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Just as PICTORIAL models and motion diagrams are useful in solving problems about MOTION,

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Presentation on theme: "Just as PICTORIAL models and motion diagrams are useful in solving problems about MOTION,"— Presentation transcript:

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2 Just as PICTORIAL models and motion diagrams are useful in solving problems about MOTION,

3 similar representations will help you analyze how FORCES affect motion.

4 Take the example of a ball tied to a string and sketch the situation to represent the forces acting on the ball.

5 CIRCLE the system and identify every place the system touches the EXTERNAL world.

6 It is at these places the contact forces are EXERTED.

7 Identify the CONTACT forces. Force from string

8 Then identify any FIELD forces on the system. This gives you the pictorial model. Gravity? Magnetic? Electric? Force of gravity

9 To make a pictorial representation of the forces acting on the ball, apply the particle model and represent the object with a DOT.

10 Represent each force with an arrow (VECTOR) that points in the direction that the force is applied.

11 Try to make the LENGTH of each arrow proportional to the SIZE of the force.

12 Always draw the force arrow pointing away from the object, even if it is a PUSH.

13 Make sure that you label each force.

14 Use an uppercase F with a SUBSCRIPT describing the force. F string F gravity

15 This type of physical model, which represents the forces acting on a system, is called a FREE- body diagram.

16 Draw free-body diagrams for the following objects.

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19 The only RULE for drawing free-body diagrams

20 is to depict all the forces which exist for that object in the given situation.

21 Thus, to CONSTRUCT free- body diagrams, it is extremely important to know the various types of forces.

22 If necessary, refer to the list of forces and their description in order to understand the various force types and their appropriate SYMBOLS.

23 Applied Force F app Contact Force An applied force is a force which is APPLIED to an object by a person or another object.

24 Gravity force F grav Field The force of GRAVITY is the force with which the earth, moon, or other MASSIVELY large object attracts another object towards itself. By definition, this is the WEIGHT of the object. The force of gravity on earth is always equal to the weight of the object as found by the equation: F grav = mg, where g = 9.8 m/s 2 (on Earth) and m = mass (in kg).

25 Normal Force F norm Contact The normal force is the SUPPORT force exerted upon an object which is in contact with another stable object. For example, if a book is resting upon a SURFACE, then the surface is exerting an upward force upon the book in order to support the weight of the book.

26 Friction Force F frict Contact The friction FORCE is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force - SLIDING and static friction. Thought it is not always the cast, the friction force often opposes the motion of an object.

27 Tension Force F tens Contact The tension force is the force which is TRANSMITTED through a string, rope, cable or WIRE when it is pulled tight by forces acting from opposite ends.

28 Spring Force F spring Contact The spring force is the force exerted by a compressed or STRETCHED spring upon any object which is attached to it.

29 Electrical F elec Field The electrical force is the force exerted by CHARGED particles on each other. Depending on the type of charged particles, F elec could be ATTRACTIVE or repulsive.

30 Magnetic F mag Field The magnetic force exerted between magnetic POLES, producing magnetization.


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