Motion physics (Term 1)

Newton’s Law 1

If vector sum of all forces on an object is zero (no net force), the velocity of that object won’t change (no force no movement)

Newton’s Law 2

If there is net force the object will accelerate according to f = ma

Newton’s Law 3

Equal and opposite forces

F_{A on B} and F_{B on A}

• are equal in magnitude

• opposite in direction

• have same fundamental nature

• each act on a different object

Forces

• operate in fields

• fields mediate all forces, so all forces act at a distance

Centre of Mass

• where gravitational field emanates from

• strength decreases as distance from centre increases

• Earth is a sphere so all points on surface are roughly same distance from centre of mass thus experiencing the same strength

Weight Force Near Earth’s Surface

• Force on object due to gravity

• force applied to a mass within gravitational field is determined by field strength

• Near Earth, 1kg mass has 9.8N of force applied to it by Earth’s gravitational field

• 1kg mass and Earth have equal and opposite force, however the effect is proportional to mass, so 1kg falls

Gravitational Field Strength Formula

g = F_grav/m Nkg^-1

Weight Force Formula

• calculated using gravitational field strength

• F_w = mg

Gravitational Acceleration

F_w = mg is similar to F = ma, so if weight is the only force acting on an object (F_net = F_w) then a = g

Therefore gravitational field strength is the gravitational acceleration

When close to Earth’s surface g = 9.8Nkg^-1 = ms^-2 (same as gravity)

Vertical Motion

Any object in freefall will experience gravitational force in a vertical downwards direction, making it accelerate (change motion) vertically

Can swap a (accel) for g (grav field strength) and s (displacement) for y in equations

If object is falling (down) make value negative in equations