Physics 211 Lecture 4

1. SR1 SR2 Physics 211 Lecture 4 SR3 SR4 SR5 • Today's Concepts: • Newton’s Laws: • Acceleration is caused by forces • Force changes momentum • Forces always come in pairs • Good reference frames

2. Stuff you asked about: The directions of force, velocity, and acceleration Nothing, lets go on to Rotation!! Third newtons law i understood it all Inertial/non-inertial reference frames. none. This is old material. I did 98% of this in high school: We just didn't call things derivatives. But I would be interested in an example where the mass of an object changes. This actually isn't something that was discussed in the prelecture, but something that has always bothered me is the derivation of the centripetal force on a banked ramp (it's inverse tangent of v^2/r or something along those lines. centrifugal force

3. Acceleration is caused by force. A bigger mass makes this harder

4. B) Acceleration is in the same direction as the net force, but the net force tells us nothing about its velocity. C) acceleration is in the same direction as the force, and velocity is the same direction as acceleration, so all must be parallel Preflight/Act • Suppose the net force on some object is parallel to the x axis. Which of the following statements best describes the motion of the object: • Its velocity is parallel to the x axis • Its acceleration parallel to the x axis • Both its velocity and its acceleration are parallel to the x axis • Neither its velocity or its acceleration need be parallel to the x axis Vote again

5. Preflight/Act • Suppose the net force on some object is parallel to the x axis. Which of the following statements best describes the motion of the object: • Its velocity is parallel to the x axis • Its acceleration parallel to the x axis • Both its velocity and its acceleration are parallel to the x axis • Neither its velocity or its acceleration need be parallel to the x axis Your preflight results:

6. Act Net Force A force F is applied to a small block, that pushes a larger block. The two blocks accelerate to the right. Compare the NET FORCE on the block w/ mass M, to the net force on the block with mass 5M. A) FM < F5M B) FM = F5M C) FM > F5M Same acceleration, so larger mass has larger net force. 5M a F M

7. Preflight/Act • You are driving a car with constant speed around a horizontal circular track. • The net force acting on your car • Points radically inward toward the center of the circular track • Points radically outward, away from the center of the circular track • Points forward in the same direction your car is moving • Points backward, opposite to the direction your car is moving • Is zero.

8. You are driving a car with constant speed around a horizontal circular track. • The net force acting on your car • Points radically inward toward the center of the circular track • Points radically outward, away from the center of the circular track • Points forward in the same direction your car is moving • Points backward, opposite to the direction your car is moving • Is zero. A) the acceleration is pointing in, so the force must too . B) the force points outward, explaining why your body shifts to the right when making a left turn. E) Because there is no acceleration, the force is 0. .

9. Centripetal acceleration is always pointed inward, and the net force points in the same direction as acceleration. Preflight/Act • You are driving a car with constant speed around a horizontal circular track. • The net force acting on your car • Points radically inward toward the center of the circular track • Points radically outward, away from the center of the circular track • Points forward in the same direction your car is moving • Points backward, opposite to the direction your car is moving • Is zero.

10. Aside: Centripetal acceleration and force 1) Objects moving in a circle always have a component of acceleration, called centripetal, which is toward the center of the circle.* 2) Centripetal acceleration must be caused by a force: - Friction, gravity – whatever force keeps it moving in a circle. - This force is often called the “centripetal force” 3) There is no “new” kind of force here. 4) There is no such thing as centrifugal force. Wineglass demo *They can have also have tangential acceleration if their speed is not constant

12. Tablecloth & Dishes

13. The momentum points in the same direction as the velocity, which is forward. Preflight/Act • You are driving a car with constant speed around a horizontal circular track. • The momentum of your car • Points radically inward toward the center of the circular track • Points radically outward, away from the center of the circular track • Points forward in the same direction your car is moving • Points backward, opposite to the direction your car is moving • Is zero.

14. Ice-puck Fake Forces: Coriolis (YouTube)

15. Hurricane Catrina And makes hurricanes rotate CCW (CW) in the Northern (Southern) hemisphere. This effect can make cannons miss their targetif you don’t takeit into account.

16. Forces come in pairs! Fire-cart

17. A small guy and a large football player moving at the same speed collide head-on. Which person experiences the larger force during the collision? The small guy. The football player. They experience the same force.

18. A small guy and a large football player moving at the same speed collide head-on. Which person experiences the larger acceleration during the collision? The small guy. The football player. The accelerations are the same.

19. A small guy moving at a high speed collides with a stationary large football player. Now, which person experiences the larger force during the collision? The small guy experiences the larger force. The football player experiences the larger force. Both experience the same force.