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FORCES Definitions A force is a push or a pull. All types of forces can be categorized either a push or a pull or a combination of the two.

Types of forces 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Friction Weight Air resistance (drag) Tension Magnetic force Electric force Gravitational force Centripetal force Lift Thrust (driving force)

What can forces do? Forces can: 1. 2. 3.

Change the speed of an object Change the direction of an object Change the shape of an object

BUT FORCES CANNOT CHANGE THE MASS OF AN OBJECT!

Friction It is a contact force between two surfaces. You can feel friction when you rub your hands together. It is an opposing force to motion. FRICTION ALWAYS OPPOSES MOTION Friction has some good effects and some bad effects.

Good effects 1. 2. 3. 4. 5.

Provides the gripping force needed to hold an object Provides the accelerating force for a car Helps us to walk (walking on oily surface is difficult) Used in the braking system of vehicles Helps us to write.

Bad effects 1. Causes heating effect which can cause fire 2. Causes wear and tear.


How to reduce friction? 1. Use of lubricants such as oil, grease, wax and water Lubricants such as oil, grease and wax are used in car engines or moving metal parts of machines in factories to reduce the wear and tear effect of friction. This way the moving metal parts are protected from damage and work longer. Water is used as a lubricant in skiing. As a person skis, some of the ice under the ski melts and a layer of water forms which makes the movement of the skier easier and faster due to reduced friction. 2. Use of air cushion Air is used as cushion in hovercrafts and maglev trains both of which travel above the ground on a cushion of air. There is no contact between the hovercraft / maglev train and the ground so there is negligible friction. This makes the movement of the two easier and faster. Watch frictionless motion on http://www.physics.com.pk/reiki. 3. Use of ball bearings Ball bearings are used in between two surfaces which are rolling against each other such as wheel and axle of a car or bicycle. The ball bearings reduce the friction between the two surfaces thus making it easier for them to roll over each other. 4. Use of rollers. Rollers are used in bridges. When bridges expand they can crumble if they are fixed at the ends. Rollers at the ends make the bridge roll over acting like ball bearings and prevent crumbling. Weight It is pull of gravity on an object. Weight acts downwards towards the centre of the earth ( for detail see chapter on Mass and Weight). Air resistance (drag) It is the friction caused by air on moving objects. When the speed of an object increases the air resistance also increases. You can feel air resistance when you put your hand out while driving a car. As the car’s speed increases, the air resistance on your hand will also increase. (See the document Free Fall for more details). Tension It is a reaction force. If you hang a brick with a rope on the ceiling the weight of the brick which acts downwards is opposed by the tension in the rope in the opposite direction.

T (tension)

W (weight)


Magnetic Force It is the force between two poles of a magnet (See the document Magnetism for more details). Electric force It is the force between two charges (see the document Static Electricity for more details). Gravitational Force It is the force of attraction between two objects such as the moon and the Earth. Any two objects experience this force but it is more noticeable in large objects. It is this force which keeps the moon in orbit around the Earth. Centripetal Force It is the force that keeps objects moving in a circle. It is always directed towards the centre of the circle. Examples: 1. A car taking a turn or going round a roundabout (friction provides the centripetal force). 2. An electron orbiting an atom (electrostatic attraction provides the centripetal force). 3. Moon going around the earth (gravitational force provides the centripetal force). Lift It is the reaction force exerted by air on the wings of an airplane or a gliding bird. Thrust (driving force) It is the forward driving force provided by the engine of a car.

Balanced Forces 6N

Unbalanced Forces 6N

6N

Resultant Force, F = 6 – 6 = 0 N

10 N

Resultant Force, F = 10 – 6 = 4 N to the Right

If there is no resultant force then the object stays at rest or keeps on moving with a constant speed. To produce acceleration you need resultant force. This is also known as Newton’s 1st law of motion. So in the first case the object does not move but in the second case the object moves to the right with increasing speed.

Newton’s 2nd law of motion It states that “the acceleration of an object is directly proportional to force and inversely proportional to its mass”.

∝ ∝

1

F = ma This is a common formula that can be used to do calculations involving mass, force and acceleration. Mass in kg;

acceleration in m/s2;

force in N.


REMEMBER: F stands for resultant force not just force. Example: A box of mass 5 kg is being pulled across a surface with a force of 10 N. If the force of friction is 4 N calculate the acceleration of the box. Solution: Resultant force, F = 15 – 5 = 10 N F = ma 10 = 5 a a = 2 m/s2. 5N

15 N

Newton’s 3rd law of motion It states that “to every action there is an equal but opposite reaction”. NOTE: the action and reaction must be acting on two different bodies. Examples: 1. Air escaping from a balloon.

AS AIR ESCAPES OUT OF THE BALLOON, THE TROLLEY MOVES FORWARD. Action:

air out of balloon trolley moves forward

Reaction:

2. An oar pushing against the water.

AS THE OAR PUSHES THE WATER BACKWARD, THE BOAT IS PROPELLED FORWARD. Action:

oar pushes the water backward

Reaction:

boat is pushed forward


3. Rocket

AS THE HOT GASES ESCAPE DOWNWARD OUT OF THE ROCKET, IT IS PROPELLED UPWARDS. Action:

hot gases move downward

Reaction:

rocket moves upwards

(See how to make a balloon rocket at http://www.physics.com.pk/). Weight of a parachutist and air resistance acting are not related to the 3rd law because they both act on the same body: the parachutist.

Note:

By Shafaq Hafeez shafaq@physics.com.pk


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