Summary

• Projectiles
• Law of inertia
• Newton's second law

• Reading Quiz
  
• Three kinds of friction
  • air
    Example #A
    Example #B
    
  • static
  • sliding
  • applet
  • antilock brakes    
  
  
• Practice questions
  Weight and mass Example #1
  
  Sliding friction Example #2
  
  Sliding friction Example #3
  
  Atwood machine Example #4
  
  
• Newton's third law
  • definition
  • suspended apple
    Example #5
    Example #6
    
    
  • rockets
  
  
  
  

wk4
Reading quiz Two canoes meet in the middle of a lake. After a brief visit, a person in canoe 1 (m₁ = 100 kg) pushes on canoe 2 (m₂ = 200 kg) and in turn gets pushed. How are the magnitudes of the two forces related?
A. F₁ = F₂
B. F₁ = ½F₂
C. F₁ = 2F₂
D. F₁ = 4F₂
Answer
















A 80 kg skydiver achieves terminal velocity after 30 seconds. At that time, what is her acceleration?
A. zero
B. 9.81 m/s² down
C. 9.81 m/s² up
D. can't say
Answer
















What is the net force on the skydiver?
A. zero
B. 785 N down
C. 785 N up
D. can't say
Answer















 
If a 30-kg instrument has a weight of 50 N on the moon, what is its acceleration when it is dropped?
A. zero
B. 0.60 m/s²
C. 1.67 m/s²
D. 1.50 km/s²
Answer















 
What applied force is needed to accelerate 50 kg at 2.0 m/s² if the frictional force is 150 N?
A. 100 N
B. 250 N
C. −50 N
D. 50 N
Answer















 
A 2.0-kg book sliding along a horizontal table has a constant frictional force of 4.0 N acting on it. If it has an initial speed of 8.0 m/s, how much time will it slide before stopping?
A. 1 s
B. 2 s
C. 3 s
D. 4 s
Answer















sj6 5.28
Two masses of m₁=3.0 kg and m₂=5.0 kg are connected as shown. What will their acceleration be?
A. 4.90 m/s²
B. 7.35 m/s²
C. 2.45 m/s²
D. 9.80 m/s²
Answer
















If the force exerted by a horse on a cart is equal and opposite to the force exerted by the cart on the horse, how does the horse manage to move the cart?
A. Gravity provides added force
B. Friction provides added force
C. Net force on cart is nonzero
D. N's 3^rd L doesn't apply here
Answer















sj6 5.13
You stand on the seat of a chair (y₀ = 0.50 m) and then hop off. Through what distance does the Earth (M = 6×10²⁴ kg) rise up to meet you?
A. 10⁻²³ m
B. 10⁻¹⁹ m
C. 10⁻¹⁵ m
D. 10⁻⁹ m
Answer















 



A. F₁ = F₂

Action/reaction forces are always equal in magnitude and opposite in direction.




















 



A. zero

Terminal velocity implies that the acceleration is zero.




















 



A. zero
An acceleration of zero means the net force is zero.




















 



C. 1.67 m/s²





















 



B. 250 N





















 



D. 4 s





















 



C. 2.45 m/s²





















 


C. Net force on cart is nonzero
is the best answer although one could also argue for
B. Friction provides added force

since the friction between the horse's hoofs and the ground is what propels the horse-cart combo.




















 



A. 10⁻²³ m