Forces

Chapter 6.1

What You Already Learned

• Velocity is the ratio of the change in position of an object over a period of time.

• Acceleration describes a change in velocity of an object.

Acceleration

• What may cause an object to experience a change in velocity?– Force

• Forces are often described as a push or pull.

• A force may cause an object to speed up.• A force may cause an object to slow down.• A force may also cause an object to

change direction.

Types of Forces

• Gravitational• Electromagnetic• Normal• Frictional• Tension

Force

• Force is a vector quantity, therefore it has both magnitude and direction.

• The SI unit for force is the Newton.– The Newton is a derived unit where:

1N = 1kg•m/s2

– 1 Newton is the amount of force required to accelerate a 1 kg mass 1 m/s2.

The System and Environment

• The object of interest is called the System.

• The area around the object is called the Environment.

Fg

FN

System

Environment

Contact vs. Long Range

• Some forces act over distances while others act only when two objects are in contact with one another.– Contact forces exist when two objects

are in contact with one another.– Long-range (FIELD) forces act over

distances without a need for direct contact. Electromagnetic forces and gravity are long-range forces.

Free Body Diagram

Fg

FN

=

FN = Force of Desk on Book

Fg = Force of Gravity on Book

Free-Body diagrams provide a means by which all the external forces acting on a system can be summarized and accounted for, and the resultant vectors determined.

System

Newton’s 2nd Law• Newton determined that the

acceleration of an object is directly proportional to the net unbalanced external force applied to move it and inversely proportional to the mass of the object.

• a = Fnet/m

Where F = Force in Newtons m = mass of the object a = acceleration of the object

Newton’s 2nd Law and Force vs. Acceleration

• How does acceleration relate to force?– As the force on an object increases,

the rate of acceleration will increase.

Acceleration

m1

m2

m2 > m1

Note: The slope of the line in a F vs. a graph will equal the mass of the object.The greater the slope, the greater the mass

Newton’s 2nd Law and velocity vs. time (constant force)

• Newton’s 2nd Law indicates that a constant force applied to an object will cause the speed to change at a constant rate.

Time

Note: Remember from kinematics that the slope of the line is equal to a.

Newton’s 2nd LawForce Constant / Different Mass

• If the applied force to a system is held constant and the mass is increased from m1 to m2, what would a velocity vs. time graph look like?

Time

m1

m2

More Mass = Less Acceleration.

m2 > m1

Newton’s 2nd LawMass Constant / Different Force

• If the mass is held constant and the force applied to a system is increased from F1 to F2, what would a velocity vs. time graph look like?

Time

F2

F1

More Force = More Acceleration.

F2 > F1

Example: What is the rate of acceleration?

• Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a 275 N force while the other applies a 395 N force. A third force of 560 N acts in in the opposite direction compared to the two people. What is the acceleration of the car?

Diagram the problem

Fnet = Fperson 1 + Fperson 2 – Fopposing force

State the Known and Unknowns

• What is known?– Mass (m) = 1850 kg– Force of person #1 (N) = 275 N– Force of person#2 (N) = 395 N– Opposing Force (N) = 560 N

• What is not known?– Acceleration (a) = ?

Perform Calculations

• Fnet = Fperson 1 + Fperson 2 - FopposingWhere:o Fnet = ma

• Substitute for Fnet and solve for a:o ma = Fperson 1 + Fperson 2 - Fopposing

o a = (Fperson 1 + Fperson 2 - Fopposing)/mo a = (275 N + 395 N – 560 N)/1850 kgo a = 0.059 m/s2

o If there was no opposing force, how would the rate of acceleration change?

o What do you think is the source of the opposing force?

It would increase.

Friction.

Key Ideas

• Force is a vector quantity that is a measure of the magnitude of a push or pull in Newtons.

• Forces exist as contact or long range.• Use free body diagrams to represent forces when

problem solving.• Newton’s 2nd Law of Motion states that the rate of

acceleration of an object is proportion to the force applied and inversely proportional to its mass.– A constant force applied to an object will cause it to

accelerate at a uniform rate.– As force increases, acceleration increases.– As mass increases, acceleration decreases.