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Review. Discussion Question 3. T. Which equations of equilibrium are correct for a wagon being pulled by a force T, moving at a constant speed? (F=friction, N=normal, W=weight) N = W ; T = F N + Tsin θ = W ; F = Tcos θ N + Tcos θ = W ; F = Tsin θ N = W + Tsin θ ; Fsin θ = T

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Review

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  1. Review

  2. Discussion Question 3 T Which equations of equilibrium are correct for a wagon being pulled by a force T, moving at a constant speed? (F=friction, N=normal, W=weight) • N = W ; T = F • N + Tsinθ = W ; F = Tcosθ • N + Tcosθ = W ; F = Tsinθ • N = W + Tsinθ ; Fsinθ = T • none of the above

  3. What do you know… • About an object in static equilibrium • About an object in translational equilibrium • About an object that is accelerating?

  4. A ball is thrown straight up in the air… • Is there a net force?

  5. A 2 kg ball is falling. Air resistance acting on the ball in 10N. Has the ball reached terminal velocity?

  6. F m M Example 2 Find the tension in the string connecting the blocks and the acceleration of the system. Ignore any friction. Express answer in terms of givens variables and known constants only.

  7. Mr. Moos uses a horizontal force Fto pull a 5-kg table across the floor at a constant speed of 3 m/s. The frictional force between the block and the floor is 8N. What is the value of the coefficient of kinetic friction, µk?

  8. Example A block with mass m is at rest on a ramp. Draw a FBD and write the equations for equilibrium. m θ

  9. Which object has a net force?

  10. A ball falls straight down through the air under the influence of gravity. There is a retarding force R on the ball with magnitude given by R= gs, where sis the speed of the ball and gis a positive constant. What is the magnitude of the acceleration a of the ball?

  11. Find tension in horizontal rope

  12. Two blocks are pushed along a horizontal friction­less surface by a force of 20 newtons to the right, as shown above. What is the force that the 2‑kilogram block exerts on the 3‑kilogram block?

  13. Solution a= 20N/5kg = 4 m/s/s for the system However, a 20N force on just the 3kg block should accelerate the block at a higher rate. The difference comes from the force that the 2kg block pushes back which is equal and opposite… F =ma = 4m/s/s * 2kg = 8N

  14. Good problems to look at • Homework 23, #2 and #4 • Object accelerating upward on a string • Atwood machine

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