CHAPTER 4 FORCES IN 1-D

FORCE Force is anything which causes a body to start moving when it is at rest, or stop when it is moving, or deflect once it is moving. Force is a push or pull on an object.

KINDS OF FORCES Gravitational Force: is an attractive force that exists between all objects. It is a weak force. Electromagnetic Force: consists of electric & magnetic forces. These forces give materials their strength, their ability to bend, squeeze, stretch, or shatter.

KINDS OF FORCES Nuclear Force: hold particles in the nucleus together. It is the strongest kind of force. Weak Force: is a form of electromagnetic force and is involved in the radioactive decay of some nuclei.

TYPES OF FORCES Weight (W): A field force due to gravitational attraction between two objects. (Ex. You and Earth) Normal Force (FN): The contact force exerted by a surface on an object. (Ex. You and the floor)

TYPES OF FORCES Friction (Ff): A contact force that acts to oppose sliding motion between surfaces. (Ex. Sliding a box across carpet) Tension (T): The pull exerted by a string, rope or cable when attracted to a body and pulled taut. (Ex. A climber hanging from a rope)

TYPES OF FORCES Spring Force (FS): A restoring force; the push or pull a spring exerts on an object. Thrust (Ftrust): A general term for the forces that move objects such as rockets, planes, cars and people.

Newton’s First Law states that an object at rest remains at rest, and an object in motion continues moving in a straight line at constant speed, unless acted upon by an unbalanced force. Inertia is the tendency of an object to remain at rest or to keep moving in a straight line at constant speed.

Newton’s Second Law states that a net force acting on an object causes the object to accelerate in the direction of the force. A larger mass requires a greater force than a smaller mass would require to achieve the same acceleration.

If an object has a net force exerted on it, it will accelerate If an object has a net force exerted on it, it will accelerate. Force and acceleration both have direction and magnitude. The acceleration is in the same direction as the force causing it. (Ex. Picking up a stack of books)

F = m x a F = Force (N) m = mass (kg) a = acceleration (m / s2) One Newton: is the amount of force needed to accelerate an object with a mass of 1 kg at an acceleration of 1 m/s2. 1 N = 1 kg x 1 m / s2

EXAMPLE 1 What is the net force required to accelerate a 1500 kg race car at 3 m/s2.

EXAMPLE 2 An artillery shell has a mass of 55 kg. The shell is fired from a gun, leaving the barrel with a velocity of 770 m/s. The gun barrel is 1.5 m long. Assume the force, and thus the acceleration of the shell is constant while the shell is in the gun barrel. What is the force on the shell while it is in the gun barrel?

Weight of an object W, is the force of gravity acting on its mass. FALLING OBJECTS Near the Earth’s surface gravity causes all falling objects to accelerate at 9.8 m/s2. Weight of an object W, is the force of gravity acting on its mass. W = m x g

W = m x g W = weight (N) m = mass (kg) g = acceleration due to gravity (9.8 m/s2)

EXAMPLE 3 Find the weight of a 2.26 kg bag of sugar. (g=9.8 m/s2). The direction of the weight is downward.

ELEVATOR PROBLEMS A person is standing on a scale in the elevator. What does the scale read?

STEPS TO SUCCESS 1. Draw Free-Body Diagram 2. Consider what’s happening 3. Follow the GUESS method 4. Remember the scale reads normal force 5. Put a box around your final answer

WHAT’S HAPPENING A) Elevator is at rest B) Elevator is moving up C) Elevator is moving down D) Elevator is in free fall 

A) Elevator is at rest N W Scale reads W of object Elevator acceleration is zero (a=0) The two opposing forces are the normal force and weight of the object. N – W = 0  N = W (zero indicates no accel) N Scale reads W of object W

B) Elevator is moving up Elevator has acceleration The person on the scale is supported by the scale so they are balanced, however we must take the acceleration of the elevator into consideration N a W

REMEMBER N a W The elevator has to exert a force to move the object, the scale reading is affected by the elevator’s acceleration N – W = ma  N = ma + W (accel is positive here) N a W

C) Elevator is moving down Elevator has acceleration The person on the scale is supported by the scale so they are balanced, however we must take the acceleration of the elevator into consideration N a W

AGAIN N a W The elevator has to exert a force to move the object, the scale reading is affected by the elevator’s acceleration N – W = - ma  N = W – ma (accel is negative here) N a W

D) Elevator is in free fall Example of when the support cable breaks  Elevator’s acceleration is equal to gravity in this case N – W = -mg N = W – mg (where W = mg N = mg – mg N = 0 N a W

Air Resistance: is the force air exerts on a moving object. This force acts in the opposite direction to that of the objects motion. Force of Gravity Air resistance

AIR RESISTANCE Air resistance pushes up as gravity pulls down. The amount of air resistance depends on the size, speed, shape, and density of the object. The larger the surface area the greater the amount of air resistance on it.

Newton’s Third Law of Motion states that when one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude but opposite in direction.

FRICTION STATIC FRICTION is the force that opposes the start of motion. KINETIC FRICTION is the force between surfaces in relative motion. Kinetic Friction < Static Friction The static friction of an object is greater than its kinetic friction

1 BODY SYSTEMS IN 1-D Pushing down on an object with a force (F) F N W

In the y- direction: N – W – F = 0 object is not moving so forces balance W

1 BODY SYSTEMS IN 1-D Consider an object hanging from a chain. T W

T W In the y- direction: T – W = 0  T = W object is not moving so forces balance W

2 BODY SYSTEMS IN 1-D T T a m2 m1 Consider two objects hanging over a pulley. T T a m2 m1

T T T a a T m2 m2 a m1 m1 m2g Object 2 m1g Object 1

T m2a T m2 m1 m2g m2a = T – m2g m1g m1a m1a = m1g – T