Forces & Newtons Law

What is a force?

A force is a push or a pull. Force is a vector quantity with SI unit newton (N).

What are the two categories of forces?

Contact forces (require physical contact) and action-at-a-distance forces (e.g., gravity, electrical forces).

What is mass?

A measure of the amount of matter in an object; a scalar with SI unit kilogram (kg).

State Newton’s 1st Law.

An object remains at rest or moves at constant speed in a straight line unless acted on by a net force.

What is a net force?

The vector sum of all forces acting on an object (found by vector addition).

State Newton’s 2nd Law.

a= f(net)/m. Acceleration is proportional to the net force and inversely proportional to mass; direction is that of the net force.

State Newton’s 3rd Law.

For every action, there is an equal and opposite reaction—forces come in equal-magnitude, opposite-direction pairs.

Example: A car (800 kg) accelerates from 8 m/s to 32 m/s in 16 s. What is the required force?

Use a=Δv/ta = \Delta v / ta=Δv/t then F=ma.(Student computes during practice.)

Example: A 0.01 kg bullet stops from 110 m/s in 1.18 ms. What force does the tree exert?

Compute deceleration a, then use F=ma

What is a free-body diagram (FBD)?

A picture showing an object and all forces acting on it.

What is weight?

The gravitational force Earth exerts on an object: W=mg, directed downward.

What are the three fundamental forces in nature?

Gravitational, electroweak, and strong nuclear forces.

What are non-fundamental forces?

Forces that arise from fundamental forces, such as friction, tension, and normal force.

State Newton’s Law of Universal Gravitation.

F=G m1m2/r2, The gravitational force is proportional to the masses and inversely proportional to the square of their separation.

What is the value of the gravitational constant G?

6.67 × 10(-11) N\ccdotpm(2)/kg(2)

What is the centre of mass?

The point where the mass of an object can be considered concentrated for gravitational calculations.

How is local acceleration due to gravity g related to G?

From universal gravitation:

g=GM/r(2)

Example: Given Moon’s Mm = 7.35 × 10(22)kg and radius 1.74 × 10(6)m, compute gM

Use the formula g = GM/r(2).

What is the normal force?

A force exerted by a surface on an object, perpendicular to the surface.

What is friction?

A force parallel to the surface that opposes motion or attempted motion.

What is static friction?

Friction when surfaces are not sliding; maximum value fs≤μsN.

What is kinetic friction?

Friction when surfaces slide; magnitude fk=μkN

Example: A 50 kg sled moving at 4 m/s on ice—how long to stop?

Use kinetic friction to find deceleration and stopping time.

What is tension?

A pulling force transmitted through a rope, cable, or chain.

How does tension behave in a massless rope?

It is transmitted undiminished throughout the rope.

How does a massless, frictionless pulley affect tension?

It transmits the same tension to both sides of the rope.

What major topics were learned in Topic 4?

• Force & mass

• Newton’s 3 laws

• Net force

• Free-body diagrams

• Gravitational force

• Universal gravitation

• Weight

• Friction

• Tension