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Physics 151: Lecture 11, Pg 1
Physics 151: Lecture 11Physics 151: Lecture 11
Homework #4 (10/2/06, 5 PM) :
Problems from Ch.6 on the webassign
TopicsReview: Forces and uniform circular motion - Ch
A split highway has a number of lanes for traffic. For traffic going in one direction, the radius for the inside of the curve is half the radius for the outside. One car, car A, travels on the inside while another car of equal mass, car B, travels at equal speed on the outside of the curve. Which statement about resultant forces on the cars is correct ?
a. The force on A is half the force on B.b. The force on B is half the force on A.c. The force on A is four times the force on B.d. The force on B is four times the force on A.e. There is no net resultant force on either as long as they stay on the road while turning.
Physics 151: Lecture 11, Pg 3
Lecture 11,Lecture 11, ACT 2ACT 2
When a car goes around a circular curve on a level road,
a. no frictional force is needed because the car simply follows the road.
b. the frictional force of the road on the car increases when the car’s speed decreases.
c. the frictional force of the road on the car increases when the car’s speed increases.
d. the frictional force of the road on the car increases when the car moves to the outside of the curve.
e. there is no net frictional force because the road and the car exert equal and opposite forces on each other.
Earlier we saw that for an object movingin a circle with nonuniform speed then
a = ar + at .
dt
vda
rv
a
t
r
2
ar
at
What are Fr and Ft ?
Physics 151: Lecture 11, Pg 5
My match box car is going to do a loop the loop. What must be its minimum speed at the top so that it can make the loop successfully ??
Example Exercise 1Example Exercise 1
See Example 6.7
Physics 151: Lecture 11, Pg 6
• My match box car is going to do a loop the loop. What must be its minimum speed at the bottom so that it can make the loop successfully ??
• This is a difficult problem with just forces. We will deal with it with energy considerations in Ch. 9.
Example Exercise 2Example Exercise 2
See Example 6.8
Physics 151: Lecture 11, Pg 7
• My match box car is going to do a loop the loop. If the speed at the bottom is vB, what is the normal force at that point ??
Example Exercise 3Example Exercise 3
See Example 6.8
Physics 151: Lecture 11, Pg 8
• I am building a roller coaster. I design it so that when I do a loop by myself I am weightless at the top, and I have speed v1.
• Next my two friends get in with me so that the total weight of car and people doubles. How fast must the car go so we are still weightless at the top ??
When a pilot executes a loop-the-loop (as in figure on the right) the aircraft moves in a vertical circle of radius R=2.70 km at a constant speed of v=225 m/s. Is the force exerted by the seat on the pilot:
ExampleExampleGravity, Normal Forces etc.Gravity, Normal Forces etc.
Consider a women on a swing:
1. When is the tension on the rope largest ? 2. Is it : A) greater than
B) the same asC) less than
the force due to gravity acting on the woman(neglect the weight of the swing)
Animation
Physics 151: Lecture 11, Pg 12
Lecture 11,Lecture 11, ACT 5ACT 5
A rock attached to a string swings in a vertical circle. Which free body diagram could correctly describe the force(s) on the rock when the string is in one possible horizontal position ?
Your first job is with General Motors. You are working on a project to design an accelerometer. The inner workings of this gadget consist of a weight of mass m that is hung inside a box that is attached to the ceiling of a car. You design the device with a very light string so that you can mathematically ignore it. The idea is that the angle the string makes with the vertical is determined by the car’s acceleration. Your preliminary task is to think about calibration of the accelerometer. First you calibrate the measurement for when the car travels on a flat road.
You are a passenger in a car and not wearing your seatbelt. Without increasing or decreasing speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is a correct description of the situation ?
A) Before and after the collision there is a rightward force pushing you into the door.
B) Starting at the time of the collision, the door exerts a leftward force on you.