Question bank: Dynamics: Main Forces

The law of conservation of energy states that the total energy of the universe is constant, that is, no energy can be created or destroyed. The elastic force also follows this law: when a body undergoes elastic deformation, all the energy needed to perform this deformation is recovered when it returns to its original shape. Which of these examples does not illustrate this law?

The magnitude of the friction force depends directly on the surface of the bodies involved in contact, as the rougher the surface, the greater the magnitude of this force. In addition, there are also some external factors that can influence the magnitude of this force such as temperature, pressure, and humidity present in the environment where the bodies are located. Which of the alternatives below incorrectly presents the external influences on the magnitude of the friction force?

During a Physics exercise, a student pushes a 20 kg box up a 30° ramp with a constant force of 100 N, parallel to the inclined plane. The ramp is on a rough surface, and the coefficient of kinetic friction between the box and the surface is 0.2. Disregard air resistance and g = 10 m/s². (1) Calculate the friction force acting on the box as it moves up the ramp. (2) Determine the acceleration of the box during the movement.

A block of mass m is initially at rest on a rough horizontal floor, subject to the action of a horizontal force F that tends to move it. The maximum static friction force between the block and the floor is equal to a coefficient of static friction µ₁ times the magnitude of the normal force N exerted by the floor on the block. The block is connected to an ideal spring of negligible natural length and elastic constant k, which is fixed on the floor, and exerts an elastic force Fₓ on the block, which is opposite to the displacement of the spring relative to its natural length. The maximum elastic force that the spring can exert is equal to a coefficient of kinetic friction µ₂ times the magnitude of the normal force N, where µ₂ < µ₁. Consider g the acceleration due to gravity. Based on this information, 1) Describe all the forces acting on the block when it starts to move and then when it is in motion, indicating directions and senses. 2) Calculate the acceleration of the block immediately after the start of the movement, assuming that the block does not move vertically and that the force F is applied appropriately for the block to move. Consider the situation as a problem of Newtonian Mechanics, without considering dissipative effects such as air resistance.