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Net force

Net force. SPH3U – Unit #2 Dynamics. Learning goals. After this topic, I will be able to… Determine the effect of forces acting on an object, Determine the net force acting on an object, Solve problems related to net force. What are the effects of many forces?.

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Net force

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  1. Net force SPH3U – Unit #2 Dynamics

  2. Learning goals After this topic, I will be able to… • Determine the effect of forces acting on an object, • Determine the net force acting on an object, • Solve problems related to net force.

  3. What are the effects of many forces? What happens when a single forces acts on an object? This is a tricky question that took very clever people about 2000 years to answer. What happens when an object is under the effect of many forces?

  4. What are the effects of many forces? Using the spring scales and the cart, exert two equal forces on the cart, but in opposite directions. How would you describe the motion of the cart? What would the FBD look like for this situation? How would you describe the total effect of the two forces on the cart?

  5. Net force Since a single force seldom acts on an object, it is often necessary to find the sum of several forces acting on an object. The vector sum of all the forces acting on an object is called the net force (Fnet).

  6. Net force Net Force (Fnet) • The vector sum of all the forces acting on an object • For ease, we will separate the net force into the horizontal and vertical components

  7. Motions involving net force Using your cart, spring scales, some masses, and your brains, let’s come up with a table that summarizes the effects the net force has on the motion of an object. One Single Force: Using a single spring scale and a cart, place a mass on the cart and exert a steady 2 N force using the spring scale. • Draw a FBD to show what’s happening (ignore friction). • What is the net force for the horizontal motion? • What is the motion of the cart? Uniform, non-uniform, or no motion? *Be very careful with this answer!

  8. Motions involving net force Let’s look more closely at the two spring scale model. Two Forces (balanced): Using two spring scales and a cart, exert a force using the spring scale that is equal yet opposite. • Draw a FBD to show what’s happening (ignore friction). • What is the net force for the horizontal motion? • What is the motion of the cart? Uniform, non-uniform, no motion? *Be very careful with this answer!

  9. Motions involving net force Observe the demo to better show two balanced forces. Two Forces (balanced): Draw the demonstration setup at the front of the room. The force on each side is ________ N. • Draw a FBD to show what’s happening to the cart (ignore friction). • What is the net force for the horizontal motion? • What is the motion of the cart? Uniform, non-uniform, no motion? *Be very careful with this answer!

  10. Motions involving net force Observe the demo to better show two unbalanced forces. Two Forces (unbalanced): Draw the demonstration setup at the front of the room. The force on side A is ________ N and the force on side B is ________ N. • Draw a FBD to show what’s happening to the cart (ignore friction). • What is the net force for the horizontal motion? • What is the motion of the cart when released? Uniform, non-uniform, no motion? *Be very careful with this answer!

  11. Motions involving net force Let’s up the ante to THREE forces! Three Forces: Using the cart, place two spring scales to one side and one on the other. Pull the lone scale with 4 N of force and the other two with 2 N of force on each. Ensure that you are pulling all three on the same line (as horizontal as possible). • Draw a FBD to show what’s happening to the cart (ignore friction). • What is the net force for the horizontal motion? • What is the motion of the cart when released? Uniform, non-uniform, no motion? *Be very careful with this answer!

  12. Motions involving net force Let’s summarize what happens when we have either a balanced or unbalancing of net force…

  13. Practice • From the FBD given, determine the net force acting on each object. + + FN = 100 N a) b) + + Fair = 20 N Fa = 70 N Fa = 20 N Ff = 20 N Ff = 20 N Fg = 100 N

  14. Practice • Determine the missing value from the FBD. + + a) b) Fnet,x = 3.2 kN + + 1.2 kN 150 N 70 N 0.5 kN 0.7 kN Fa = ? 1.2 kN Fg = ?

  15. Practice • The game tug-of-war is all about the balancing and unbalancing of forces. Two teams are playing tug-of-war. Team Awesome is pulling with everything they’ve got (2000 N), while Team Constant has been holding with an unchanging 1700 N of force. Using a FBD, determine the net force.

  16. Homework Read: pg. 112 – 121 Practice problems: Pg. 122 #15 Quiz #1 on Tuesday Oct 21st.

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