related to the normal force you are experiencing. What happens when a non zero net force acts on an object?If the net force acting on an object is non zero, then there is an acceleration or deceleration. Either the object is increasing or decreasing its speed.
What kind of motion does a non zero resultant force produce on an object?Resultant forces change motion
When the forces do not add to zero (there is a non-zero resultant force), the object will either speed up, slow down or change direction.
What is a constant nonzero net force?There can be several forces acting on a body at the same time. The vectorial sum of all these forces is called the net force. If after the sum, all these forces do not cancel each other, the remaining force is called the non-zero net force. Let us assume, F 1 = F 2 = F 3 , where the magnitudes of the forces are equal.
When a constant non zero net force acts on an object the object moves with?Answer and Explanation: According to Newton's Second Law, every non-zero force is associated with an acceleration in the same direction as the force. Sometimes it happens that the system of forces acting on an object cancel out and there is no net force.
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What kind of motion does a constant, non-zero net force produce on an object of constant mass?
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