FORCE

The cause of acceleration, or change in velocity.

F = m x a (mass times acceleration) units are in Newtons (N)

Ex. A truck with a mass of 1500kg is decelerated at a rate of 5m/s2. How much force did this require?

F = ma = (1500kg)(5m/s2) = 7500 N

A hunter shoots a 2 kg arrow accelerating it 15m/s2. How much force does this require?

F = ma = (2kg)(15m/s2)

A guy pushes some 100kg punk with 300N of force. How much is the punk accelerated?

a = F / m = 300 N / 100 kg = 3 m/s2

-Piano scenario-

The direction of force is important, as well as the size of the object

-Tug of war game-

Balanced forces – forces combine to produce net (overall) force of 0.

Unbalanced forces – the net force is nonzero

Cars have unbalanced, contact force acting on them – friction. Friction is a force between two objects in contact that opposes the motion of either object. For anything to move, it has to have an unbalanced force acting on it.

Air resistance is a type of friction.

What is the overall force acting on a piano if two people are each putting 100 N of force on it?

WORK – What is work?

Common sense of the word = everyday things we have to do

Scientific sense of the word = work is done BY a force ON an object, changing the motion on that object.

Work – a quantity that measures the effects of a force over a distance. (Therefore, there must be movement in order for work to be done.)

Work = force x distance W = F x d

Because work is measured in force x distance, it is measured in units of Newtons x meters, N x m. These units are called joules (J).

Example – A parent lifts small child into the air, repeatedly. Calculate the work being done by the parent with each lift if he lifts the child 2.0m and exerts an average force of 190N?


W = F x d W = 190N x 2.0 m

= 380 J

Ex. The brakes on a bicycle apply 125 N of frictional force to the wheels as the bicycle travels 14.0m. How much work has the brakes done on the bicycle?


W = F x d W = 125N x 14.0m

= 1750 J ______

Power – a quantity that measures the rate at which work is done.

Power = work / time P = W / t

The units for power is the WATT (W)

You must know the WORK involved in order to solve for POWER.

Example: While rowing in a race, John does 3960J of work on the oars in 60.0s. What is his power output in watts?

P = W / t P = 3960J / 60.0s

= 66.0 W


Ex. A 1500 W hairdryer is operated for 2 minutes. How much work is that hairdryer doing?

W = P x t W = (1500W)(120sec) = 180,000 J