Chapter 4

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Chapter 4 . 4-1 Changes in motion. Objectives . Describe how force affects the motion of an object. Interpret and construct free body diagrams. What is force?. What comes to mind when you think about force? What is force?

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Chapter 4

4-1 Changes in motion

Objectives
• Describe how force affects the motion of an object.
• Interpret and construct free body diagrams
What is force?
• What comes to mind when you think about force?
• What is force?
• A force is an actionexerted on an object which may change the object’s state of rest or motion.
Three ways that force change motion

Force can make:

• An stationary object to move like throwing a ball
• a moving object to stop like catching a ball
• A moving objet to change direction like hitting a ball

Notice that in all cases the force is responsible for a change in velocity with respect of time-acceleration.

Si unit of force
• It is the newton’s
• Name after Sir Isaac Newton , whose work contributed much to the understanding of force and motion.
• The newton(N) is defined as the amount of force that, when acting on a 1 kg mass , produces an acceleration of 1
Different classes of forces
• When you pull on a spring , it stretches . If you pull on a wagon, the wagon moves . The pushes and pulls ae examples of contact forces.
• Contact forces results between physical contact between two objects.
• Field forcedoes not involve physical contact between two objects. One example is gravitational force . Whenever an object falls to Earth, the object is accelerated by earth gravity. , In other words, earth exerts a force on the object even when earth is not in immediate physical contact.
Classes of forces
• Another example for field force is the attraction or repulsion between electric charges.
forces
• When you push a car toy, it accelerates. If you push the car harder, it’s acceleration increases.
• In other words, the acceleration depends on the force’s magnitude.
• The direction in which the car moves depends on the direction of the force.
Free body diagram
• Drawing Free-Body Diagrams
• Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. A free-body diagram is a special example of the vector diagrams that were discussed in an earlier unit. These diagrams will be used throughout our study of physics. The size of the arrow in a free-body diagram reflects the magnitude of the force. The direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labeled to indicate the exact type of force. It is generally customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction that the force is acting. An example of a free-body diagram is shown at the right.
Example#3
• book is at rest on a tabletop. Diagram the forces acting on the book. See answer.
• A girl is suspended motionless from the ceiling by two ropes. Diagram the forces acting on the combination of girl and bar. See answer.
• An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it is falling. See answer.
• A flying squirrel is gliding (no wingflaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel. See answer.
• A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. See answer.
• A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. See answer.
• A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. Diagram the vertical forces acting on the backpack. See answer.
• A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver. See answer.
• A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration. Diagram the forces acting upon the sled. See answer.
• A football is moving upwards towards its peak after having been booted by the punter. Diagram the forces acting upon the football as it rises upward towards its peak. See answer.
• A car is coasting to the right and slowing down. Diagram the forces acting upon the car. See answer.