Kinetic friction or static friction

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SUMMARY

The discussion centers on the dynamics of kinetic and static friction in relation to a moving box subjected to varying forces. When a box is pulled with a force F and experiences kinetic friction of 0.4F, it accelerates in the direction of the applied force, resulting in a net force of 0.6F. If the pulling force decreases to 0.3F while the box is still in motion, kinetic friction continues to act, causing the box to decelerate. This illustrates that kinetic friction is dependent on the velocity difference between the box and the surface, confirming that a reduction in driving force below the frictional force leads to a slowdown.

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Knowledge of friction types: static and kinetic friction
  • Familiarity with force diagrams and net force calculations
  • Basic principles of acceleration and deceleration
NEXT STEPS
  • Study the mathematical models of kinetic friction, including coefficients of friction
  • Explore the relationship between normal force and frictional force in detail
  • Learn about the implications of friction in real-world applications, such as vehicle dynamics
  • Investigate the transition between static and kinetic friction in various materials
USEFUL FOR

Physics students, mechanical engineers, and anyone interested in understanding the principles of friction and motion dynamics.

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Can anyone explain this case ?
If a moving box is pulling with a force F and the kinetic friction is 0.4F, of course, it will move in the direction of applied force with an acceleration and the net force is 0.6F
But what if the force suddenly decreases to 0.3F ?
The box is still moving so the friction is still kinetic friction?? But if it is the case, then the box will slow down! This seems quite strange to me@@
Can anyone explain please?
 
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1. " the kinetic friction is 0.4F, of course,"
Of course??
Doesn't seem like an ofcoursity to me!
Why should the frictional force be proportional to the pulling force?
It might be, obviously, but generally, friction is proportional to the NORMAL force between the surface and te box.

2. "But if it is the case, then the box will slow down!"
So what?
Kinetic friction occurs whenever there is a strict velocity difference between the surface and the box.
 
Do you really find that surprising? If the driving force drops below the friction force, naturally the object will slowdown. If you suddenly ease on the accelerator, do you find it surprising that your car coasts down to a stop?
 

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