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Problem-Solving Methods in Physics: Calculating Unbalanced Forces

Learn step-by-step approaches for solving dynamic problems in physics, including finding forces, accelerations, mass, tension, and more. Understand concepts like free-body diagrams and force components. Apply principles of Newton's second law to determine unknowns.

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Problem-Solving Methods in Physics: Calculating Unbalanced Forces

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  1. Solving problems when  F = ma

  2. The cyclist has a mass of 50 kg and is accelerating at 0.9 m/s2. What is the size of the unbalanced force acting on the cyclist?

  3. How to approach a dynamic problem: • Draw a free-body diagram • Choose a coordinate axis and resolve all forces into components. • Set the sum of the force components each equal to ma. • Solve the resulting equations for the unknowns.

  4. Read the problem. (identify givens, look for “hidden” knowledge) Draw a free-body diagram (identify all forces acting upon object) Add all forces in one direction together (x?)  F = F1 + F2 + F3 + … (determine sum of forces, maybe Fnet = 0 or Fnet = ma) Add all forces in other direction together (y?) (determine sum of forces, maybe Fnet = 0 or Fnet = ma) Solve for what you don’t know

  5. If each person is pushing forward against the 1,500 N car, find: • The Normal force • The acceleration of the car

  6. If the 10 Newton crate is being pushed forward by a • force P of magnitude 80 N at an angle of 300 as shown • find: • The mass of the crate • The Normal force • The acceleration of the crate

  7. If m1=20 kg and m2=70 kg find: • The tension in the cable • The acceleration of the masses

  8. 600N 1 m • Given: The car is accelerating • forward at 2m/s2 and  = 25° • Find: The forces in the ropes AB • and AC.

  9. A stream of water strikes a stationary turbine blade, as the drawing illustrates. The incident water stream has a velocity of +18.0 m/s, while the exiting water stream has a velocity of -18.0 m/s. The mass of water per second that strikes the blade is 25.0 kg/s. Find the magnitude of the average force exerted on the water by the blade.

  10. Robin Hood (m =82 kg) is escaping from a dangerous situation. • With one hand he is gripping the rope that holds up a chandelier • (m =220 kg). When he cuts the rope where it is tied to the floor, the • chandelier will fall, and he will be pulled up to the balcony. Find: • the acceleration with which Robin is pulled upward • the tension in the rope while Robin escapes.

  11. What is the net force acting on the mule? • What is the approximate answer you expect to get? • Begin by calculating the components of each force.

  12. A 3.0 kg mass hangs at one end of a rope that is attached to a support on a railroad car. When the car accelerates to the right, the rope makes an angle of 4.0° with the vertical.  Find the acceleration of the car.

  13. In the vertical direction In the horizontal direction

  14. A block S (the sliding block) has a mass M = 3.3 kg. The block is free to move along a horizontal frictionless surface and is connected by a cord that wraps over a frictionless pulley to a second block H (the hanging block), with mass m = 2.1 kg. The cord and pulley are “massless”. The hanging block H falls as the sliding block S accelerates to the right.

  15. An inclined plane making an angle of 25o with the horizontal has a • pulley at its top. A 30 kg block on the plane is connected to a freely • hanging 20 kg block by means of a cord passing over the pulley. • Compute the distance that the 20 kg block will fall in 2.0 seconds • starting from rest. Neglect friction. • We now cut the cord. As the block then slides down the inclined • plane, does it accelerate? If so, what is its acceleration?

  16. Positive direction is down the inclined plane

  17. Given: M1 = 12.0 kg M2 = 24.0 kg M3 = 31.0 kg T3 = 65.0 N What is the tension T2

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