2.  To describe how a force affects the motion of an object.  To interpret and construct free body diagrams.  To recognize Newton's laws of motion in real life situations.

3. When you push a grocery cart, a motor lifts an elevator, or a hammer hits a nail, a force is being exerted. If an object is at rest, to start it moving requires force, that is a force is needed to accelerate an object from zero velocity to moving. A force exerted in different directions has a different effect. So force has a direction as well as magnitude, and is indeed a vector. Intuitively, Force is any kind of push or pull on an object.

4. Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it. The tendency of an object to maintain its state of rest or uniform motion in a straight line is called inertia. http://www.youtube.com/watch?v=Q0Wz5P 0JdeU&feature=related

5. http://www.stevespanglerscience.com/experiment/egg-drop-inertia-trick Watch the following video and then discuss it in groups. Share your comments with the rest of the group You may try this at home, if and only if, you clean any mess you make.

6. The acceleration of an object is directly proportional to the net force acting on it, and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on the object. http://www.youtube.com/watc h?v=WzvhuQ5RWJE&featur e=related

8. For next class please bring the following materials per team: Tablecloth A full bottle of water (Closed) Unbreakable dinner plates, saucers (The heavier the better) Spoons (metallic preferably) Perform the magician’s tablecloth trick. Explain your results according to Newton’s first law of movement.

10. Whenever an object exerts a force on a second object, the second exerts an equal force in the opposite direction of the first. This law is sometimes paraphrased as “to every action there is an equal and opposite reaction” This is perfectly valid. But to avoid confusion it is very important to remember that the “action” force and the “reaction” force are acting on different objects. http://www.youtube.com/watch?v=cP0Bb3 WXJ_k&feature=related

11. In groups discuss: What exerts the force on a car? What makes the car go forward? Share your results with the rest of the group. http://www.youtube.com/watch?v=r8E5dUnLmh4&feature=related

12. The magnitude of the force of gravity on an object is commonly called the object’s weight. An object’s weight will vary according to the force of gravity effects on different planets or in space. For example de acceleration due to gravity on the moon is about one sixth the one on Earth.

13. The net force on an object at rest is zero according to Newton’s Laws. Therefore the downward force of gravity on an object must be balances by an upward force. When a contact force acts perpendicular to the common surface of contact, it is referred to as the Normal Force.

14. When solving problems involving Newton’s laws and force, it is important to draw a diagram showing all the forces on each object involved. Such diagram is called a free-body diagram. Choose one object, and draw an arrow to represent each force acting on it. If the problem involves more than one object, a separate free-body diagram is needed for each object.

15. Hint: Draw a free body diagram like this:

16. Solve for components of the force pulling the box. Now apply Newton’s Second Law in x to find the acceleration.

17. Note that the normal force is lower than the weight because part of the pull of the cord is in the upward direction.

18. Two boxes connected by a light cord (weightless) are resting on a frictionless table. The boxes have a mass of 12 kg and 10 kg. A horizontal force of 40.0 N is applied to the 10 kg box as shown. Find the acceleration of each box and the tension in the cord connecting the boxes.

19. Friction exists between two solid surfaces because of its roughness. Even the smoothest looking surface is quite rough on a microscopic scale. Sliding friction is usually called kinetic friction (kinetic is from the Greek for moving). When an object slides along a rough surface, the force of kinetic friction acts opposite to the direction of the object’s velocity.

22. There is also static friction, which refers to a force parallel to the two surfaces that can arise even when they are not sliding. Suppose you are pushing a desk and it does not move. You are exerting a force on the desk, but it isn’t moving so there must be another force on the desk keeping it from moving. (The net force is zero on an object that doesn’t move). This is the force of static friction exerted by the floor on the desk. If you push hard enough, the desk will eventually move, and kinetic friction takes over. So Static Friction:

23. Pull a 10.0 kg box with a cord as shown on the figure. The magnitude of the force is 40.0 N and is exerted at a 30 ̊ angle. Calculate the acceleration of the box. This time assume a coefficient of kinetic friction of 0.30.

24. www.youtube.com/watch?v=EgXOVGjIeyI The inclined plane is one of the original six simple machines. It is a flat surface whose endpoints are at different heights. By moving the object up the plane rather than vertically the amount of force needed is reduced at the expense of increasing the distance the object is moved.

25. Solving problems is usually easier if we chose the xy coordinate system so the x axis points along the incline and the y axis is perpendicular to the incline as shown in the figure. Remember that the normal force is not vertical but perpendicular to the sloping surface of the plane, yet weight is indeed vertical and pointing down.

26. The skier in the figure has just begun descending the 30 ̊ slope. Assuming the coefficient of kinetic friction is 0.10, calculate (a) her acceleration and (b) the speed she will reach after 4.0 s.

27. Watch the following video about static friction: www.youtube.com/watch?v=jSu0Tvlm6LY&feature=related www.youtube.com/watch?v=QMW_uYWwHWQ&feature=related Then answer the questions.

28. 1.What are the wooden clamps for? 2.In the first tug-o-war Adam and Jaime find that the main problem is traction, explain why? 3.What is the scale measuring? 4.What was the problem with using cars to pull the interleaved books? 5.What was the equivalent force needed to separate the books equivalent to? 6.Can you think of another real life situation where friction plays an important role?

29. Giancoli, Douglas C. Physics Sixth Edition. USA Pearson 2005 Serway, Raymond A. Essentials of College Physics. USA Thomson 2007