 # Physics: Newton’s Laws

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Physics: Newton’s Laws
Mr. Fox’s Science Class The insanity moves on! Newton’s balls anyone?

Sir Isaac Newton The fig newton is not named after him!
Was in a heated competition with Robert Hooke over who was better for England. One of the best Scientists to date Came up with Laws on Motion and mechanics

The Nature of Force Force – a push or a pull.
Unbalanced forces – cause an object to start moving, stop moving, or change direction. Balanced forces – equal forces acting in opposite directions on an object Net Force – overall force on an object after all the forces are added together.

Unbalanced Forces An unbalanced force acting on an object will change the object’s motion.

Balanced Forces Balanced Forces acting on an object will not change the object’s motion.

Newton’s First Law of Motion
Inertia – the tendency of an object to resist change in it’s motion. Newton’s first law states - An object at rest will stay at rest, or an object in motion will stay in motion, unless acted upon by another unbalanced force.

Mass vs. Inertia Amount of Inertia depends on the mass of the object.
The more mass an object has, the more resistant to change in motion the object is going to have.

Newton’s Second Law of Motion
The net force on an object is equal to the product of its acceleration and its mass. OR Force = Mass x Acceleration Units: kg x m/s/s or N (for newton)

Practice Problems What is the force on a 1000kg elevator accelerating at 2m/s/s? How much force is needed to accelerate a 55kg runner at 15m/s/s?

Gravity Obviously the force that holds us to the Earth.
m/s/s is acceleration due to gravity. How much do you weigh in newtons? 1lb. = kg

Gravity Confusion If two objects of different masses, are dropped in a vacuum they would hit the ground at the same time. If one object is dropped straight down, and another shot forward they will still hit the ground at the same time. ( I will prove this on Monday. ) So why don’t a leaf and an acorn hit the ground at the same time when they both drop at the same time?

Weight Remember weight is caused by gravity! Everything has mass, but only objects that are being pulled by gravity have weight. Weight = Mass x (Acc. Due to gravity) How much would you weigh on the moon? (Gravity = 1.65m/s/s) How much would you weigh on Jupiter? (Gravity = m/s/s)

Friction The force that one surface exerts on another when the two rub against each other. The strength depends on the type of surface and how hard the surfaces are pushed together. Is friction useful?

Types of Friction Sliding Friction – when solid surfaces slide over each other. Rolling Friction – when an object rolls over a surface Fluid friction – friction that occurs when an object moves through a fluid.

Air Resistance Objects falling through air experience fluid friction.
The greater the surface area an object has the more air resistance it encounters.

Newton’s 3rd Law of Motion
If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. For every reaction… there is an equal and opposite reaction.

Momentum Explains why someone would want to catch a baseball at 90mph rather than a car at 90mph. Momentum = Mass x Velocity kg · m/s No fancy unit name. Think of momentum as damage!

Practice 1. If a car with a mass of 500kg has an acceleration of 120m/s/s. How much force is acting upon the car? 2. Mr. Fox is exerting a force of 3135N. We know he has a mass of 122kg. How much is he accelerating? 3. An unknown object has a mass of 0.14kg and a velocity of m/s. Another unknown has a mass of 0.042kg and a velocity of 935m/s. Which has more momentum? Which one would cause more damage? What do you think the two objects are?