Newton’s Laws of Motion
1. Newton’s First Law
a. An object at rest remains at rest and an object in motion remains in motion at a constant speed and in a straight line unless acted on by an unbalanced force.
b. Also called the Law of Inertia
c. Inertia is the tendency of an object to resist any change in motion. If not in motion, it will stay motionless. If moving it will continue at the same speed and direction.
d. When a car traveling at 60 mph comes to an abrupt stop, anything not strapped in will continue at that speed and direction until a force acts on it.
2. Newton’s Second Law
a. The acceleration of an object depends on the mass of the object and the amount of force applied.
b. Force = Mass x Acceleration
a = F ÷ m
m = F ÷ a
c. SI unit for force is the newton (N)
d. A newton is equal to one kilogram accelerated at one meter/second2 (1 kg∙m/s2)
e. A medicine ball has a mass of 8 kg and is accelerated at 25 m/s2. How much force has been applied?
m = 8 kg
a = 25 m/s2
f = ?
f = m x a
? = 8 kg x 25 m/s2
F = 200 N
3. Newton’s Third Law
a. To every action force, there is an equal and opposite reaction force.
Action (person pushing water with arms)
Reaction (water exerts a force on the swimmer)
b. Also called Action-Reaction pairs
c. Examples: recoil from shooting a gun, jumping from a boat, throwing a ball from a wagon
d. While driving down the road, Anna Litical observed a bug striking the windshield of her car. Quite obviously, a case of Newton’s third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?
e. A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is a(n)…
f. Rockets are unable to accelerate in space because…
i. There is no air in space for the rockets to push off of.
ii. There is no gravity in space.
iii. There is no air resistance in space.
iv. Nonsense!! Rockets do accelerate. They accelerate because burning fuel pushes the inside of the rocket (action) and comes out the back (reaction).
4. Momentum
a. Property of a moving object based on the object’s mass and velocity
b. Mass x velocity = momentum
c. Mo / velocity = mass
d. Mo / mass = velocity
e. Mo units: kg ∙ m/s
f. Momentum can be transferred