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Newton’s 2 nd Law of Motion. True or False?. In your journal, re-write the statements below, state whether you think it is true or false, and provide 1-2 sentences of your reasoning. When a ball has been thrown, the force of the hand that threw it remains on it.
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True or False? In your journal, re-write the statements below, state whether you think it is true or false, and provide 1-2 sentences of your reasoning. • When a ball has been thrown, the force of the hand that threw it remains on it. • A force is always needed to keep an object moving. • Inertia is a force. • Air does not exert a force. • The quantity ma(Mass x Acceleration) is a force.
Equilibrium • A system is in equilibrium if and only if the net force acting on it is equal to zero. • Static Equilibrium: Fnet = 0 N and the system is at rest • Dynamic Equilibrium: Fnet = 0 N and the system is moving at a constant velocity
Net Force • The net force (Fnet)acting on an object is the vector sum of all the forces acting on an object. • To find the vector sum, we need to separate forces into vertical and horizontal components, • This is similar to what we did for component method in vector addition and the velocities during projectile motion. • If the net force is 0 N, then we say that the forces acting on the object are balanced forces.
Equilibrium Example • A sign is hanging from the ceiling by 2 cables. The cables are equal lengths and are placed at each of the 2 top corners of the sign. The weight of the sign is 490 N. If the cables attached to the sign are hanging straight down, what is the tension in each cable?
The Connection • Newton’s 1st Law: • No unbalanced forces → Fnet = 0 N • Stays in motion or at rest → v = constant • In other words → a = 0 m/s2 • This begs the question of the “other” case • Unbalanced forces → Fnet ≠ 0 N • From 1st Law, we can deduce → v is changing! • In other words → a ≠ 0 m/s2!
Acceleration and Force • How does acceleration depend upon the force? • Force and acceleration are directly proportional: the larger the force, the larger the acceleration • Linear relationship • a α F so if a ↑ then F ↑
Acceleration and Mass • How does acceleration depend upon the object? • As the mass (amount of object) increases, a greater force will be needed to accelerate the object. • Inverse relationship • i.e. - same force with twice as much mass half as much acceleration • a αso if m↓ then a ↑
Newton’s 2nd Law of Motion • “The acceleration of an object is proportional to the net force exerted on the object and inversely proportional to the mass of the object being accelerated.” • In other words: A net force exerted on an object will cause the object to accelerate in the direction of that net force. The acceleration will be inversely proportional to the mass of the object. a α F and a α→
Solving 2nd Law Problems • Follow a few steps to solve many problems: • Draw a FBD • Find and label a and/or Fnet • FindFnet • Use our new equation from Newton’s 2nd Law:
Sample Problem 1 • As a dutiful older sibling, Pat pulls Pete on a sled across the snow. When they get closer to home, Pat starts running! If the mass of the sled (with Pete on board) is 35 kg and their acceleration is 1.5 m/s2, what is the net force that Pat is exerting on the sled? (Assume Pat is pulling horizontally on the sled)
Sample Problem 2 • Clyde’s job is to determine how safe cars are. When he is testing a new model of car, he starts the car out at 18 m/s and brings it to a stop in an astonishing 0.20s! If the mass of the car is 1200 kg, what is the net force experienced by the car? (Hint: use our kinematics equations to find the acceleration first!).
