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Phys151_PPL5 - Net Force Force being related to...

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Net Force Force, being related to acceleration, is a vector quantity. If I push a stationary block to the right, it does not move to the left or anywhere else! What happens if you have more than one force acting at the same time ? Compounding forces: You cannot just simply add them numerically! (See Fendt) Example 1: 10 N to right with 8N to left. Shown through force diagram, net force is 2N to right . F Motion with a 10 N 8 N Net is 2N to right. Motion with a Example 2: Two forces at right angles. Represent forces by arrows, to be “added” tip to end. Net force
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Fun with Forces An object is being pushed horizontally with a force of 50 N and no motion is observed. How is that possible? A car is traveling at a constant speed along a straight road. What is the net force on it? What if the road is bending? There must be (an invisible) friction force of 50 N against it. There is then no unbalanced force. Constant velocity means no acceleration, therefore zero net force. Engine’s “force”is counterbalanced by road’s friction. Though speed is constant, direction is not. Therefore velocity is changing . There is a non-zero net force!
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More Fun with Forces If gravity is acting on the ball in my hand, why does the ball move only when I release it? The brakes of a moving car are applied. Why is the car’s motion similar to that of a ball thrown upwards? What is the purpose of the headrest on a car seat? Because your hand exerts an equal and opposite force to gravity. Net force is zero! The car will have a force acting opposite to its direction of motion, like gravity on the ball moving upwards. For safety in rear-end collisions. Because of the Law of Inertia, your head will “snap back”.
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Elevator Physics Why does your “weight” change when you ride up and down an elevator? Your “weight” (S) is given by the bathroom scale on which you stand. The push of the floor -transmitted through the scale - and your true weight (W) - measuring the Earth’s attraction on you - are the only forces acting on you during the motion. They account for the acceleration you experience. a = 0, i.e. v is constant so elevator is stopped or coasting up or down, Hence S = W S W a = - , i.e. v is speeding up going up; v is slowing down coming down.
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