Problem Solving Suggestions

n Where appropriate, break each problem response into the parts shown below.

 

       a. Diagram to include coordinate system with positive direction shown

       b. Variables that are known along with values - indicate on above diagram

       c. Variable(s) whose values need to be found

       d. Equation(s) to use: note that you may need to solve only 1 since an equation with more than 1 unknown does not have a single solution

       e. Calculations

 

n Action-reaction forces always act on two different bodies and they never cancel each other

n The normal force is always perpendicular to the surface.

n The force of gravity always acts vertically downward

n When a surface is involved, there is typically no acceleration in the normal direction. The sum of the normal components of the forces must be zero

n For an object moving on a flat surface, balance the forces in the normal direction of the force. Sometimes, but not always, this is equal to the weight of the object.

n If you need to resolve forces, keep in mind that the horizontal and vertical directions may not always be the simplest.

n For inclined planes, it is convenient to choose x and y axes parallel and perpendicular to the inclined surface, respectively.

n Find the normal force in order to calculate the force of friction (when present).

n If the object is at rest, calculate the force of static friction.

n If the object is moving with a constant velocity, calculate the force of kinetic friction.

n If the forces are neither parallel nor perpendicular to the actual or impending motion, resolve the forces into x and y components. Find the net force åF_x  

       and åF_y  in the x and y directions.

n Pulleys are used to change the direction of tension. IF the mass of a string is negligible, the tension in the string has the same magnitude throughout its length.

n If an object is in equilibrium (that is, at rest or moving at a constant velocity), the net acceleration = 0. This means that åF_x  and åF_y = 0. Use these

       equations to solve the problems for the objects in equilibrium.

n For problems involving systems moving with a constant acceleration, apply Newton's second law separately for the motion of each object.