Forces and the Laws of Motion Changes in Motion

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

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

3. 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

4. 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/s2 • Therefore, 1 N = 1 kg x 1m/s2

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. Force Diagram Free-Body Diagram

15. 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