Forces & Newton 1

What Is a Force?• A Force is an interaction between

two bodies.– Convention: FFa,b means

“the force acting on a due to b”.• A Force is a push or a pull.• A Force has magnitude & direction

(vector).• Adding forces means adding

vectors.

Forces - 1151• Contact (fundamentally E+M)

– Normal: Perpendicular to surface– Friction: Parallel to surface– Anything touching the object

• Rope: Tension• Spring F = -kx• Person

• Non-Contact or Action at a Distance (these are field forces)– Gravity

g

Physics

Force & Mass• Force

– Symbol: F or F– Units:

• SI – Newton (N) (~0.22 lb)• English – Pound (lb)• cgs – dyne (10-5 N)

• Mass– Symbol: m or M– Units:

• SI – kilogram (kg)• English – slug (~14.5 kg)• cgs – gram (g)

Weight Compared to Mass• Weight is a Force

– Depends on the gravitational field strength at the location

• Mass is the amount of “stuff” – Mass is independent of location!

(although it is often measured by comparing weights)

– Mass is also the inertia of the object•Inertia is resistance to a change in

motion – See Newton 1

On Earth

W = (9.8N/kg) m

1.Every force must have an agent that produces it;

2.Every contact force must act only at the point of contact;

3.The normal force acts only perpendicular to the surface in contact;

4.The friction force acts only parallel to the surface in contact;

5.The tension force from a string or rope acts only along the line of the string or rope.

Remember

Tension

• Spring scale reads the force exerted on each end.

• Tension is force transmitted by rope and is the force exerted by each end of rope.

Tension ACT

A. 2000 N

B. 500 N C. 1000 N

D. 0 N E. 2000 kg

A pair of tug-of-war teams are pulling on the ends of a rope, each team with a force of 1000 N. The tension in the rope is:

Tension ACT

a. T1>T2 b. T1=T2 c. T1<T2 d. depends on pulley radius

Free Body Diagrams

Identify all forces acting on the object. Draw a coordinate system. Use the axes defined in your pictorial representation. If those axes are tilted, for motion along an incline, then the axes of the free-body diagram should be similarly tilted. Represent the object as a dot at the origin of the coordinate axes. This is the particle model. Draw vectors representing each of the identified forces. Be sure to label each force vector.

Drawing a FBD

Examples of Force Vectors

Pull(contact force)

Push(contact force)

Gravity(long-range force)

Identifying Forces Identify “the system” and “the environment.” The system is the object whose motion you wish to study; the environment is everything else. Draw a picture of the situation. Show the object—the system—and everything in the environment that touches the system. Ropes, springs, and surfaces are all parts of the environment. Draw a closed curve around the system. Only the object is inside the curve; everything else is outside. Locate every point on the boundary of this curve where the environment touches the system. These are the points where the environment exerts contact forces on the object. Name and label each contact force acting on the object. There is at least one force at each point of contact; there may be more than one. When necessary, use subscripts to distinguish forces of the same type. Name and label each long-range force acting on the object. For now, the only long-range force is weight.

Forces on a Bungee Jumper

T

wx

y

nfk

The Forces on a Skier

T

w

x

y

The Forces on a Rocket

Fthrust

wD x

y

Inertial Reference Frame• The following statements can be

thought of as the definition of inertial reference frames.– An IRF is a reference frame that is not

accelerating (or rotating) with respect to the “fixed stars”.

– If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector!

Newton 1• Newton’s First Law• An object subject to no external

forces is at rest or moves with a constant velocity if viewed from an inertial reference frameinertial reference frame.– If no net forces act, there is no

acceleration.

0 0netF a