PHYS16 – Lecture 13 Ch. 7 Work and Energy

Announcements Test – Week of Feb. 28 during lab Formal reports and Notebooks due Tues. 5pm for M/T lab In-class Review on Friday – Ch. 1-6

Circular Motion Post-Questions

Circular Motion post-question A bicycle speedometer sensor is mounted on the bike at a fixed location, usually the fork, and pulses when the spoke-mounted magnet passes by. If the speedometer sensor is mounted at a radius of 0.50 m and counts 320 pulses, how far has the cyclist traveled? A)0.50 km B)0.80 km C)1.0 km D)1.5 km E)1.8 km sensor magnet

Circular Motion post-question At the point P indicated in the figure the string suddenly breaks. What is the trajectory of the ball?

Circular Motion post-question Tarzan is swinging across a lake on a vine that may not support his weight. What is the most likely point for the vine to break at? A C B D

Circular Motion post-question An object is swung in uniform circular motion vertically. Where is the tangential acceleration the greatest? A)At the top of the circle B)At the bottom of the circle C)At the point midway between top and bottom D)At both the top and bottom of the circle E)None of the above

Circular Motion post-question A pickup has a crate of apples in its bed. The pickup makes a right turn and the once stationary crate accelerates to the left with respect to the truck bed and hits the side of the pickup. In which direction is a force acting on the crate? A)To the right B)To the left C)Not enough information

Description of Motion – What else do we need? We have: – Laws of Calculus – Displacement, Velocity and Acceleration – Newton’s Laws – F=ma

We need energy…

Mechanical Energy – Potential Energy – Kinetic Energy Work – Constant Force → Dot Product – Variable Force → Integral Work-Mechanical Energy theorem Ch. 7 Work and Energy

Energy Pre-question A force F is used to raise a 4-kg mass from the ground to a height of 5 m, as shown in the diagram below. What is the work done by the force F? Ignore friction and assume the rope and pulleys are massless. A)50 J B)100 J C)174 J D)200 J E)None of the above F θ = 60 degrees mass

Demo – Name that Energy

Definition of Energy Energy – A quantity whose expenditure or transformation allows for physical activity – An ability to drive motion – A capacity for action Scalar Quantity Unit = Joule (J) = kg·m 2 /s 2 Comes in many forms – Thermal – Chemical – Mechanical!!!!!

Mechanical Energy Kinetic Energy (K)– energy stored in the movement of an object Potential Energy (U) – energy stored in the configuration of a system – Gravitational Potential Energy – Spring Potential Energy

Practice Question A 0.50 kg vase falls from 3.0 m. What is the kinetic energy of the vase just before it hits the ground? A) 0 J B) 15 J C) 1.5 J D) 2.3 J

Work

Definition of Work Mechanical Work (W) – energy transferred to an object due to the action of a force (+) transfer to object (-) transfer from object

Aside on Dot Product Dot Product is one way to multiply two vectors – Basically just multiply components and add – Dot Product is a scalar A B

Aside on Dot Product Dot Product is one way to multiply two vectors – Basically just multiply components and add – Dot Product is a scalar A B

Aside on Dot Product Dot Product is one way to multiply two vectors – Basically just multiply components and add – Dot Product is a scalar – Or multiply magnitudes and cosine angle between the vectors A B θ=56°

Work with a Constant Force Force = Constant, then can take force outside integral

Work with a Variable Force Force = Variable, then need to break into components and integrate FxFx x

Practice Questions I pull a 4.0 kg sled a distance of 5.0 m. I pull the sled using a rope at a 30.0 degree angle with a force of 5.0 N. The kinetic coefficient of friction is 0.1. What is the work done by me? A force is given by F x = 3x What is the work done by the force for moving an object from x=0.0 m to x=4.0 m? 22 J 200J

Conclusions Mechanical Energy – Potential Energy, Gravitational – Potential Energy, Spring – Kinetic Energy Work – Constant Force – Variable Force