Chapter 4

Physics Practice                                                         Name_____________________

                                                                                    Class/time_________________

 

Free-Body Diagrams and

Summation of Force-Vector Components with Newton’s 2nd Law   (SF=ma)

 

Problem-solving strategy:  1) draw free-body diagram for each object in problem, then

2) draw all forces acting upon it, 3) label the drawing with all given information. 

4) Then sum the components of the forces (SFx and SF)y separately, setting these sums =ma.  5) Perform algebra to find unknowns. 

 

If an object is at rest, v=0 and a=0

If an object is moving with constant v, then a=0

Acceleration down is (-), up is (+)

 

Fn-normal force; perpendicular to surface object is resting upon

Ff-frictional force; opposes motion and acts parallel to surface object is upon

Fapp-applied force

T-tension; a force in a cable, rope, string, etc.

 

Directions:  Draw free-body diagrams with all forces and given information.  Write summation of x and y components.   Then stop.  Do not solve the problems at this time. 

 

  1. A block of mass 5.00 kg rests on a horizontal surface where the coefficient of sliding kinetic friction between the two is 0.200. A string attached to the block is pulled horizontally, resulting in a 2.00-m/s2 acceleration by the block. Find the tension in the string and the normal force. (g = 9.80 m/s2)

 

Free-body diagram                   

 

 

 

 

 

 

Summation of components                     SFx                                                                                        SFy      (each set =ma)                                    

 

 

 

 

 

 

 

 

 

 

  1. A 15-kg block is on top of a level frictionless surface and is attached by a light string to a 5.0-kg hanging mass where the string passes over a massless frictionless pulley. If g = 9.8 m/s2, what is the tension in the connecting string?  Also find the normal force and acceleration of the system when in motion.

 

 

Free-body diagrams (two)                     

 

 

 

 

 

 

 

 

Summation of components    SFx                                        SFy                                    SFy                                                                   

(each set =ma)                                     

 

 

 

 

 

 

 

 

 

  1. A horizontal force of 750 N (Fapp) is needed to overcome the force of friction between a level floor and a 250-kg crate. If g = 9.8 m/s2, what is the coefficient of friction if moving with a constant velocity?

 

Free-body diagram                   

 

 

 

 

 

 

Summation of components                     SFx                                                                                        SFy      (each set =ma)                                    

 

 

 

 

 

 

 

 

 

 

 

4.  Doug hits a hockey puck, giving it an initial velocity of 6.0 m/s. If the coefficient of kinetic friction between ice and puck is 0.050, how far will the puck slide before stopping?  (Hint:  don’t need mass of puck, set up the formulas and set what happens.  There is no applied force.)

 

Free-body diagram                   

 

 

 

 

 

 

 

 

Summation of components                     SFx                                                                                        SFy      (each set =ma)                                    

 

 

 

 

 

 

 

 

 

 

 

 

5.  A man pulls a 100 kg sled at a constant velocity across a horizontal snow surface. If a force of 80 N is being applied to the sled rope at an angle of 53° to the ground, what is the force of friction between sled and snow?  Find normal force also. 

 

Free-body diagram                   

 

 

 

 

 

 

 

 

Summation of components                     SFx                                                                                        SFy      (each set =ma)                                    

 

 

 

 

 

 

 

 

 

6.  A sled weighs 100 N. It is held in place on a frictionless 20° slope by a rope attached to a stake at the top; the rope is parallel to the slope. What is the normal force of the slope acting on the sled?

 

Free-body diagram                   

 

 

 

 

 

 

 

 

 

 

Summation of components                     SFx                                                                                        SFy      (each set =ma)                                    

 

 

 

 

 

 

 

 

 

 

 

 

 

7.   A 150-N sled is pulled up a 28° slope at a constant speed by a force of 100 N. What is the coefficient of kinetic friction between sled and slope?

 

Free-body diagram                   

 

 

 

 

 

 

 

 

 

 

 

 

Summation of components                     SFx                                                                                        SFy      (each set =ma)                                    

8.  A 20‑kg traffic light hangs midway on a cable between two poles 40 meters apart. If the sag in the cable is 4.0 meters, what is the tension in each side of the cable (T1, T2, T3)?  The angle between the horizontal and the sagging cable on each side is 11o. 

 

 

Free-body diagram                   

 

 

 

 

 

 

 

 

 

 

 

 

Summation of components                     SFx                                                                                        SFy      (each set =ma)