Transcript Powerpoint

Physics 151 Week 9 Day 1
Topics: Newton’s 2nd Law
 Force and Motion Graphs
Questions from last time
 Net force vs. acceleration
 Mass vs. Acceleration
 Applying Newton’s 2nd Law
Wednesday’s Class: All about Forces & friction
Newton’s Third Law of Motion Question
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.
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 and Motion Graphs
Net Force vs. Acceleration Graphs
Net Force vs. Mass Graphs
Constant 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
Example Problem
A 100 kg block with a weight of 980 N hangs on a rope. Find
the tension in the rope if
A. the block is stationary.
B. it’s moving upward at a steady speed of 5 m/s.
C. it’s accelerating upward at 5 m/s2.
Slide 5-15
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
Friction Brainstorm
One person in each team takes out a sheet of paper and
records their group brainstorming everything they know
about friction.
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