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Unformatted text preview: 4-1 FORCES AND NEWTONS LAWS OF MOTION4Q4.1. Reason: If friction and air resistance are negligible (as stated) then the net force on the puck is zero (the normal force and gravitational force are equal in magnitude and opposite in direction). If the net force on the puck is zero, then Newtons first law states that it will continue on with constant velocity. So no force is needed to keep the puck moving; it will naturally keep moving unless a force acts on it to change its velocity. Assess: This question demonstrates the difference between Aristotelian thinking and Newtonian thinking. Objects do not need forces on them to keep them moving; forces are only required when we want to changethe velocity of the object. The reason one has to normally keep pushing an object to keep it moving is because of friction; to keep it at constant velocity your pushing force must be equal in magnitude to the friction force. But in the case of this question, there is no friction, so there is no force needed to keep it moving. Q4.2.Reason:Even if an object is not moving forces can be acting on it. However, the netforce must be zero. As an example consider a book on a flat table. The forces that act on the book are the weight of the book (a long ranger force) and the normal force exerted by the table (a contact force). There are two forces acting on the book, but it is not moving because the net force on the book is zero. Assess:The net force, which is the vector sum of the forces acting on an object, governs the acceleration of objects through Equation 4.4. Q4.3.Reason:Newtons first law does notstate that there can be no forces acting on an object moving with constant velocityonly that the sumof the forces must be zero (sometimes worded as no unbalanced forces). There can be forces acting on an object with constant velocity, but the vector sum of those forces must be zero. Assess:When we say there is no netforce on an object with constant velocity, we are not saying that there are no forces. Net force means the vector sum of the forces. Q4.4.Reason:The ball is initially and rest and will stay at rest unless some unbalanced (net) force acts on it. As the wagon moves forward, static friction tries to pull the ball along with the wagon. This static friction causes the ball to roll backwards in the wagon. Assess:Put a marble or some other small spherical object on your text and then push your text back and forth. The sphere will always roll in the direction opposite the direction you are moving the text. Q4.5. Reason: No. If you know all of the forces than you know the direction of the acceleration, not the direction of the motion (velocity). For example, a car moving forward could have on it a net force forward if speeding up or backward if slowing down or no net force at all if moving at constant speed....
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