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 Physics 211 Lecture 4

Acceleration is caused by force. A bigger mass makes this harder Newton’s 2 nd Law 10

Checkpoint The net force on a box is in the positive x direction. Which of the following statements best describes the motion of the box : 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 15

Newton's second law states that the direction of the net force on an object is the same as the acceleration of that object. We don't know the initial velocity of the box B) Newton's second law states that the direction of the net force on an object is the same as the acceleration of that object. We don't know the initial velocity of the box If the force is parallel to the x-axis then the box will accelerate in that same direction. Therefore, the velocity will also be in that direction. C) If the force is parallel to the x-axis then the box will accelerate in that same direction. Therefore, the velocity will also be in that direction.. Checkpoint The net force on a box is in the positive x direction. Which of the following statements best describes the motion of the box : A) Its velocity is parallel to the x axis B) Its acceleration is 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 19

ACT 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 with 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 M F a Net Force Same acceleration, so larger mass has larger net force. 5M5M 22

Checkpoint 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. Since acceleration is directed towards the center of motion, so is the force. 24

Good Question! Please explain why, even though the force due to centripetal acceleration points inwards, the force we feel pushes us towards the outside edge of the circular path, and the cell phone and dice all move away from the center of the circle of travel. 25

Good Question! Please explain why, even though the force due to centripetal acceleration points inwards, the force we feel pushes us towards the outside edge of the circular path, and the cell phone and dice all move away from the center of the circle of travel. 25 An observer inside the car is in a NONINERTIAL reference frame. We are intuitively familiar with F = ma and that is valid only in an INERTIAL reference frame. In a NONINERTIAL reference frame, like a rotating reference frame, unexpected “forces” show up. These are usually called inertial forces; sometimes, even fictitious forces. We will only analyze or investigate motion as seen by INERTIAL reference frames.

Mechanics Lecture 4, Slide 9 A) Force is in the same direction as acceleration (in this case, centripetal). B) the acceleration is outwards, therefore the force is outwards E) Because the car is moving at a constant speed, so there is no a. We get the net force is zero. C) The car is moving forward so the net force must be forward. Checkpoint Responses 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.

Mechanics Lecture 4, Slide 10 A) Force is in the same direction as acceleration (in this case, centripetal). B) the acceleration is outwards, therefore the force is outwards E) Because the car is moving at a constant speed, so there is no a. We get the net force is zero. C) The car is moving forward so the net force must be forward. Checkpoint Responses 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.

* They can have also have tangential acceleration if their speed is not constant 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. (Important!) 4) There is no such thing as centrifugal force. 27

Momentum & Force 30

Checkpoint You are driving a car with constant speed around a horizontal circular track. The momentum of your car A) Points radially inward toward the center of the circular track B) Points radially 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. 32

A) The momentum is in the same direction as the force. C) It points in the same direction as v, which is forward Checkpoint Responses You are driving a car with constant speed around a horizontal circular track. The momentum of your car A) Points radially inward toward the center of the circular track B) Points radially 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. 34 Remember, Momentum is mass x velocity, P = m v So momentum always point the same as velocity.

A) The momentum is in the same direction as the force. C) It points in the same direction as v, which is forward Checkpoint Responses You are driving a car with constant speed around a horizontal circular track. The momentum of your car A) Points radially inward toward the center of the circular track B) Points radially 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. 34

Circular Motion 35

Momentum and Force 36

Ice-puckFake Forces: Coriolis Newton’s 1 st Law Inertial reference frames PLEASE 38 Demos: dishes, bear

Earth’s Rotation is important over large distances 38 And makes hurricanes rotate CCW (CW) in the Northern (Southern) hemisphere.

Forces come in pairs! Demo:Fire- cart Newton’s 3 rd Law 41

Remember, the forces in these “pairs” act on different objects. That is very important but is easy to overlook. Newton’s 3 rd Law Concerns 43

ACT 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. 45

ACT 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. 45

ACT 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. 47

ACT 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. I know everything has an opposite and equal reaction but how does a bug exert that much force on a car. 47

ACT 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? A) The small guy experiences the larger force. B) The football player experiences the larger force. C) Both experience the same force. 48

ACT 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? A) The small guy experiences the larger force. B) The football player experiences the larger force. C) Both experience the same force. 48