CHAPTER 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 action exerted on an object which may change the object’s state of rest or motion.

Three ways that force change motion Force can make: 1. An stationary object to move like throwing a ball 2. a moving object to stop like catching a ball 3. 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

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 force does 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.in an earlier unit

Types of force Applied Force F app An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Types of force Gravity Force (also known as Weight) F grav The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation: Fgrav = m * g where g = 9.8 N/kg (on Earth) and m = mass (in kg)

Types of force Normal Force F norm The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.

Friction Force F frict The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force - sliding and static friction. Thought it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below: F frict = µ F norm

Example#3 book is at rest on a tabletop. Diagram the forces acting on the book. See answer.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.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.See answer. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel. See answer.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.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.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.See answer. A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver. See answer.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.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.See answer. A car is coasting to the right and slowing down. Diagram the forces acting upon the car. See answer.See answer. Answers

Student guided practice Do practice problems 1-2 from you book page 122

closure Today we learned about the changes in force as well force diagrams We are going to continue with Newton’s First Law