Unit 6: Applications of Newton's Laws
Friction, tension, drag, and circular motion — Newton's second law applied to the messy forces of the real world.
6 topics · 22 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Static and Kinetic Friction
- How it worksHow do the coefficients of static and kinetic friction set the maximum and steady friction force?
- Worked exampleFind the maximum static friction force on a 20 kg crate with a coefficient of static friction of 0.4.
- Common mistakeWhy doesn't friction always point in the direction opposite to an object's motion?
- Going furtherWhat happens to the friction force in the instant a stationary block finally starts to slide?
Your turn1 of 2
A crate of mass m sits on a horizontal floor with coefficient of static friction mu, under gravitational acceleration g. Write an expression, in terms of mu, m, and g, for the maximum static friction force before the crate starts to slide.Tension in Ropes and Pulleys
- How it worksHow does a massless, ideal pulley just redirect a rope's tension without changing its size?
- Worked exampleFind the tension in a rope holding a 15 kg mass in equilibrium, hanging straight down.
- Common mistakeWhy doesn't a heavier object hanging from a rope automatically mean more tension everywhere in a pulley system?
Your turn1 of 2
A rope holds a 22 kg mass motionless, hanging straight down. Taking g = 9.8 m/s2, what is the tension in the rope, in newtons?Drag Force and Terminal Velocity
- How it worksHow does a drag force that grows with speed eventually balance gravity to create a terminal velocity?
- Worked exampleFind the terminal velocity of a 70 kg skydiver given a drag relation F_drag = 0.5v2 in SI units.
- Common mistakeWhy doesn't reaching terminal velocity mean an object's acceleration turns negative and it starts slowing down?
- Going furtherWhat happens to a skydiver's terminal velocity in the instant they open their parachute?
- Ties togetherHow does the balance of forces at terminal velocity connect to the same equilibrium condition used in static problems?
Your turn1 of 2
A skydiver of mass 85 kg experiences a drag force F_drag = 0.589 v2 (SI units). Taking g = 9.8 m/s2, what is her terminal velocity, in meters per second?Centripetal Force
- How it worksHow does a real force like tension, friction, or gravity end up supplying the centripetal force in circular motion?
- Worked exampleFind the tension in a string swinging a 0.5 kg ball in a horizontal circle of radius 1 m at 4 m/s.
- Common mistakeWhy isn't centrifugal force a real outward push acting on an object moving in a circle?
Your turn1 of 2
A 1.11 kg ball swings in a horizontal circle of radius 1.8 m at 8 m/s, held by a string. What is the tension in the string, in newtons?Banked Curves and Nonuniform Circular Motion
- How it worksHow does the normal force on a banked curve supply centripetal force without any help from friction?
- Worked exampleFind the ideal banking angle for a curve of radius 100 m taken at 25 m/s with no friction needed.
- Going furtherWhat happens to the net force on a car when it both speeds up and turns a corner at the same time?
Your turn1 of 2
A curve of radius 154 m is banked so that a car moving at 28 m/s needs no friction to turn safely. Taking g = 9.8 m/s2, what is the ideal banking angle, in degrees?Vertical Circles and Apparent Weight
- How it worksHow does the normal force change at the top and bottom of a vertical circle to keep supplying centripetal force?
- Worked exampleFind the minimum speed at the top of a 10 m radius loop needed for a roller coaster car to stay on the track.
- Common mistakeWhy doesn't gravity stop pulling down on you at the top of a loop just because you feel light?
- Ties togetherHow does the vertical-circle problem connect the idea of centripetal force to the apparent weight you feel in an elevator?
Your turn1 of 2
A roller coaster loop has radius 15 m. What is the minimum speed at the top of the loop needed for the car to maintain contact with the track (g = 9.8 m/s2)?