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Forces and the Laws of Motion

Forces and the Laws of Motion. Newton’s Laws. Newton’s First Law (Law of inertia). An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force. Inertia.

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Forces and the Laws of Motion

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  1. Forces and the Laws of Motion Newton’s Laws

  2. Newton’s First Law(Law of inertia) • An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force.

  3. Inertia • Inertia is the tendency of an object not to accelerate. • In other words, Newton’s First Law says that when the net external force on an object is zero, the object’s acceleration (change in velocity) is zero.

  4. Net force • The sum of forces action on an object is called the net force. F ground F resistance F forward F gravity A car moving at a constant velocity has a vector sum of zero.

  5. Force Pairs • Forces always come in pairs 1. Applied Force…force ON an object 2. Frictional Force…opposes motion • Normal Force…upward forces, opposes weight • Gravity/Weight…acts downward • Fa Fn Ff Fg

  6. Inertia • Inertia is proportional to an objects mass. • The greater the mass, the less the body accelerates under an applied force. • Ex. Basketball and bowling ball – Which has more inertia? • Bowling Ball! Why? Because it has more mass! • Therefore, mass can be defined as the amount of matter in an object and also as a measure of the inertia of an object.

  7. Equilibrium • Definition: objects that are either at rest or moving with constant velocity are said to be in equilibrium. Equilibrium means the net force acting on a body in equilibrium must be equal to zero. • It the net force is zero, the body is in equilibrium. IF there is a net force, a second force equal and opposite to this net force will put the body in equilibrium.

  8. Law of Acceleration – Newton’s Second Law • Relates acceleration of an object to the force applied on it and the mass of the object • A is directly proportional to F • A is indirectly proportional to M • Newton’s 2nd law of motion – which states: The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass. Force = mass x acceleration

  9. Formula • F = ma • Units: m….kg a….m/s2 F….kg-m/s2 or N (Newton)

  10. Mass vs. Weight • Mass…the amount of matter • Weight…the gravitational attraction for matter • 1 N is the “weight” of one medium apple! • So, W = mg where g= 9.8 m/s2 • Is a vector … + is up - is down

  11. VI. Net Force = Fnet = MAnet • Fnet is the sum of all of the forces acting on an object VERTICAL: Fnet = Fup + Fdown = Fa + W = Fa + mg HORIZONTAL: Fnet = Fforward + Ffriction = Fa + μW = Fa + μmg

  12. Ex. 13. A 1090 N person, standing on a scale, is moving upward in an elevator at a constant velocity. Find the reading on the scale. + 1090 N

  13. Fnet = Fup + Fdown MAnet = Fscale + W Fscale = MAnet – W Fscale = 0 – ( - 1090) = + 1090 N

  14. Ex. 14. A 88.2 kg person standing on a scale in an elevator is moving upward with an acceleration of 1.55 m/s2. Find the reading on the scale. + 1.00 x 103N

  15. Fnet = Fup + Fdown MAnet = Fscale + mg Fscale = MAnet – MG = M (a-g) = 88.2 [1.55 – (-9.80)] = 88.2 (11.35) = 1001.07 = + 1.00 x 103N

  16. 15. A 36 kg object is accelerated upward at 2.0 m/s2 . Calculate the force applied on the object. 420 N

  17. Fnet = MAnet = Fup + W Fup = MAnet – W = MAnet – mg = m (a-g) = 36 [2.0 – (-9.80)] = 36 (11.8) = 424.8 = 420 N

  18. 16.. A rocketship ( m = 1.75 x106 kg) has a lift-off force of 3.50 x107 N. Calculate the net acceleration of the spacecraft. + 10.2 m/s2

  19. Fnet = MAnet = Fup + W Anet = (Fup + W) / M = (Fup + mg) / m = [3.50 x107 N + (1.75 x106 kg)(-9.80)] / 1.75 x106 kg

  20. 19.. A 4.55 kg cat is shot upwards with an overall acceleration of 15.5 m/s2. Calculate the thrust needed for the launch. 115 N

  21. Fnet = MAnet = Fup + W Fup = Fnet – W = ma – mg = m (a-g) = 4.55 [15.5 –(-9.80)] = 4.55(25.3) = 115 N

  22. Fnet = MAnet = Fup + W Fup = MAnet – W = MAnet – mg = m (a-g) = 36 [2.0 – (-9.80)] = 36 (11.8) = 424.8 = 420 N

  23. 16.. A rocketship ( m = 1.75 x106 kg) has a lift-off force of 3.50 x107 N. Calculate the net acceleration of the spacecraft. + 10.2 m/s2

  24. Fnet = MAnet = Fup + W Anet = (Fup + W) / M = (Fup + mg) / m = [3.50 x107 N + (1.75 x106 kg)(-9.80)] / 1.75 x106 kg

  25. 17.. A 36 kg rock is pushed against a surface where μ = 0.800. What force must be applied to give an net acceleration of 0.25 m/s2? 290 N

  26. Fnet = MAnet = Fa + Ff MAnet = Fa + μmg Fa = MAnet – μmg = m (a – μg) = 36 [0.25 – (0.800)(-9.80)] = 36 [0.25 + 7.84] = 36(8.09) = 291.24 = 290 N

  27. 18.. A 788 kg rock is pushed along a road, using 6850 N of force to achieve 0.250 m/s2 of acceleration. Calculate the COF of the road and the rock. 0.862

  28. Fnet = MAnet = Fa + μmg μmg = MAnet – Fa μ = (ma – Fa) / mg = [(788)(0.250) – 6850] / (788)(-9.80) = -6653/-7722.4 μ = 0.862

  29. Friction Friction…any force that opposes the motion of an object • Due to bonding forces between the 2 surfaces…”microwelds”

  30. Types of friction: 1. Static Friction…force necessary to start the motion of an object at rest …break the microwelds 2. Sliding Friction…F necessary to keep an object in motion at a constant velocity …less than the static friction

  31. 3. Rolling Friction…friction between a rolling object and the surface it is rolling on 4. Fluid Friction…friction between a moving fluid and the surface of the object it is flowing within …”fluids” include liquids and gases

  32. Coefficient of Friction = μ • The ratio of the applied force to the weight • High μ…high resistance to motion • Low μ…low resistance to motion • μ = -Fa(Ff) / Fgor -Fa(Ff)=μFg

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