Ch 7 PhysicsYR 7 CH 7 FORCES WIKI

Introduction to Forces

Honda the COG - Rube-Goldberg Machine youtube video 2:00 min

This too shall pass - Music video – Rube-Goldberg Machine youtube 3:58 min

Refer to the description to the right to draw a surfer on a wave and label it with 4 arrows indicating the forces working on him/her.

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Good demo Interactive on forces

Forces page 246

Force A push, pull or twist that can change an object’s motion – make it speed up, slow down, change direction or change shape.

Acceleration Increase in speed

Deceleration Decrease in speed

Measuring forces Spring balances can measure the stretching caused by a force and bathroom scales can measure the compression caused by a force.

Newton (N) Unit used to measure force

Drawing forces page 246

Balanced forces page 247

If the forces acting on a body are balanced, it will either remain still, or continue at its original speed until an unbalanced force acts on it.

Inertia The tendency of an object to resist any change in its motion – the greater its mass or speed, the greater its inertia/momentum.

Draw diagrams 7.1.5 and 7.1.6

Balanced forces page 247

Unbalanced forces page 247

Whenever the forces are unbalanced, the motion will change: Draw and label both the accelerating and decelerating diagrams.

nb With this in mind – redraw your original surfer with accurate arrows.

Inertia page 247

Inertia is the tendency of an object to resist any change in its motion (eg a school left on your bedroom floor will stay there unless you apply an unbalanced force to pick it up, or a space ship to continue at a very high speed and course unless gravity or thrusters are applied to change its direction or speed.)

Newton’s First Law states: An object at rest will stay at rest unless an unbalanced force

makes it move

An object in motion will continue moving at the same speed in the same direction unless an unbalanced forced is applied to it.

As the mass of an object increases, so does its inertia (momentum)

Assessing forces – draw and complete this table

Situation Forces – Balanced/Unbalanced

Explanation

Parachute falling at a constant speed

Car accelerating

Team losing a tug-of-war

Car parked on a hill.

Balanced

Balanced

Unbalanced

Unbalanced

Gravity = Air resistance therefore constant speed

Driving force of the engine is > than friction.The stronger team applied > force

Gravity = park break

When you finish, read ‘Car safety features p 249’

Page 252

Friction page 255

Friction Force that acts against an object’s motion

Contact Force Force that acts between two objects that touch, or are in contact; for example friction

Friction depends upon: How rough the contact surfaces are

How hard the surfaces are pushed together

The mass of the object

Air resistance and drag

Air Resistance or drag Friction that acts on an object moving through the air

Lubricants Fluid, such as oil, used to reduce friction between moving parts

Lift, weight, propulsion and drag