Applying Newton s Laws

Transcription

1 Applying Newton s Laws

2 Free Body Diagrams Draw and label the forces acting on the object. Examples of forces: weight, normal force, air resistance, friction, applied forces (like a push or pull) Velocity is NOT a force. Only label forces.

3 Air resistance and Terminal Velocity Air resistance is result of an object trying to go through air Air resistance has a greater effect on lighter objects Air resistance depends on the shape of the object and the speed that it falls through the air

4 Air resistance and Terminal Velocity An object will accelerate (speed up) until it reaches a terminal velocity At terminal velocity, the net force is zero and the acceleration is zero At terminal velocity... F air = weight

5 An 8.0 kg bowling ball reaches a terminal velocity of 65 m/s. What is the acceleration of the bowling ball at this point? a) 0 m/s 2 b) 8.1 m/s 2 c) 9.8 m/s 2 d) 65 m/s 2

6 An 8.0 kg bowling ball reaches a terminal velocity of 65 m/s. What is the acceleration of the bowling ball at this point? a) 0 m/s 2 (because terminal velocity means the object is falling at a constant velocity and the object is no longer accelerating) b) 8.1 m/s 2 c) 9.8 m/s 2 d) 65 m/s 2

7 An 8.0 kg bowling ball reaches a terminal velocity of 65 m/s. What is the approximate net force on the bowling ball at this point? a) 0 N b) 80 N c) 520 N d) 800 N

8 An 8.0 kg bowling ball reaches a terminal velocity of 65 m/s. What is the approximate net force on the bowling ball at this point? a) 0 N (because terminal velocity means the object is falling at a constant velocity and the object is no longer accelerating. By definition, the net force is zero and the acceleration is zero.) b) 80 N c) 520 N d) 800 N

9 An 8.0 kg bowling ball reaches a terminal velocity of 65 m/s. What is the approximate force of air resistance on the bowling ball at this point? a) 0 N b) 80 N c) 520 N d) 800 N

10 An 8.0 kg bowling ball reaches a terminal velocity of 65 m/s. What is the approximate force of air resistance on the bowling ball at this point? a) 0 N b) 80 N (if the net force is zero, then the weight and air resistance must add up to zero. So if the weight = (8kg)(9.8 m/s 2 ), the weight is 79.2 N and air resistance up is 79.2 N. c) 520 N d) 800 N

11 Friction Friction always opposes motion or the applied force (called a non-conservative force) Force due to the rubbing of two surfaces against each other μ = the coefficient of friction (no units) Typically, 0< μ < 1 0 would be frictionless and μ=1 is really sticky

12 Friction Force of Friction = (coefficient of friction)(normal Force) The Normal force is the perpendicular force of one surface on another. (In our examples, this is typically the floor pushing up on the object.)

13 The road from static to kinetic friction

14 Types of Friction Static friction friction when an object is at rest Kinetic friction friction when sliding Static friction > Kinetic friction Both friction types are defined by the coefficient of friction, μ (either kinetic, μ k or static, μ s )

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17 A 5.0 kg dog is being pulled by a horizontal force of 4.0 N. The dog is sliding along the icy ground with a constant velocity of 1.0 m/s. Determine all of the values below. weight = normal force = friction = net force = acceleration = =

18 A 5.0 kg dog is being pulled by a horizontal force of 4.0 N. The dog is sliding along the icy ground with a constant velocity of 1.0 m/s. Determine all of the values below. weight = normal force = friction = net force = acceleration = =

19 An 5 N force is pulling a 2.0 kg block to the right at a constant velocity of 3 m/s. The force of friction is a) Less than 5 N b) Exactly 5 N c) Greater than 5 N

20 An 5 N force is pulling a 2.0 kg block to the right at a constant velocity of 3 m/s. The force of friction is a) Less than 5 N b) Exactly 5 N (because at constant velocity, Net force = 0, so friction must be exactly the same as the pulling force) c) Greater than 5 N

21 An 5 N force is pulling a 2.0 kg block to the right at and the block is accelerating to the right at a constant rate of 1.0 m/s 2. The force of friction is a) Less than 5 N b) Exactly 5 N c) Greater than 5 N

22 An 5 N force is pulling a 2.0 kg block to the right at and the block is accelerating to the right at a constant rate of 1.0 m/s 2. The force of friction is a) Less than 5 N (because the block is accelerating to the right, so the pulling force to the right must be greater than friction) b) Exactly 5 N c) Greater than 5 N

23 An 5 N force is pulling a 2.0 kg block to the right at and the block is slowing down at a constant rate of 1.0 m/s 2. The force of friction is a) Less than 5 N b) Exactly 5 N c) Greater than 5 N

24 An 5 N force is pulling a 2.0 kg block to the right at and the block is slowing down at a constant rate of 1.0 m/s 2. The force of friction is a) Less than 5 N b) Exactly 5 N c) Greater than 5 N (because friction is greater than the pulling force, then block is slowing down)