HOW TO SOLVE PHYSICS PROBLEMS THE PROCEDURE Step

HOW TO SOLVE PHYSICS PROBLEMS

THE PROCEDURE • • Step 1: Draw a free body diagram Step 2: Write down the givens Step 3: Write down the unknown Step 4: Resolve the free body diagram

THE PROCEDURE • Step 5: Use equations for the x and y directions • Step 6: Use “sum of” substitution • Step 7: Plug in givens and solve for unknown • Step 8: Check the answer with common sense

Problem 11 (page 128) Two forces, F 1 and F 2, act on a 5. 00 kg block. The magnitudes of the Forces are F 1=45. 0 N and F 2=25. 0 N. What is the horizontal acceleration of the block? 65 F 2 F 1

Problem 11 (page 128) § Step 1: Draw free body diagram. y + x F 1 65 F 2

Problem 11 (page 128) § Step 2: Write down the givens. • m = 5. 00 kg • F 1 = 45. 0 N • F 2 = 25. 0 N

Problem 11 (page 128) § Step 3: Write down the unknown. • Horizontal Acceleration = ax

Problem 11 (page 128) § Step 4: Resolve the free body diagram. y + x F 1 F 2 y 65 F 2 x F 2

y + x Problem 11 (page 128) F 1 F 2 65 ∑Fx=ma ∑ Fy=ma § Step 5: Use equations for the x and y directions.

Problem 11 (page 128) § Step 6: Use ”sum of” substitution. F 1 x - F 2 = max F 1 y = may

Problem 11 (page 128) § Step 7: Plug in givens and solve. F 1 cos 65 -F 2 = ax m

Problem 11 (page 128) § Step 8: Check with common sense. -1. 20 m/s 2 Sounds good!

Problem 55 (page 131) A helicopter is traveling horizontally to the right at constant velocity. The Weight of the helicopter is 53800 N. The lift force L generated by the rotating blade makes an angle of 21 degrees with respect to the verticle. What is the magnitude of the lift force and determine the magnitude of the air resistance R that opposes the motion.

Problem 55 (page 131) § Step 1: Draw free body diagram. y + x 21 R W L

Problem 55 (page 131) § W = 53800 N Step 2: Write down the givens.

Problem 55 (page 131) § The lift force (L) Air resistance (R) Step 3: Write down the unknown.

Problem 55 (page 131) § y + x Lx Ly 21 R W L Step 4: Resolve the free body diagram.

Problem 55 (page 131) y + x Lx Ly R 21 W ∑Fx = ma ∑ Fy = ma § Step 5: Use equations for the x and y directions.

Problem 55 (page 131) § Lx - R = ma Ly - W = ma Step 6: Use ”sum of” substitution.

Problem 55 (page 131) § W =L (cos 21) L sin 21 = R Step 7: Plug in givens and solve.

Problem 55 (page 131) § L = 57627 N R = 20651 N Step 8: Check with common sense.

Problem 79 (page 133 ) The weight of a block on a table is 111 N and that of the hanging block is 258 N. Assuming the coefficient between the 111 N block and the table is 0. 300, find the acceleration of the blocks and the tension of the string.

Problem 79 (page 133 ) § Step N + T F 1: Draw free body diagram. T W 1 W 2

Problem 79 (page 133 ) § u =0. 300 W 1 = 111 N W 2 = 258 N Step 2: Write down the givens.

Problem 79 (page 133 ) § Acceleration of the two blocks (a) Tension of the string (T) Step 3: Write down the unknown.

Problem 79 (page 133 ) § N + T F Step 4: Resolve the free body diagram. T W 1 W 2

Problem 79 (page 133 ) § N + F T Step 5: Use equations for the x and y T directions. W 1 ∑Fx = ma W 2 ∑ Fy =0 ∑ Fx = ma

Problem 79 (page 133 ) § T-F = ma W 1 -N = 0 W 2 -T = ma Step 6: Use ”sum of” substitution.

Problem 79 (page 133 ) § T = m 1(a + ug) a = W 2 -T m 2 Step 7: Plug in givens and solve.

Problem 79 (page 133 ) § T = 100. 89 N a = 5. 974 m/s Step 8: Check with common sense. 2

Problem 111 (page 135) A 45 kg box is sliding up an incline that makes an angle of 15 degrees with respect to the horizontal. The coefficient of friction between the box and the surface of the incline is 0. 180. The initial speed of the box at the bottom of the incline is 1. 5 m/s, How far does the box travel along the incline before coming to rest.

Problem 111 (page 135) § Step + N F W 15 1: Draw free body diagram.

Problem 111 (page 135) § m = 45 kg u = 0. 180 Vo = 1. 5 m/s Step 2: Write down the givens.

Problem 111 (page 135) § Distance the box travels (d) Step 3: Write down the unknown.

Problem 111 (page 135) § N F 15 W Wy Wx Step 4: Resolve the free body diagram.

Problem 111 (page 135) § N F Step 5: Use equations for the x and y directions. 15 W Wy Wx ∑Fx = ma ∑ Fy =0

Problem 111 (page 135) § - F - Wx = ma N - Wy = 0 Step 6: Use ”sum of” substitution.

Problem 111 (page 135) § a = -4. 245 m/s 2 2 d = V - Vo 2 a 2 Step 7: Plug in givens and solve.

Problem 111 (page 135) § d = 0. 265 m Step 8: Check with common sense.

by ANDREW MEDLEY