1. Forces and Newton’s Laws of Motion

2. Forces  The natural state of an object –its behavior if free of external influences - is uniform motion with constant velocity.  “At rest” has no special significance in Galileo’s view of motion.  Influences that lead to deviations from uniform motion are called forces.

3. Newton’s First Law of Motion  Consider an object with no force acting on it. If it is at rest, it will remain at rest;  If it is moving, it will continue to move in a straight line at constant speed.  Consider the crash test dummy who continues in motion after the car has hit a wall. Always wear your seat belt! It’s just good physics!

4. Properties of forces  A force is a push or pull  A force acts on an object. Forces do not act in isolation.  A force requires an agent. An agent is something that pushes or pulls. It can be an inert object  A force is a vector with magnitude and direction. The size or strength of the force is its magnitude.

5. Properties of forces cont.  A force can be either a contact force or a long range force.  Contact forces are forces that act on an object by touching it at a point of contact (ex. Bat must hit ball to hit it)  Long range forces are forces that act on an object without physical contact (ex. Gravity or a magnet)

6. Forces  In general the object exerts a force back on the agent.  A force is really an interaction between two objects

7. Drawing force vectors

8.  F N is the normal force. Normal means perpendicular.  Include what is being acted on, by what. (object and agent) F g on the box by the earth FNFN

9. More Force Drawings  Forces can represent a push or pull. Either way they are depicted the same, acting on a particle

10. Combining Forces  The mathematical sumation is called the superposition of forces. The net force is the resultant.  F NET is not a new force – The original forces are replaced by F NET.

11. What is the net force?

12. Net force

13. Stop to Think  Two of the three forces exerted on an object are shown. The net force points directly to the left. Which is the missing third force? CCCC

14. Balanced Forces Forces are balanced if the object isForces are balanced if the object is Not moving orNot moving or At constant velocityAt constant velocity

15. Unbalanced Forces Unbalanced Forces  Forces are unbalanced if –The object starts moving –The object changes direction –The object stops If there is an unbalanced force an object will accelerate

16. Inertia  Inertia is an objects tendency to resist a change in motion  Remember the crash test dummy