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**Unformatted text preview: **Physics 100A Homework 4 Chapter 5 Newtons First Law A) If a car is moving to the left with constant velocity then the net force applied to the car is zero. B) An object cannot remain at rest unless the net force acting on it is zero. C) An object has constant acceleration if the net force acting on it is constant. Understanding Newtons Laws A) An object cannot remain at rest unless the net force acting on it is zero. B) If a block is moving to the left with constant velocity the net force applied to the block is zero. C) A block of mass 2 kg is acted upon by two forces: 3N (directed to the left) and 4N (directed to the right). What can you say about the block's motion? The net force is point to the right, so the object could - be moving to the right and accelerating to the right (speeding up) - be moving to the left and accelerating to the right (slowing down) - be at rest and on the verge of moving to the right D) A block pulled with a constant force will have a constant acceleration in the same direction as the force. E) Two forces, of magnitude 4N and10N, are applied to an object. The relative direction of the forces is unknown. The two limits for adding the forces give that the magnitude of the net force must be N F N net 14 6 So 5N is excluded Free-Body Diagrams Chadwick is pushing a piano across a level floor (see the figure). ( Intro 1 figure ) The piano can slide across the floor without friction. If Chadwick applies a horizontal force to the piano, what is the piano's acceleration? A), B) and C) The three forces acting on the piano are: Chadwick pushing to the right; gravity down; normal force from the floor up. Copyright 2010 Pearson Education, Inc. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. 5 1 Chapter 5 : Newtons Laws of Motion James S. Walker, Physics , 4 th Edition D), E) and F) When going up the inclined plane, the three forces acting on the piano are: Chadwick pushing parallel to the plane; gravity down; normal force up and perpendicular to the incline. Motion of a Block with Three Forces Three forces of magnitudes F 1 =4.0 N , F 2 =6.0 N, and F 3 =8.0 N are applied to a block of mass m=2.0 kg, initially at rest, at angles shown on the diagram. In this problem, you will determine the resultant (net) force by combining the three individual force vectors. All angles should be measured counterclockwise. A) Find the magnitude of the resultant 3 2 1 F F F F r r r r r + + = Add the x and y components independently ) 180 cos( ) 325 cos( ) 25 cos( 3 2 1 3 2 1 F F F F F F F x x x rx + + = + + = N F rx 54 . ) 180 cos( . 8 ) 325 cos( . 6 ) 25 cos( . 4 = + + = ) 180 sin( ) 325 sin( ) 25 sin( 3 2 1 3 2 1 F F F F F F F y y y ry + + = + + = N F ry 75 . 1 ) 180 sin( . 8 ) 325 sin( . 6 ) 25 sin( . 4 = + + = Copyright 2010 Pearson Education, Inc. Copyright 2010 Pearson Education, Inc....

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