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Forces and Newton’s Laws of Motion Chapter 4
All objects naturally tend to continue moving in the same direction at the same speed. All objects resist changing their velocity. (velocity is speed and direction) Resistance to changing velocity is inertia. The amount of inertia is the mass. Mass is measured in kilograms (kg). Inertia
Forces External forces can change an object's velocity. Total external force is called net force. Net force determines how quickly and in what direction the velocity changes. When net force is zero, the velocity remains unchanged. If the object is moving, it keeps moving in a straight line.
Newton's first law When net force, the velocity stays the same. (same speed and same direction) The reverse is also true. When the velocity stays the same, the net force. Newton's first law is also called the law of inertia because an object's inertia keeps it moving in the same direction at the same speed if the net force is zero.
An inertial reference frame is a coordinate system in which Newton’s law of inertia is valid. Inertial frames have a constant velocity. All accelerating reference frames are non- inertial because Newton’s law of inertia is not valid in accelerated coordinate systems. Non-inertial frame: A car when speeding up, slowing down, or turning a corner. Inertial reference frame
Individual ForcesNet Force 10 N4 N The net force is the vector sum of all external forces acting on a single object. The SI unit of force is the newton (N). Net force 6 N
Individual ForcesNet Force Net force 3 N 4 N 3 N 4 N 5 N
Newton's second law An object's acceleration vector is equal to the net force acting on the object divided by the object's mass. Acceleration direction is same direction as net force.
or Newton's second law Use x and y components when you need more than + and - to specify directions.
SI unit of force newton N is the SI unit for force
Force diagram (also known as free-body diagram) Net force is in the +x direction. mass 1850 kg ALWAYS draw a force diagram with a force vector for each individual, external force acting on a single object.
The “+” sign indicates the +x direction. Example: What is the car's acceleration? Net force is +110 N and car's mass is 1850 kg
Newton's third law Two objects always exert equal and opposite forces on each other. Forces always occur in pairs. Each object experiences the force equally, but the forces are in opposite directions. Example: You are pushing down on the chair with some force and the chair is pushing on you with the same amount of force but in the opposite direction (up).
Astronaut pushes with 36 N on a spacecraft. What are their accelerations? Example 11,000 kg 92 kg The two objects exert 36 N of force on each other.
Find the tension in the trailer drawbar and the force D that propels the truck forward. Ignore the forces of friction and air resistance. Unbalanced horizontal forcesUnbalanced horizontal force Equal and opposite forces on different objects Example
There are two general types of forces in nature, fundamental and non-fundamental. Fundamental gravity electromagnetism weak nuclear strong nuclear Forces Non-fundamental push pull support friction tension...
Universal law of gravity Every two objects in the universe attract each other with the gravity force. magnitude Equal attracting forces in exactly opposite directions. centerline universal gravity constant (Newton's 3 rd law: equal and opposite forces on two objects)
Weight is the gravity force on an object where Example: a 5 kg mass "weighs" 49 N
Contact forces Normal means perpendicular so the support force is often called the normal force. Support Perpendicular to the contact surfaces Friction Tangent to the contact surfaces Sliding friction (kinetic) Non-sliding (static)
15 N 15 N block Examples: Forces on the block
Apparent weight is the reading of the scale. It is equal to the support force the scale exerts on the person and the person exerts on the scale. Apparent weight forces on the person
Friction force direction opposes the impending relative motion between two objects. Static friction Static friction magnitude is just enough to prevent motion. static friction coefficient Static means the two surfaces are not sliding across each another.
Kinetic friction opposes sliding motion kinetic friction coefficient Kinetic friction Friction forces do not depend on contact area. Friction depends on contact force F N.
force diagram Kinetic friction force causes the sled to slow down. Example: Find sliding distance 0° get the directions analyze the x and y components
Tension force Flexible things like rope, string, cables exert tension forces. Force direction is always tangent to the rope, etc. Tension force has the same magnitude at each end.
Pulleys change the direction of a tension force, but not the magnitude of the force. Newton's third law tells us that the rope exerts the same amount of force on the man as the man exerts on the rope. To simplify things, physics ropes and cables are usually massless and pulleys are usually frictionless. The rope has no weight that needs to be considered. Tension force force of the rope on the block force of the man on the rope man
or Equilibrium Equilibrium mean balanced. For equilibrium the forces are balanced and the net force. Velocity magnitude & direction stay the same.
Select an object to analyze. Draw a force diagram showing only the forces acting on the object, but not forces that the object exerts on other things. Choose a set of x and y axes. Set up balanced force equations The sum of the x force components add up to zero. The sum of the y force components add up to zero. Solve for any unknown quantities. Equilibrium Reasoning Strategy
2.2 kg Example find F Object is the pulley, but first find the tension in the rope First, select the block as the object. If the block stays at rest, then the forces acting on it are balanced. Therefore, the tension and gravity forces are balanced. ( Second law: )
21.56 N If the pulley stays at rest, then the forces on the pulley are balanced. (Second law: ) Second, select the pulley as the object
F x =F cos θF y =F sin θ -F0 17.66 N12.37 N 17.66 N-12.37 N 00
Engine weighs 3150 N Example
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