1. Physics 103: Lecture 8 Application of Newton's Laws
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Leftovers from Chapter 4
How to solve problems 1

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4. From Lecture 1: Course Philosophy
Basic Course Philosophy
read about it (text)
untangle it (lectures)
play with it (labs)
challenge yourself (homework)
close the loop (discussion)
Do make use of discussion, office hours, etc.
Try the homework during the weekend so that you
can ask questions during the week
Homework is posted Friday and is due the following Friday.
Exam worries:
Will post practice exams by Monday
You can bring a single 8.5”x11” sheet with equations to exam

5. Preflight 8: Question 1
Consider a person standing in an elevator that is accelerating upward. The upward normal force N exerted by the elevator floor on the person is
a) larger than
b) identical to
c) less than
the downward weight W of the person.
Example 4.11 in the book! mg N
Person is accelerating upwards - net upwards force is non zero

6. Preflight 8: Question 3
You are pushing a wooden crate across the floor at constant speed. You decide to turn the crate on end, reducing by half the surface area in contact with the floor. In the new orientation, to push the same crate across the same floor with the same speed, the force that you apply must be about
a) four times as great
b) twice as great
c) equally as great
d) half as great
e) one-fourth as great
as the force required before you changed the crate orientation.
Frictional force does not depend on the area of contact. It depends only on
the normal force and the coefficient of friction for the contact.

7. Tension
Tension can be transmitted around corners If there is no friction in the pulleys, T remains the same
Tension is a force along the length of a medium

8. Example 8: Pulley Problem IW
Example 8: Pulley Problem I
What is the tension in the string?
A) T<W
B) T=W
C) W<T<2W
D) T=2W
Pull with
force = W W
Same answer

9. Example 9: Pulley Problem II
What is the tension in the string?
A) T<W
B) T=W
C) W<T<2W
D) T=2W W 2W a a

10. Preflight 8: Question 5
In the 17th century, Otto von Guricke, a physicist in Magdeburg, fitted two hollow bronze hemispheres together and removed the air from the resulting sphere with a pump. Two eight-horse teams could not pull the halves apart even though the hemispheres fell apart when air was readmitted! Suppose von Guricke had tied both teams of horses to one side and bolted the other side to a heavy tree trunk. In this case, the tension on the hemisphere would be
a) twice what it was
b) exactly what it was
c) half what it was Fh T
2Fh 2T

11. Reminder: Procedure for Solving Problems
Identify force using Free Body Diagram
Set up axes
Write Fnet=ma for each axis
Calculate acceleration components
Use kinematic equations
Solve!
the most crucial step!

12. Problem 4.27
What force does a trampoline have to apply to a 45 kg gymnast to accelerate her up at 7.5 m/s2? mg Ft

13. Problem 4.21
A flea jumps by exerting a force of 1.2 x 10-5 N straight down on the ground. A breeze blowing on the flea parallel to ground exerts a force of 0.5 x N on the flea. Find the direction and magnitude of the acceleration of the flea if its mass is 6 x kg. mg F f

14. Problem 4.33
Suppose you have a 120 kg wooden crate resting on a wood floor. (a) What maximum force can you exert horizontally on the crate without moving it?
What kind of friction is impeding movement?
static kinetic mg N F fk
(b) If you continue to exert this force once the crate starts to slip what will its acceleration be?
What is the net force acting on the crate?
Force exerted
Force exerted - Static friction
Force exerted - Kinetic friction

15. Find acceleration of block =500
Example 3
T=50 N
M = 5 kg
k = 0.2
Find acceleration of block
=500
Similar to example 4.8 in book
Hints:
Draw FBD
Resolve T in x and y components
Normal force is smaller than when pulling along horizontal
Tension component along horizontal is also smaller
Answer: 6.0 m/s2

16. Find acceleration and normal force
Example 6
mg cos ()
mg sin ()  
Find acceleration and normal force
Choose axis and calculate components
Answers: a= g sin() FN=mg cos ()

17. Problems 4.59 and 4. 60 ms (ice-on-ice)=0.1
A contestant in a winter games event pushes a 45 kg block of ice across a
frozen lake. Calculate the minimum force, F, he must exert to get the bock
moving. What is the force, F’, he needs to exert if he were to pull the
bock with a rope over his shoulder at the same angle above the horizontal?
Is F = F’?
1) Yes
2) No
25o
F cos25o
F’ cos25o
-F sin25o
F’ sin25o F’ F
ms (ice-on-ice)=0.1

18. Problem 4.59 and 4.60 Continued ms (ice-on-ice)=0.1 F’ 25o F cos25o
-F sin25o
F’ sin25o F
ms (ice-on-ice)=0.1 N N’
Case B
Case A mg mg