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Friction Affects Motion

Friction Affects Motion. Static. Friction. Friction. is a force that opposes the motion between two surfaces that are in contact There are two types STATIC friction: opposes the start of motion between two surfaces that are not in relative motion.

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Friction Affects Motion

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  1. Friction Affects Motion

  2. Static Friction Friction • is a force that opposes the motion between two surfaces that are in contact • There are two types • STATIC friction: opposes the start of motion between two surfaces that are not in relative motion. • KINETIC friction: the force between two surfaces that are in relative motion. It always acts in the oppositedirection to the motion of the object. Static friction is ALWAYS bigger than kinetic friction

  3. Friction

  4. Flat Surface Friction Ex: 1 A westward force of 50 N is applied to a 50 kg mass resting on a surface that will exert a frictional force of 16 N. If the force is applied for 5 seconds, what is the resulting velocity? Fnet = ma 34 N [W] = (50kg)(a) a = 0.68 m/s2 a = vf – vi t 3.4 m/s [W] • Fnet = Fa + Ff • Fnet = 50 N [W] + 16 N [E] • Fnet = 34 N [W]

  5. Flat Surface Friction Ex 2: A 30 kg mass is dragged over a surface with a frictional force of 12 N by pulling on a rope attached to the mass. The rope makes an angle of 30o to the horizontal and the applied force is 60 N. What is the resulting acceleration? Vertical No acceleration Horizontal Fnet = Ff + Fa(h) • Fnet = -12 N + (cos30 • 60) Fnet= 39.96 N (in the positive direction) Fnet = ma 39.96 = (30)(a) a = 1.3 m/s2

  6. Flat Surface Friction Ex 3: The magnitude of the applied force in Figure 3.71 is 165 N and 30.0o. If the desk remains stationary, calculate the force of static friction acting on the desk. Desk is stationary so …Fnet = 0 in both x and y HORIZONTAL Fnet = Fa + Ff 0 = (165)(cos 30.0) + Ff Ff = - 143 N [horizontal]

  7. Calculating Friction • Friction is directly proportional to the weight of an object (FN) = the Greek letter “mu” = force due to friction (N) “mew” = coefficient of friction = normal force (N) The coefficient of friction is a number (no units) that depends on the two surfaces in contact. The larger the number is, the more “sticky” the situation. So, a very slippery surface (ice) would have a low coefficient of friction.

  8. 0.06 – 0.20 0.8 – 1.2 1.0 – 1.6

  9. Calculating Friction • Ex 4: A 12 kg piece of wood is placed on top of another piece of wood. There is 35 N of maximum static friction measured between them. Determine the coefficient of friction between the two pieces of wood. • FN = Fg = mg • FN = (12 kg)(9.84m/s2) • FN = 1.2x 102 N • Ff = μ FN • 35 N = (μ)(1.2 x 102 N) • μ = 0.30 0.30

  10. Calculating Friction • Ex 5: What frictional force must be overcome to start a 50 kg object sliding across a surface with a static coefficient of friction equal to 0.35? • FN = Fg = mg • FN = (50 kg)(9.84m/s2) • FN = 490.5 N • Ff = μ FN • Ff = (0.35)(490.5 N) • Ff = 171.675 N 1.7 x 102 N

  11. Calculating Friction • Ex 6: A 10 kg box is dragged over a horizontal surface by a force of 40 N. If the box moves with a constant speed of 0.5 m/s, what is the coefficient of kinetic friction for the surface? 0.41

  12. Calculating Friction • Ex 7: A 10 kg box is dragged across a level floor with a force of 60 N. The force is applied at an angle of 30o above the horizontal. If the coefficient of kinetic friction is 0.20, what is the acceleration of the box? 3.2 m/s2

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