Physics Ch. 5 Review: Forces & Newton's Laws

Force

An interaction that, when unopposed, changes the motion or shape of an object, commonly experienced as a push or a pull. It is a vector quantity measured in newtons (N).

Friction

A force that opposes the relative motion of two surfaces in contact, caused by microscopic surface irregularities. It acts parallel to the contact surface and is measured in newtons (N).

Everyday forces

Forces commonly acting on objects in daily life, including:

• Gravity (pulling objects downward)

• Air pressure (atmospheric force exerted on all exposed surfaces)

• Normal force (support force exerted by surfaces like floors, which also includes friction)

Net force

The vector sum of all individual forces acting upon an object, calculated as Ftotal = F1 + F2 + F3 + ...

Mass

A measure of the amount of matter in an object. Measures an object’s inertia, or how difficult it is to change its velocity. Ex. A car has more mass than a piece of chalk, so it takes more force to accelerate or slow down the car. A scalar quantity (SI = kg), only has magnitude no direction.

Newton’s First Law

An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. Objects naturally resist changes in their motion (inertia). If net force = 0: The object’s velocity does not change → no acceleration. Ex. A book sitting on a table stays still because the net force is zero.

Newton’s Second Law

An object’s acceleration is directly proportional to the net force acting on it and inversely proportional to its mass. Formula: Fₙₑₜ = ma. Force: vector quantity → magnitude + direction. Mass: scalar quantity → magnitude only

What is acceleration’s relationship to net force?

Directly proportional to net force: As net force increases, acceleration increases, if mass stays constant. a ∝ Fnet​

Why does an object fall?

Gravity creates a net downward force, causing the object to accelerate toward the ground. Force: Gravity pulls downward. Acceleration: The object’s downward velocity increases over time. a = g ≈ 9.81 m/s²

How are the directions of the Force and acceleration related?

Acceleration is always in the same direction as the net force. Net force → right → acceleration → right; Net force → left → acceleration → left; Net force = 0 → acceleration = 0.

What is the SI unit of Force?

Measured in Newtons (N), where 1N = 1kg ⋅ m/s²

Weight

Force of gravity acting on an object, W = mg. Weight = gravitational force. Direction: downward, toward Earth.

Normal force

A perpendicular support force a surface exerts on an object. Ex. A book sitting on a table → the table pushes upward on the book with a normal force.

Is normal force always equal to weight?

No, normal force equals weight only in certain situations, such as an object resting on a level surface with no other vertical forces. Ex. Partially lifted book, additional downward pressure, or an incline (NF not = to weight).

Is weight and mass the same everywhere?

Can change with gravity, but mass stays the same everywhere.

What can cause a scale reading to change?

Can change due to tilt, external forces, or different gravity

Einstein travels upward and slows down. What is the acceleration?

Downward; velocity is upward but slowly means the acceleration is opposite.

Einstein travels downward and speeds up. What is the acceleration?

Downward; velocity and acceleration are in the same direction.

Einstein travels upward at a constant speed. What is the acceleration?

Zero; constant velocity means no acceleration.

How do you find acceleration when multiple forces act on an object?

Ex. Car: 275 N + 395 N forward, 560 N friction, 1850 kg. Find a.

Find net force: Fnet = forces with motion − opposing forces. Then apply a = Fnet​ / m.

Answer: 0.0595 m/s²

How do you combine forces in the same direction? What about opposing forces?

Add them together to find the sum. Subtraction is needed to find the sum of opposing forces.

How do you find acceleration from net force and mass?

a = Fnet / m

What is the net force? 275 N + 395 N forward, 560 N friction

110 N forward

What determines the direction of acceleration?

The direction of the net force.

If the net force is zero, what is the acceleration?

Zero; the velocity does not change.