PHYS 201 Problem Notes for Force Friction Tension and Newton s Laws Frictional Force and Static and Kinetic Friction The frictional force of an object at rest is 0N If a force push or pull is acting on the object but the object is not moving the frictional force equals the pull push force but is opposite in direction If a force push or pull causes an object to move the frictional force is less than the force of the push pull When an object is just about to move use static friction to find frictional force When an object is moving sliding use kinetic friction The frictional force is the opposite direction of the push or pull force The larger the mass the higher the frictional force Inclined Planes For inclined planes make the y axis go along the normal force The x axis will go along the frictional force If the force of the pull Fp is straight you only need to worry about the components for the gravitational force Fg Fgy and Fgx Fgy normal force Fn mgcos theta Fgx Fgsin theta Acceleration in the y direction is 0 m s 2 since the object is not moving upwards or downwards If Fp is along the x axis the Fgsin theta Fgx Freely Falling Objects If an object is freely falling use one of these equations 1 Velocity Initial velocity acceleration x time 2 Direction x Initial direction initial velocity x time 1 2 x acceleration x time 2 3 Velocity 2 Initial velocity 2 2 x acceleration x displacement You may have to use the quadratic equation to solve for the unknown If an object jumps from a height the peak of when the object is in the air is when v 0 m s The acceleration is always 9 80 m s 2 can be negative depending on if the object is falling or being thrown up Tension If 2 objects are attached by a string and are pulled horizontally with a constant force the tension in the 2nd object is more than the tension in the first because the second object has the weight of both objects If the force of tension Ft is in the y direction and the object is at rest or constant velocity the tension equals the Fg In hanging masses tension is greater above both objects because of mass In a pulley system the Ft in the right direction for the object on the table equals the Ft upwards for the hanging object Objects Stopping or Coming to a Rest Use 1 D kinematic equations and Newton s Second Law Sum of forces mass x acceleration The final velocity will equal 0 m s Acceleration can be found by using velocity 2 initial velocity 2 2 x displacement if time is not given Newton s First Law Newton s Second Law Newton s Third Law States that every object continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces acting on it If one object with a large mass is dropped at the same time as an object with a much smaller mass they hit the ground at the same time because the larger object has more inertia Valid with an inertial reference frame States that the rate of change of momentum of an object is directly proportional to the net force applied to the object The sum of forces change in momentum change in time mass x velocity change in time mass x acceleration The acceleration and net force are directly proportional If an object moves at constant velocity the acceleration and net force are both 0 If an object moves with constant speed the acceleration and net force are not necessarily 0 because the objects can change directions Whenever one object exerts a force on a second object the second object exerts an equal force in the opposite direction on the first The force pairs exert their forces on different objects When a small object comes in contact with a much larger object the smaller object has a much larger acceleration both objects have equal forces in opposite directions