What do we want to do today?! Sunday: 27-1-1440 A bit of housekeeping Push forward with chapter-7, including some problems Prof. Zain Yamani Office: 15-3100/ 3102; Phone: 860-4363; Mobile: 0504608515 E-Mail: zhyamani@kfupm.edu.sa; HP: http://faculty.kfupm.edu.sa/phys/zhyamani

A bit of housekeeping Attendance If you come late, please make me aware during the (end of) the same lecture.

Is it time for another evening Help Session? A bit of housekeeping Is it time for another evening Help Session?

Challenge: add friction!!  A racing car speeds on a circular (radius: R) track that is banked at angle (q) with the horizontal. The car mass is (m) and the coefficient of static friction between car and track is m. Draw a schematic of the problem. Calculate the maximum speed that the car can run without skidding off. In what ways can this “maximum” speed be improved? As always, Legible handwriting No broadcasting

Chapter-7: Energy Kinetic Energy Work Work done by Weight (gravity) Work done by Spring Power

Energy, kinetic energy and units of energy Work, force, displacement For point-like particles The work depends on the angle between force and displacement!

Work - (kinetic) Energy Theorem What if the force is not constant during the displacement (i.e. variable force)? Net work Work - (kinetic) Energy Theorem

Chapter-7 A 5.00 kg block moves in a straight line on a horizontal frictionless surface under the influence of a single force that varies with position x as shown in the Figure. If the block has a speed v = 3.00 m/s at x = 0.00 m, find the speed of the block at x = 8.00 m. A) 4.36 m/s B) 2.16 m/s C) 3.62 m/s D) 7.73 m/s E) 9.55 m/s 141-F

What if the motion is not vertical?? Let’s apply these ideas to work done by the earth’s gravitational force, as you throw up a ball for example. Explain what happens as the ball goes up, momentarily stops, and then falls down. What if the motion is not vertical??

What if the work is done by a spring? The relaxed: non-stretched/ non-extended, non- compressed state. The spring force is a (linear) “restoring” force. Spring (or Force) constant: k F = - k d What does big “k” mean? SI units for “k”?

Two balls are launched from the same spring-loaded cannon with the spring compressed the same distance each time. Ball A has a 40 kg mass and ball B has a 60 kg mass. The relation between their speeds at the instant of launch is: 141-2

Work done by a general variable force (omitted)

Average and Instantaneous Power P, units of P Pavg Pinstanaeous The horsepower = 746 watt

A constant tension force is used to pull a 50 A constant tension force is used to pull a 50.0 kg box up a frictionless plane inclined at 30.0° relative to the horizontal. The tension force is parallel to the incline. The box is moved a distance of 30.0 m along the incline with a constant speed of 1.00 m/s. At what rate is work done by the tension force? A) 245 W B) 49.3 W C) 98.0 W D) 292 W E) 495 W 101-2

A rope exerts a force F on on a 20 A rope exerts a force F on on a 20.0 kg crate in lifting it vertically upward above the floor. The crate starts from rest and accelerates upward at 5.00 m/s2. How much work was done by the force F in raising the crate 4.00 m above the floor? A) 1.18 kJ B) 1.98 kJ C) 2.50 kJ D) 1.71 kJ E) 0.704 kJ 101-2

A 0.50 kg object, moving along the x-axis, experiences the force shown in Figure 1. The object’s velocity at x = 0.0 m is v = 2.0 m/s, and at x = 4.0 m is v = 8.0 m/s. What is Fmax? A) 5.0 N B) 7.2 N C) 9.7 N D) 3.2 N E) 1.8 N 101-2

Chapter-7 slides 

Mathematica can choose 

!!Check official Phys-101 website!!

Have you ever used Mathematica?

A light object and a heavy object are initially sliding with equal speeds along a horizontal frictionless surface. Then, they both slide up the same frictionless incline. Which object rises to a greater height? A) They both slide to the same height. B) The heavy object, because it has greater kinetic energy C) The light object, because it has smaller kinetic energy. D) The light object, because it weighs less. E) The heavy object, because it weighs more. 101-2

A block is dropped from a high tower and is falling freely under the influence of gravity. Which one of the following statements is true? (Ignore air resistance). A) The kinetic energy increases by equal amounts over equal distances. B) As the block falls, the net work done by all of the forces acting on the block is zero joules. C) The kinetic energy of the block increases by equal amounts in equal times. D) The potential energy of the block decreases by equal amounts in equal times. E) The total energy of the block increases by equal amounts over equal distances. 141-2

Starting from rest, an elevator with a mass of 1 Starting from rest, an elevator with a mass of 1.00×103 kg moves 100 m vertically upward in 50.0 s. At what average rate does the force from the cable do work on the elevator?   A) 19.6 kW B) 11.1 kW C) 29.7 kW D) 31.3 kW E) 15.6 kW 141-2

Two balls are launched from the same spring-loaded cannon with the spring compressed the same distance each time. Ball A has a 40 kg mass and ball B has a 60 kg mass. The relation between their speeds at the instant of launch is: 141-2