Newton's Laws (Ch 5, 12, 6)

Question 1

Force of gravity (F_g)

Answer 1

• The force of attraction between two masses
• F_g = (Gm₁m₂)/r²
  
  • G = 6.67 x 10^-11

Question 2

Weight (W)

Answer 2

• The force of attraction due to the mass of the earth/planet
• Also known as “true weight”
• W = mg
  
  • g = 9.81 m/s²

Question 3

Apparent Weight

Answer 3

• The normal force (N)
• What the scale actually measures; how heavy you actually feel

Question 4

Normal Force (N)

Answer 4

• The force that a surface exerts on an object in contact with that surface
  
  • Always *perpendicular* to the surface
• It is the apparent weight
  
  • Sometimes it is the true weight, but NOT always
• It cannot be calculated from a specific equation

Question 5

Friction (f)

Answer 5

• Contact force which opposes the direction of motion of an object
• Depends on two factors
  
  • coefficient of friction (μ) - the roughness of the surface in contact
    
    • unitless quantity between 0 and 1
  • normal force - how close the surfaces are pushed together
• Two types

Question 6

Static friction (f_s)

Answer 6

• Opposes the direction the object wants to move while the object is at rest relative to the surface
  
  • Will only have enough magnitude to counter force and keep the object at rest
  • Right before the object moves, it will be max
• f_s ≤ μ_sN
• fs, max ≤ μ_sN

Question 7

Kinetic friction (f_k)

Answer 7

• Opposes the direction of motion while the object is moving relative to the surface
• f_k = μ_kN

Question 8

Relationship between the coefficients of static friction and kinetic friction

Answer 8

• μ_s > μk
• It takes more force to get an object to move than it takes to keep the object moving

Question 9

Tension (T)

Answer 9

• Force transmitted to an object through a rope or a wire when a force is applied on the other end of the rope or wire
• The magnitude is equal to the force being applied
  
  • T = F
• The direction is always along the rope and will always pull on the object connected to the rope

Question 10

Spring Force (F_s)

Answer 10

• The restoring force which acts to bring a spring or elastic material back to the unstretched position (equilibrium)
• Hook’s Law: the restoring force is proportional to the displacement
  
  • F = (-)kx
    
    • Spring constant (k): tells us how stiff the spring is
    • The direction is always opposite the direction of displacement