Physics: Forces on a System

Grade 9 physics

[5:31 PM, 4/11/2026] Leen Qurban: * In a system, multiple objects interact with each other, and these forces can affect the motion of every object involved.

When you are in an elevator accelerating upward.

Your apparent(opposite) weight will be greater than your actual weight.

External Force

A force exerted by an object outside a defined system

Tension in the rope

Force that causes the dolly to accelerate in a pulley system with a sandbag pulling a dolly.

Internal forces

Forces within a system that cancel each other out in pairs according to Newton's Third Law

Equilibrium

A term which clockwise and counterclockwise torques must be equal when a meter stick is balanced at its center and two weights hung at different distances.

Mass A is heavier than mass B

In an Atwood Machine when mass A goes down and mass B goes up.

Torque

How effective a force is in producing rotation.

F = m x a

The equation that correctly represents Newton's second law for a block with mass (m) and acceleration (a).

An internal force

The force of Block A pushing on Block B In a system of two blocks (A and B) pushed by a hand.

A free-body diagram for a block sitting on a horizontal surface with a rope pulling it to the right.

A box that represents the block. From the box arrows extend for each force: an arrow pointing up for "normal force” an arrow down for "weight," an arrow to the right for "tension from the rope", and an arrow to the left for "friction" if needed.

Equilibrium (clockwise torque = counterclockwise torque)

The state of balance that causes internal forces to not change the overall motion of the entire system.

1.5m

Where you should place a 0.2N weight so that the meter stick is in static equilibrium. With a 0.6N weight on the meter stick 0.5m to the left of the center.

Hand pushing the first box(1). Box A pushing on Box B(2).

When you are pushing a stack of three boxes across a floor. The external force acting on the system of boxes(1). The internal force acting between the boxes(2).

free-body

A diagram that shows all the forces acting on an object is called a [blank] diagram.

tension

In a system with two blocks connected by a rope, the force that the rope applies to each block is called [blank].

kinetic friction

The force that opposes the motion of an object sliding on a surface is called [blank].

mechanical

When all the forces and torques on an object are balanced, the object is in [blank] equilibrium.

string/rope

In an Atwood machine, the two masses are connected by a [blank] that passes over a pulley.

magnitude

When two objects are connected and move together, they have the same [blank] of acceleration.

ignored/canceled out

In a free-body diagram, internal forces between objects in the same system are [blank] when considering the system as a whole.

friction

The force that opposes motion between two surfaces in contact is called [blank].

constant

In a pulley system, the tension throughout an ideal rope (with no friction or mass) is [blank] everywhere along the rope.

torques

For a meter stick balanced at its center, the clockwise and counterclockwise [blank] must be equal for equilibrium.

free-body

A [blank] diagram is a tool used to model all the forces acting on an individual object.

acceleration

The sum of all external forces acting on a system is equal to the total mass of the system multiplied by the [blank]

static

When an object is not rotating and not moving, it is said to be in [blank] equilibrium.

normal

In the elevator model, the [blank] force from the floor is what we feel as our weight.

acceleration

If two blocks are connected by a string and move together, they must share the same [blank].

internal force

Force between two blocks inside a system is [blank].

lever/momentum arm

[blank] is the perpendicular distance to the pivot.

torque

The "twist" or rotational force is [blank].

tension

[blank] is the force exerted by a string or rope.

center of mass

[blank] is the point where a system's mass is concentrated.

newtons (N)

Force is measured in [blank].

Internal forces

[blank] don't change the system's motion because they come in pairs that cancel each other out (Newton's Third Law).