Chapter 4 - Newton’s Laws of motion. 4.1 force and interactions. A force is a push or a pull. It is an interaction between two bodies or between a body and its environment. Force as a vector quantity.
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Note: we draw a wiggly line through the force vector F to show that we have replaced it by its x and y components.
It’s important to note that Newton’s first law is under the condition that the net force on the object is zero.
According the Newton’s 1st Law, an object in motion will remain in motion. The spaceship will not slow down, it will travel at constant velocity forever.
You are driving a Porsche Carrera GT on a straight testing track at a constant speed of 150 km/h. you pass a 1971 Volkswagen beetle doing a constant 75 km/h. For which car is the net force greater?
Since both cars are in equilibrium because their velocities are constant, therefore the net force on each car is zero.
Suppose you are in a bus that is traveling on a straight road and speeding up. If you could stand in the aisle on roller skates, you would start moving backward relative to the bus as the bus gains speed. If instead the bus was slowing to a stop, you would start moving forward down the aisle. In either case, it looks as though Newton’s first law is not obeyed; there is no net force acting on you, yet your velocity change. What wrong?
Initially, you and the vehicle move at constant velocity. Your body tends to move at constant velocity as the vehicle slows down you.
Initially, you and the vehicle stay at rest. Your body tends to stay at rest as the vehicle accelerating around you.
Newton’s 1st law tells us the when a body is acted on by zero net force, it moves with constant (including zero) velocity and zero acceleration.
But what happens when the net force is not zero?
Newton’s 2nd law - A net force acting on a body causes the body to accelerate in the same direction as the net force.
If the magnitude of the net force is constant, then the magnitude of the acceleration is also constant. In fact, the magnitude of the acceleration is directly proportional to the magnitude of the net force acting on the body.
These conclusions about net force and acceleration also apply to a body moving along a curved path.
One Newton is the amount of net force that gives an acceleration of 1 m/s/s to a body with a mass of 1 kilogram.
For the same net force, the ratio of the masses of two bodies is the inverse of the ratio of their acceleration.
1. ∑F means the sum of all external forces.
2. the equation is only valid if mass is constant.
3. the equation is only valid in the inertial frame of reference
1 dyne = 10-5 N
1 pound = 4.48 N
w = mg
w = mg
A body’s weight acts at all times
If we take a standard kilogram to the surface of the moon, its weight is 1.62 N, but is mass is still 1 kg.
An 80.0 kg astronaut weighs 784 N on Earth, but weighs only 130 N on the moon.
CAUTION: the two forces in an action-reaction pair act on different bodies. Unlike in Newton's 1st or 2nd Law, which involve the forces act on one body.
The action and reaction forces can be contact forces or long-range forces.
When you drop a ball, both the ball and the earth accelerate toward each other. The net force on each body ahs the same magnitude, but the earth’s acceleration is microscopically small because its mass is so great. Nevertheless it does move!
In both cases, the force you exert on the car is equal in magnitude and opposite in direction to the force the car exerts on you.
An apple sits on a table in equilibrium. What forces act on it? What is the reaction force to each of the forces acting on the apple? What are the action-reaction pairs?
Action: Man on Rope
Reaction: Rope on Man
Action: Rope on Box
Reaction: Box on Rope
Not Action-Reaction Pair
Man on Rope
Box on Rope
Man on Rope
Rope on Box
We saw in example 4.10 that the stonemason pulls as hard on the rope-block combination as that combination pulls back on him. Why, then, does the block move while the stonemason remains stationary?
They are the same in magnitude and opposite in direction.
Whyis the mosquito splattered while the car is undamaged?
The car’s mass is much bigger than the mosquito, therefore, its acceleration a = Fnet / mass is much smaller than that of the mosquito.