1. Physics 103, Spring 2003, U.Wisconsin
Example 4 T1
T=24 N
M2 = 4 kg
M1 = 2 kg
Find acceleration of blocks and tension T1
Note friction is not specified, only external force T pulls blocks
Acceleration, a = Fnet / M = T / (M1+M2) = 24 / 6 m/s2 = 4 m/s2
Tension, T1, is causing block M2 to accelerate at 4 m/s2
T1 = M2a = 4 * 4 N = 16 N
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

2. Example 5 T1 T=24 N M2 = 2 kg M1 = 4 kg Turn blocks around….
Find acceleration of blocks and tension T1
Answers: a= 4 m/s2 (the same)
and T1= M2a = 8 N (smaller)
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

3. Example 7 s M1 M2 a
Find maximum acceleration such that upper block does not slip
M1 is fixed with respect to M2 - therefore, static frictional force on M1 at the M1-M2 contact surface is, f = s M1 g
Answers: a= f/M1 = sg
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

4. Example 6 mg cos () mg sin ()   Find acceleration and normal force
Choose axis and calculate components
Answers: a= g sin() FN=mg cos ()
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

5. Physics 103, Spring 2003, U.Wisconsin
Example 8
What force does a trampoline have to apply to a 45 kg gymnast to accelerate her up at 7.5 m/s2? mg Ft
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

6. Physics 103, Spring 2003, U.Wisconsin
Example 9
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
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

7. Physics 103, Spring 2003, U.Wisconsin
Example 10
mwoodOnWood= 0.5,0.3
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
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

8. Physics 103, Spring 2003, U.Wisconsin
Example 13
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
9/22/2018
Physics 103, Spring 2003, U.Wisconsin

9. Physics 103, Spring 2003, U.Wisconsin
Example 13 - Contd. F’
25o
F cos25o
F’ cos25o
-F sin25o
F’ sin25o F
ms (ice-on-ice)=0.1 N N’
Case B
Case A mg mg
9/22/2018
Physics 103, Spring 2003, U.Wisconsin