Force, Mass, and

Acceleration

Is it easier to push an empty shopping cart at the grocery store than it is to push a cart with all the week’s groceries in it? Why?

The cart is lighter when there is no food in it, right? Therefore it has less mass.

Less mass means that you don’t have to work as hard to push the cart, and because of that you can push it faster.

• This is a perfect example of Newton’s 2nd law of motion, which states that net force of an object is equal to the product of its acceleration and it’s mass OR

F=massXacceleration (F=ma).

• The units scientists use for force are Newtons (N).

• The newton is composed of units for acceleration, which are meters/second/second (m/s/s) and units for mass, which are kilograms (kg). • F(N)=m(kg)a(m/s/s)

• If you double the acceleration of an object, then the force must also double.

• EX: when you want to run faster do you work more or less? More, right? Because it takes more work, or more force, for you to move faster.

If you double the mass of an object, then the acceleration decreases by half.

• EX: If you are running and someone jumps on your back, do you speed up or slow down? Slow down, right? Because it’s harder, or it takes more force, for you to go faster.

Friction • Touch the table top of your

chair. • Does it feel smooth? It does,

doesn’t it?

Although surfaces may seem smooth, if you look closely with a microscope, you’ll find that any smooth surface still has many irregularities (cracks, hills, valleys, bumps, etc.)

These irregularities get caught up on each other when they rub against other irregularities on other surfaces (when two surfaces rub together) and they create a force we like to call friction.

Friction opposes motion, without it, an object in motion would remain in motion forever.

The amount of friction depends on two factors:• Types of surfaces (ice vs. sand)• How hard the surfaces push together (hand friction)

There are three types of friction:• Sliding friction (Solid surfaces

slides over another)• Rolling friction (object rolls

over another)• Fluid friction (object moves

through liquid or gas)

• Sliding friction examples: skis sliding through snow, socks sliding on carpet, your hand sliding over the tabletop.

• Rolling friction: roller blading, cars driving on pavement, skatebording.

• Fluid friction: skis moving through water, jumping out of an airplane (air resistance), adding oil to a squeaky door hinge.

Gravity • The force that pulls objects towards the center of the earth is called gravity.• The force of gravity pulls us with is a whopping 9.8m/s.

The measure of the force of gravity on an object is called weight. • CAUTION: a smart person mistake

would be to think that weight is the same as mass, but weight and mass are completely different.

•Weight changes with the gravity pull and mass changes with the amount of matter an object has.

• EX: When an astronaut weighs herself here on earth she weighs 150lbs, but when she weighs herself on the moon she weighs less. Her body didn’t loose any matter, but the force of gravity on the moon is less, so her weight changed—not her mass.