The stretch of the spring is a measure of the applied force.

1. The stretch of the spring is a measure of the applied force.

2. Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change the state by forces impressed upon it.

3. Net Force The combination of forces that act on the object is the net force

4. Units of Force 1 newton (N) = 1 Kg *m/s2

5. acceleration ~ net force a ~ F

6. You push on a crate that sits on a smooth floor and it accelerates. If you apply four times the pushing force, how much greater will be the acceleration? • Same for a rough surface.

7. Friction If you apply a force to an object, a force of friction usually reduces the net force and resulting acceleration. The direction of friction force is always in the direction opposing motion. The force of dry friction between solid surfaces doesn’t depend on speed or area of contact.

8. Fluid friction is called drag Drag does depend on the speed and area of contact. For slow motion through water or air drag is approximately proportional to the speed of the object.

9. Question • A jumbo jet cruises at constant velocity of 1000 km/h when the thrusting force of its engines is constant 100000 N. • What is the acceleration of the jet? • What is the force resistance on the jet?

10. Mass and Force of Gravity Mass: The quantity of matter in an object. It is also the measure of the inertia or sluggishness, that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way. Force of Gravity= mg

11. Newton’s Second Law The net force on an object is equal to its mass times its acceleration and points in the direction of acceleration Fnet = ma

12. Important: acceleration of a body is always in the direction of net force!!!!

13. Free-body diagrams

14. Free fall revisited

15. When Acceleration is Less Than g – Nonfree Fall

16. The force of air drag experienced by the falling object depends on two things: • Frontal area of the falling object • The speed of the object

17. When the force of air resistance completely balances the force of gravity we say, that an object has reached its terminal speed.

18. Examples • Forces of 4 N and 6 N act on the object. What is the minimum value for the sum of these two forces? • Two ropes are being used to pull a car out of a ditch. Each rope exerts a force of 700 N on the car. Is it possible for the sum of these two forces to have a magnitude of 1000N? Explain your reasoning. • If the net force on a boat is directed due east, what is the direction of the acceleration of the boat? Would your answer change if the boat had a velocity due north but the net force still acted to the east?

19. Examples • Describe the force(s) that allow you to walk across the room. • You are analyzing a problem in which two forces act on an object. A 200 N force pulls to the right, and a 40 N force pulls to the left. Your classmate asserts that the net force is 200 N because that is the dominant force that is acting. What is wrong with that assertion?

20. Summary of Terms Mass Newton (units of force) Force Friction Free fall Terminal speed