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Physics 151 Week 10 Day 3
Topics: Newton’s 2nd Law and Applications
 Applying Newton’s 2nd Law
 Nature of Friction
 Kinetic Friction
 Static Friction
 Coefficients of Friction
General Force Model
Newton 0th Law
Objects are dumb - They have no memory of the past and cannot predict the
future. Objects only know what is acting directly on them right now
Newton's 1st Law
An object that is at rest will remain at rest and an object that is moving will continue
to move in a straight line with constant speed, if and only if the sum of the forces
acting on that object is zero.
ax = 0 m/s2 IFF Sum Fx = 0 N
(IFF => if and only if)
Newton's 3rd Law
Recall that a force is an interaction between two objects. If object A exerts a force
on object B then object B exerts a force on object that is in the opposite direction,
equal in magnitude, and of the same type.
Visualizations:
• Force Diagrams
• System Schema
Net Force Model
Newton's 2nd Law
acceleration of an object = sum of forces acting on that object / the mass of the
object
Remainder of week:
Friction Model
Apparent Weight
Slide 4-19
Rank the Forces
Three identical bricks are pushed across a table at constant speed.
The arrow pushes horizontally. (Note: there is friction between the bricks
and the table.)
• Draw separate free-body diagrams for each of the systems A and B.
• Rank the magnitudes of all the horizontal forces that you identified
above in order from largest to smallest. (Recall that the bricks are
pushed so that they move at constant speed.)
Static & Kinetic Friction - Part I
Slide 4-19
Static & Kinetic Friction
Describe what is happening to the forces on the box and the
effect of the forces on the motion of the box from the pictures.
Slide 4-19
Static & Kinetic Friction - Part III
Below is graph of the friction force exerted by the table on
the box.
A. Label times a-f that match the free-body diagrams in the
previous problem.
B. If the mass of the box is 3.0 kg, the maximum Ffs is 10 N,
and Ffk has an average of 6.0 N, find the coefficients of
static and kinetic friction.
Slide 4-19
Coefficients of Friction
What can you deduce/generalize about friction forces
from this table?
Describe 3-4 real world situations that can be explained
by this table
Slide 4-19
Clicker Question
The coefficient of static friction is
A.
B.
C.
D.
E.
smaller than the coefficient of kinetic friction.
equal to the coefficient of kinetic friction.
larger than the coefficient of kinetic friction.
equal to or larger than the coefficient of kinetic friction
not discussed in this chapter.
Slide 5-9
Answer
The coefficient of static friction is
A.
B.
C.
D.
E.
smaller than the coefficient of kinetic friction.
equal to the coefficient of kinetic friction.
larger than the coefficient of kinetic friction.
equal to or larger than the coefficient of kinetic friction
not discussed in this chapter.
Slide 5-10
Parking on a Hill
A. If you park on a hill with a 10 degree slope with the car
held by the parking brake, what is the magnitude of the
frictional force that holds your car in place?
B. The coefficient of static friction between your car's
wheels and the road when wet is 0.30. What is the
largest angle slope on which you can park your car in the
rain so that it will not slide down the hill?
C. The coefficient of kinetic friction between your wheels
and the wet road surface is 0.25. If someone gave your
your car a push on the wet hill and it started sliding
down, what would its acceleration be?
Slide 4-19
Example Problem
A sled with a mass of 20 kg slides along frictionless ice at 4.5 m/s.
It then crosses a rough patch of snow which exerts a friction force
of 12 N. How far does it slide on the snow before coming to rest?
Slide 5-21
Example Problem
A 75 kg skier starts down a 50-m-high, 10° slope on
frictionless skis. What is his speed at the bottom?
Slide 5-27
Example Problem
Burglars are trying to haul a 1000 kg safe up a frictionless ramp
to their getaway truck. The ramp is tilted at angle θ. What is the
tension in the rope if the safe is at rest? If the safe is moving up
the ramp at a steady 1 m/s? If the safe is accelerating up the
ramp at 1 m/s2? Do these answers have the expected behavior
in the limit θ → 0° and θ → 90°?
Slide 5-28
Example Problem
Macie pulls a 40 kg rolling trunk by a strap angled at 30° from
the horizontal. She pulls with a force of 40 N, and there is a 30 N
rolling friction force acting on trunk. What is the trunk’s
acceleration?
Slide 5-22