Forces, Net Force, Newton's Laws & Friction

What is force

A push or pull that can change an object's motion

What is the SI unit of force

The newton (N)

What is net force

The vector sum of all forces acting on an object

What are balanced forces

Forces that cancel so the net force is zero

What are unbalanced forces

Forces that produce a nonzero net force

What happens to an object's acceleration when the net force is zero

Its acceleration is zero

If net force is zero, must the object be stationary

No. It may be at rest or move with constant velocity

What is Newton's First Law

An object remains at rest or moves at constant velocity unless acted on by a nonzero net external force

What is inertia

An object's resistance to changes in its motion

How does mass affect inertia

Greater mass means greater inertia

What is Newton's Second Law

An object's acceleration depends on net force and mass according to F = ma

What is the formula for net force

Fnet = mass multiplied by acceleration

What is the SI unit relationship for a newton

1 N = 1 kg·m/s²

What is Newton's Third Law

When one object exerts a force on another, the second exerts an equal and opposite force on the first

Why don't Newton's Third Law forces cancel each other on one object

They act on different objects

A 5 kg object accelerates at 4 m/s². Find its net force

20 N

A 12 kg object experiences a net force of 36 N. Find its acceleration

3 m/s²

A net force of 45 N produces an acceleration of 5 m/s². Find the mass

9 kg

A 10 kg object has a net force of zero. What is its acceleration

0 m/s²

A 4 kg object has a 30 N force east and a 14 N force west. Find the net force and acceleration

16 N east and 4 m/s² east

A 6 kg object is pulled right with 50 N and left with 50 N. What happens to its motion

Its acceleration is zero, so its velocity remains constant

If the net force on the same object doubles, what happens to its acceleration

It doubles

If the same net force acts on twice the mass, what happens to acceleration

It becomes half as large

What is static friction

Friction that prevents surfaces at rest relative to each other from beginning to slide

What is sliding friction

Friction that opposes surfaces sliding past each other

What is rolling friction

Resistance that opposes an object rolling over a surface

What is air resistance

A force that opposes motion through air

What is traction

Grip between surfaces that helps prevent slipping

A box is pushed right with 20 N while friction pushes left with 20 N. What is the net force

0 N

Why does a sliding box eventually stop if no one keeps pushing it

Friction creates a net force opposite its motion

A rocket moves forward as gas is expelled backward. Which law explains this

Newton's Third Law