1. Everyday Forces Chapter 4 Section 4 Page 141

2. Weight vs. Mass … What’s the Difference?  Weight takes into account the force of gravity  Imagine holding a bowling ball and a tennis ball You know the bowling ball weighs more than the tennis ball This is because the bowling ball has more mass, therefore the force of gravity pulls more strongly on the bowling ball

3. Weight vs. Mass  Mass is an inherent property  Weight takes into account location Weight of an astronaut on Earth vs. weight of the same astronaut on the moon Where is the weight going to be heavier? Why?  We see this here on Earth, too Objects have less weight at higher altitudes

4. Weight  We can calculate weight when the mass and the acceleration of an object is known F g = mg F g = weight g = magnitude of acceleration due to gravity (also known as free fall acceleration) g = 9.8 m/s 2

5. Forces work in pairs, remember?  Imagine a TV resting on a table  F g is pulling downward on the TV  But the TV is in equilibrium, so what is acting in the opposite direction? Remember, forces work in pairs Equilibrium … equal but opposite  Something must be acting on the TV in an opposite direction …

6. Normal Force  A force exerted by one object on another object in a direction perpendicular to the surface of contact.  The word normal is used because one meaning of the word normal is perpendicular  F n

7. The Force of Friction  Open your book to page 142  Static Friction The restive force that opposes the relative motion of two contacting surfaces that are at rest with respect to one another  Kinetic Friction The resistive force that opposes the relative motion of two contacting surfaces that are moving past one another

8. The Force of Friction  Static friction F s If the object does not move, F s = -F applied Maximum value of static friction = F s,max  Kinetic friction When applied force exceeds F s,max, then the object begins to move The retarding (slowing force) on the object in motion is called kinetic friction F k

9. Coefficient of Friction  The quantity of friction acting on a particular surface of contact  Symbol = μ mu (Greek)  It is a ratio of between the force of friction and the normal force between the two surfaces

10. Coefficient of Friction  Turn to page 144 – Look at Table 4-2  There are different coefficients of friction for different forces