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Newton’s Law Review Problems
1. Think about a person who jumps up in the air, then falls back to the ground.
Draw force diagrams for three specific points in this journey: 1) When the
person first pushed off the ground
2) When the person is in the air
3) When the person reaches the ground again
2. Why is it easier to walk on a carpeted floor than on a smooth, polished floor?
Frictional forces actually push you forward when you walk (just try
walking on ice if you don’t believe me), so more friction means more
friction pushing you forward
3. You push a 150kg refrigerator across a rough surface, where it experiences
50N of friction. If the refrigerate accelerates at 2.0m/s/s, what is your pushing
Push = 350N in the positive direction
4. A falling skydiver (mass = 70kg) accelerates at 6.5 m/s/s. What is the force of
air resistance acting upon her?
Force of Air Resistance = 230N
5. A loaded truck can accelerate at 1 m/s/s. If it loses its load and has threefourths of the original mass, what acceleration can it attain from the same
driving force?
1.3 m/s/s
6. A car passenger can survive a car crash if the acceleration is less than 30 g’s.
Calculate the force on a 70kg person experiencing this acceleration.
20,600 N
7. A Porsche (m = 1200 kg) experiences 500N of air resistance. If the ground
pushes it forward with 4000N of force, what is its acceleration?
2.9 m/s/s
Challenging Conceptual Questions:
1. As the Earth rotates about its axis, it takes three hours for the United States 3
hours to pass a point above Earth that is stationary. Upon hearing this fact, a
person states that it would be extremely easy to get from Washington DC to
San Francisco: simply ascent in a helicopter high over Washington and wait
three hours until San Francisco passes below. Is this person correct?
No. Objects that are on the ground are still moving forward with the
Earth, including the helicopter. As it lifts off, no external forces act
upon it in a direction opposite Earths’ motion.
2. At high speeds, the air resistance experienced by an object is proportional to
velocity squared (if an object goes twice as fast, it experiences four times as
much resistance). Now think about a rocket, whose mass decreases as it
burns through its fuel.
At time = 0s, the rocket has mass m and a velocity v. At time = 100s, it has
mass 1/2m and velocity 2v. At which time does the rocket have a greater
acceleration? Explain.
At time = 100s, the air resistance has increased by a factor of 4. This
outweighs the increase in acceleration due to the mass reduction