Forces and the Laws of Motion Changes in Motion Chapter 4:Section 1

Learning Targets Describe how force affects the motion of an object Interpret and construct free body diagrams P3.1A, P3.2A

What is Force??? Forces describe the interactions between objects and their environment –You exert a force on a ball when you throw it or kick it –You exert a force on a chair when you sit in it

Units of Force The SI unit of force is the newton –The newton (N) is defined as the amount of force that, when acting on a 1 kg mass, produces and acceleration of 1 m/s 2 Therefore, 1 N = 1 kg x 1m/s 2

Contact Forces Forces can act through contact or at a distance Contact forces result from physical contact between two objects –Stretching a spring, pulling a wagon, catching a football –Usually easy to identify

Field Forces Field forces do not involve physical contact between two objects –Gravity, attraction or repulsion between electric charges According to the theory of fields, masses create gravitational fields in the space around them

Reviewing Forces Remember that a force is a push or pull on an object that results from the interactions between objects and their environment Force is a vector quantity because it has both a magnitude and a direction

Balanced vs. Unbalanced Forces An object that is experiencing a balanced force, will remain at rest –Since balanced forces are of equal magnitude and in opposite directions, they balance each other Objects experiencing unbalanced forces will undergo a change in motion

Forces and Acceleration Forces can cause accelerations –In many situations, a force exerted on an object can change the object’s velocity A change in velocity means an object is accelerating –Forces can cause stationary objects to move, moving objects to stop, or moving objects to change direction

Relating Force and Weight The weight of an object is a measure of the magnitude of the gravitational force exerted on the object –Many of the terms and units used every day to talk about weight are really units of force that can be converted to newtons

Force Diagrams Force has both a magnitude and a direction –Therefore it is a vector Diagrams that show force vectors as arrows are called force diagrams

Drawing Force Diagrams When drawing a force diagram,the tail of the vector arrow is attached to the object on which the force is acting A force vector points in the direction of the force, and its length is proportional to the magnitude of the force

Free-Body Diagrams While force diagrams represent all the forces acting in a situation, free-body diagrams represent only the forces acting on the object of interest –The force exerted by the object on other objects are not included in the free-body diagram because they do not affect the motion of the object

Force Diagram Free-Body Diagram

Drawing Free-Body Diagrams A physics book is at rest on a desk. Gravitational force pulls the book down. The desk exerts an upward force on the book that is equal in magnitude to the gravitational force. Draw a free-body diagram of the book. Draw a free-body diagram of a person pulling a sled. The magnitudes of the forces acting on the sled are 60 N by the string, 130 N by the Earth (gravitational force), and 90 N upward by the ground