Chapter 5 Newton’s Second Law of Motion- Force and Acceleration

1. Chapter 5Newton’s Second Law of Motion- Force and Acceleration Conceptual Physics Hewitt, 1999

2. 5.1 Force Causes Acceleration • To change velocity, you need to accelerate, decelerate or change direction • The only way to change something’s state of motion is to apply a force • The more you apply a force, the more you can change something • Acceleration ~ Force • ~ means directly proportional • As one changes, the other changes in the same manner • If force doubles, acceleration doubles • Example: Hours preparing for class (HW & studying) ~ grade earned • Net Force- the sum of all forces; dictates acceleration • Recall: acceleration = (change in velocity) / (time interval) • a=Dv/Dt

3. 5.2 Mass Resists Acceleration • If mass has inertia and inertia resists change, the more mass you have, the more resistant it is to change • Acceleration ~ 1/mass • Inversely proportional to each other • As one changes, the other changes in the opposite manner • If mass doubles, acceleration is cut in half • Example: Keeping your room clean ~ 1/(mom yelling at you)

4. 5.3 Newton’s Second Law • “The acceleration of an object is equal to the net force acting on the object, divided by the mass of the object.” • a = F/m • Also can be used as F=ma

5. 5.4 Friction • Friction- acts in the opposite direction to the applied force • Caused by surface irregularities • Rough and scratchy versus smooth and silky • High friction makes you need larger forces • Low friction allows for smaller forces to get the same acceleration • Physlet I.4.4

6. Drag • Drag force- the force exerted by a fluid on the object moving through the fluid • Gas or liquid • Dependant on size and shape of object, properties of fluid, velocity of object

7. 5.5 Applying Force- Pressure • Although the weight of a book on the table won’t change depending on which way it lays, the amount of pushing you have to do to get it to move will change • When the book is flat, you need to push harder than when it is on its edge because there is more area in contact with the table • Pressure- force per unit of area • P = F/A • Measured in Pascals (Pa)

8. Bed of Nails • Would you rather sit on a single nail? • Or would you rather sit on 900 nails?

9. 5.6 Free Fall Explained • Galileo explained that regardless of the size of the object, it will fall at the same rate (1kg vs 100kg cannon ball) • A larger mass has more gravity acting on it than a smaller mass • Since the acceleration due to gravity is always g (9.8 m/s2), F/m will always be true, regardless of m F/m = F/m • Example: Find g for both cannonballs. • For 1kg: a = F/m = weight/mass = (9.8x1)/1 = 9.8m/s2 • For 100kg: a = F/m = weight/mass = (9.8x100)/100 = 9.8m/s2

10. Elephant vs Man

11. 5.7 Falling and Air Resistance • Terminal velocity- reached when drag force equals the force of gravity • An application of drag • Only present when air is present • No air resistance in a vacuum

12. New Formulae & Assessments • P=F/A • Review Questions 1-18 • Think & Explain 1-10