Forces

Chapter 4

Force & Motion• Force-a push or a pull on an

object• System-the object(s)

experiencing the force• Environment-the world around

the system

Contact Forces & Field Forces

• How are all the ways you could move your textbook across the table top.

• Contact Forces

• Jump out of a plane. Is this a contact force?

• Field forces

Free Body Diagrams

• Pictorial model of the forces

• Forces can be applied left, right, up or down

• Forces are vectors (both magnitude and direction)

Combining Forces

• Forces can be added to find a Net force.

• If there is no net force the forces are balanced and then there is no change in motion

• If there is a net force then the forces are unbalanced and there will be a change in motion.

Newton’s Second Law of Motion

• Experiments are necessary in order to study what happens when forces are exerted

• Important to eliminate friction and gravity

• Gravity doesn’t act in the horizontal direction

Acceleration - Force

• If you apply 10 N to a 5 kg object and then 20 N to the same object, which would increase the motion more?

Acceleration - Mass

• If you apply 10 N of force to a 5 kg object and 10 N of force to a 50 kg object, which will have more motion?

Newton’s 2nd Law

• Acceleration depends upon both force and mass

m

Fa net

• a- acceleration, m/s2

• Fnet-the sum of the forces acting on the object

• m-mass, kg

The Newton

• One unit of force causes a 1 kg mass to accelerate at 1 m/s2

• Defined as a Newton, N

• SI unit for force

Newton’s First Law of Motion

• An object at rest stays at rest and an object moving will remain moving until a net force acts on the object.

Inertia• The tendency of an object to

resist a change in motion

• First law is sometimes called the law of inertia

Equilibrium• The net force acting on an

object is zero• Equilibrium doesn’t mean the

object is at rest• Equilibrium means the

acceleration is zero

Common Misconceptions

• When a ball has been thrown, the force of the hand that threw it remains on it.

• A force is needed to keep an object moving.

Using Newton’s Laws

4.2

• Early in the 20th century physicists discovered that the second law doesn’t hold for velocities close to the speed of light or masses the size of atoms

• But everyday experiences are still governed by Newton’s laws

Mass and Weight

• Galileo determined that all objects gain speed at the same rate no matter their size, shape or mass

• In other words, they have the same downward acceleration

• This has been tested and is true

Weight

• A force due to gravity.

• On Earth g=9.8 m/s2

• Weight is different on different planets, but mass stays the same

• Fg=mg

Scales

• Scales have springs that compress when a mass is placed on them

• The compression depends upon the pull of gravity

Apparent Weight

• The force exerted on a scale.

• The apparent weight changes if the object is accelerating up or down other than gravity.

• Example-elevator

Weightlessness• Doesn’t mean your weight is

zero

• Means that there are no contact forces pushing up on you

• Your apparent weight is zero

The Friction Force• The force that opposes motion

• Measured in Newtons

• Always acts in the opposite direction to motion

Terminal Velocity• Unlike friction between

surfaces, the air resistance (friction) depends upon the speed.

• Air resistance increases as the speed increases

• Eventually the upward force of air resistance equals the downward force of gravity

• Net force equals zero and the acceleration is zero and terminal velocity has been reached

• If the air resistance changes then the terminal velocity will also change