Advanced Work With Forces I

Advanced Work With Forces I Mark Lesmeister Dawson High School

Solving Force Problems • Step1: Write Givens, Unknowns and Model • Draw a free body diagram. • Choose a coordinate system so that the acceleration is along an axis. • Break any forces not on axes into components. • Identify any other quantities in the problem. • Identify whether the equilibrium model or constant force model applies in each direction (x and y) separately. • Equilibrium- object not moving or moving with constant velocity. • Constant force- object accelerating.

Solving Force Problems • Step 2- Identify the Method you will use for finding the unknowns. • Using your models, write down appropriate equations. • Constant force • Equilibrium

Solving Force Problems • Step 3: Implement your plan. • Solve all equations before substituting values. • Step 4: Evaluate the Solution. • The answer should make sense physically. Remember G U M M I E S

Example: Forces in 2 Directions • A 3 kg ball is dropped from the roof of a building 176.4 m high. While the ball is falling, a horizontal wind exerts a force of 12 N on the ball. • How long does it take to hit the ground? • How far from the building does the ball hit the ground? • What is its speed when it hits? 176.4 m

Step 1: Identify Givens, Unknowns and Models. m = 3.00 kg ∆y= -176.4 m FW = 12.0 N vi = 0 Δt = ? ∆x = ? vf= ? There is a constant force in both the x and y directions. FW mg

Step 2: Method In the horizontal direction: In the vertical direction: FW mg

Step 3: Implement the plan.

Steps 2 and 3: Final Velocity Vfx Vf Vfy

Forces on an Incline • On an inclined plane, we can use the parallel and perpendicular directions for our coordinate system. Fn Fapp q mg cos(q) mg mg sin(q) q

Example: Forces on an Incline • A 40 kg wagon is towed up a hill at an 18.5o incline. The tow rope exerts a force of 140 N. The wagon starts from rest. • How fast is the wagon going after 30 m? Fapp=140N q=18.5o

Step 1: Write down givens, unknowns and model • m = 40.0 kg • Fapp = 140 N • q = 18.5o • Dx =30 m • vi = 0 • vf = ? • This is constant force in the x direction and equilibrium in y. Fn Fapp q mg cos(q) mg mg sin(q)

Step 2: Identify the method. Fn Fapp q mg cos(q) mg mg sin(q)

Step 3: Implement the plan. Fn Fapp q mg cos(q) mg mg sin(q)

Fn Fapp q mg cos(q) mg mg sin(q) Step 4: This is about 11 mi/hr.

Advanced Work With Forces I