ReviewForce, Friction, Motion, Power and

Work

In science, a force is a push or a pull.

All forces have two properties:Direction and Size

A newton (N) is the unit that describes the size of a force.

When the forces on an object produce a net force of 0 N, the forces are balanced.

There is no change in the motion of the object.

When the net force on an object is not 0 N, the forces on the object are unbalanced.

Unbalanced forces produce a change in motion of an object.

Every time you push on something, friction pushes back. To get an object moving, you have to overcome the friction force first. If you’ve ever walked on pavement and suddenly stepped on a patch of ice, you know that different surfaces have different amounts of friction.

What determines how much friction an object experiences? Is it the type of material? The object’s weight? The amount of surface area in contact with the ground?

Friction occurs because the surfaces of any object is rough.

Even surfaces that appear smooth cause friction.

The rougher the surface, the greater the friction.

Ex. Pavement is much rougher than grass, more friction is produced when you slide on the pavement than on grass.

Objects with more mass need more force to overcome friction, objects with less mass need less force to overcome friction.

Sliding Friction- Ex. When you go sledding, or apply the brakes on a bicycle or car.

Give an example of sliding friction

Rolling Friction-Important when it comes to means of transportation. Anything with wheels using rolling friction.

Fluid Friction includes liquids and gases. For example the friction between air and

a fast moving car is the largest force opposing the motion of the car.

Static Friction is when a force is applied to the object but the object does not move.

Static friction disappears as soon as the object moves.

It is easier to drag an object over glass than sandpaper. The reason for this is that the sandpaper exerts more frictional resistance. In many problems, it is assumed that a surface is "smooth", which means that it does not exert any frictional force. A "rough" surface is one which will offer some frictional resistance. -

The coefficient of friction (COF), often symbolized by the Greek letter µ, which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one.

Imagine that you are trying to push a book along a table with your finger. If you apply a very small force, the book will not move. This must mean that the frictional force is equal to the force with which you are pushing the book.

The force of kinetic friction which is equal to the coefficient of kinetic friction times the normal force. Since the magnitude of the applied force is greater than the magnitude of the force of kinetic friction opposing it, the block is accelerating to the left

Use this equation: Ff = µ * mg and solve for µ.

µ = Ff / mg

Find coefficient of friction, given force and mass

Q: An engine provides 5.0 kN of force to keep a 1600-kg vehicle moving at a uniform speed. (Air resistance is negligible.) What is the coefficient of rolling friction between the tires and the road surface?

A: First convert 5.0 kN to Newtons: F = 5.0 kN = 5000 N. (Just like 5 kg = 5000 grams when measuring mass.)

You know it’s traveling at uniform speed, so this tells us that the right force = left force. There is no net force, because there is no acceleration!

F = 5000 Nm = 1600 kgg = 9.8= ?

A horizontal force of 400.0 N is needed to pull a 1760 N trunk across the floor at constant speed. What is the coefficient of kinetic friction (µk)?

  What force must be exceeded to start a 1.35

kg book moving across a desk if the coefficient of static friction (µs) is 0.30?

  A 1500. N force is exerted on a 200.0 kg

crate to move it across the floor. If µk is 0.250, what is the acceleration of the crate?

Work is done when a force causes an object to move in the direction that the force is applied.

The formula for work is: W = F X D If there is no movement, there is no work

Work is measured in Joules. Rule for Work Work (Joules) = Force (Newtons) X

Distance(meters)W = F X d

Power is the rate at which work is done. Power is measured in Watts. • Rule for Power • Power (Watts) = Work (Joules) / Time

(seconds)P = W / T