Forces and Newton’s Laws of Motion

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. Drawing force vectors • FN is the normal force. Normal means perpendicular. • Include what is being acted on, by what. (object and agent) FN Fgon the box by the earth

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. • FNET is not a new force – The original forces are replaced by FNET.

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

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

15. 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