# Physics 211 Lecture 4, Slide 1 PHYSICS Today's Concepts: Newton’s Laws: a)Acceleration is caused by forces b)Force changes momentum c)Forces always come.

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Physics 211 Lecture 4, Slide 1 PHYSICS Today's Concepts: Newton’s Laws: a)Acceleration is caused by forces b)Force changes momentum c)Forces always come in pairs d)Good reference frames SR1 SR3 SR4 SR2 SR5

Physics 211 Lecture 4, Slide 2 Acceleration is caused by force. A bigger mass makes this harder

Physics 211 Lecture 4, Slide 3 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: A) Its velocity is parallel to the x axis B) Its acceleration parallel to the x axis C) Both its velocity and its acceleration are parallel to the x axis D) Neither its velocity or its acceleration need be parallel to the x axis 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 Vote again

Physics 211 Lecture 4, Slide 4 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: A) Its velocity is parallel to the x axis B) Its acceleration parallel to the x axis C) Both its velocity and its acceleration are parallel to the x axis D) Neither its velocity or its acceleration need be parallel to the x axis Your preflight results:

Physics 211 Lecture 4, Slide 5 Act5M M F a 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) F M < F 5M B) F M = F 5M C) F M > F 5M Net Force Same acceleration, so larger mass has larger net force.

Physics 211 Lecture 4, Slide 6 Preflight/Act You are driving a car with constant speed around a horizontal circular track. The net force acting on your car A)Points radically inward toward the center of the circular track B)Points radically outward, away from the center of the circular track C)Points forward in the same direction your car is moving D)Points backward, opposite to the direction your car is moving E)Is zero.

Physics 211 Lecture 4, Slide 7 You are driving a car with constant speed around a horizontal circular track. The net force acting on your car A)Points radically inward toward the center of the circular track B)Points radically outward, away from the center of the circular track C)Points forward in the same direction your car is moving D)Points backward, opposite to the direction your car is moving E)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..

Physics 211 Lecture 4, Slide 8 Preflight/Act You are driving a car with constant speed around a horizontal circular track. The net force acting on your car A)Points radically inward toward the center of the circular track B)Points radically outward, away from the center of the circular track C)Points forward in the same direction your car is moving D)Points backward, opposite to the direction your car is moving E)Is zero. Centripetal acceleration is always pointed inward, and the net force points in the same direction as acceleration.

Physics 211 Lecture 4, Slide 9 Wineglass demo 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. - Friction, gravity – whatever force keeps it moving in a circle. - This force is often called the “centripetal force” - This force is often called the “centripetal force” * They can have also have tangential acceleration if their speed is not constant 3) There is no “new” kind of force here. 4) There is no such thing as centrifugal force.

Physics 211 Lecture 4, Slide 10

Physics 211 Lecture 4, Slide 11 Tablecloth & Dishes

Physics 211 Lecture 4, Slide 12 Preflight/Act You are driving a car with constant speed around a horizontal circular track. The momentum of your car A)Points radically inward toward the center of the circular track B)Points radically outward, away from the center of the circular track C)Points forward in the same direction your car is moving D)Points backward, opposite to the direction your car is moving E)Is zero. The momentum points in the same direction as the velocity, which is forward.

Physics 211 Lecture 4, Slide 13 Ice-puck Fake Forces: Coriolis (YouTube) Fake Forces: Coriolis (YouTube)

Physics 211 Lecture 4, Slide 14 This effect can make cannons miss their target if you don’t take it into account. Hurricane Catrina And makes hurricanes rotate CCW (CW) in the Northern (Southern) hemisphere.

Physics 211 Lecture 4, Slide 15 Forces come in pairs! Fire-cart

Physics 211 Lecture 4, Slide 16 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? A.The small guy. B.The football player. C.They experience the same force.

Physics 211 Lecture 4, Slide 17 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? A.The small guy. B.The football player. C.The accelerations are the same.

Physics 211 Lecture 4, Slide 18 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? 1.The small guy experiences the larger force. 2.The football player experiences the larger force. 3.Both experience the same force.

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