# Forces

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• Created by: Holly
• Created on: 03-01-13 15:16

## Forces

Reaction of a surface-Balanced Forces

1) If an objects resting on a surface, it pushes downwards (because of its weight.)

2) This causes an equal reaction force from the surface pushing up on the object.

3) The two forces are the same size, so the arrows on the diagram are the same size.

1) If an object is moving with a steady speed the forces must be in balance.

2) Just because something's moving doesn't mean that there's an overall force acting on it-unless it's changing speed or direction, the overall (resultant) force is zero.

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## Reultant Force

1) The resultant force is the overall force acting on an object-the force you get when you take into account (add up) all the individual forces and their directions.

2) It's this force that decides the motion of the object-whether it will accelerate, decelerate or stay at a steady speed.

3) Rememeber that 'accelerate' just means change velocity- and since velocity has both speed and direction, accelerating doesn't necessarily mean changing speed-it might just mean changing direction.

4) So if there's a resultant force acting on an object, its speed or direction (or both) changes.

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## Friction

1) When an object is moving relative to another one, both objects experience a force in the direction that opposes the movement-this is called friction.

2) Friction is a reaction force-it hapens as a result of an applied force.

3) The frictional force will match the size of the force trying to move an object, up to a maximum point-after this point the friction will be less than the other force and the object will move.

There are three types of friction.

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## Friction

1. Friction between solid surfaces which are gripping

This is the kind of friction that lets you walk around-the friction between your shoes and the ground allows you to push against it and move forwards. If there was no friction you would slip.

2. Friction between solid surfaces which are sliding past each other

e.g. moving bits and pieces in a car engine.

You can reduce sliding friction and gripping friction by putting a lubricant like oil or grease between the surfaces.

3. Resistance or 'drag' from fluids (liquids or gases, e.g. air)

Drag's basically just the same as other types of friction-an object moving through fluid has to force its way past all the molecules in that fluid, and that causes friction. Obviously a lorry would experience more drag or air resistance  than a streamlined car.

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## Forces and Momentum

Acceleration-Unbalanced Forces

1) If a car's engine exerts a bigger driving force (forwards) than the drag counter force (backwards), the car will acclerate.

2) The bigger the resultant force, the greater the accleration.

3) Ifthe driving force was less than the drag, the car would slow down.

4) REMEMBER the forces in the other directions (up and down) are still balanced.

e.g. A rocket taking off acclerates away from the ground, so the upward force (the thrust) must be greater than the downward forces slowing it down. In this case there are two downward forces-gravity and drag (from friciton between the rocket and the air.) If the thrust stopped, then the downward forces would be greater than the upward forces and the rocket would slow down until it stopped and the accelerate downward.

The same ideas apply to things thrown up in the air. Their motion is all about the relative sizes of the upward and downward forces.

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## Momentum

Momentum = Mass x Velocity

(kg m/s)         (kg)       (m/s)

1) The greater the mass of an object, or the greater its velocity, the more momentum the object has.

2) Momentum is a Vector quantity-it has size and direction (like velocity, but not speed.)

e.g.   A 65 Kg kangaroo is moving in a staright line at 10 m/s. Calculate its momentum.

Answer: Momentum = mass x velocity

=65 x 10 = 650 Kg m/s

3) A resultant force of zero means taht a stationary object will stay still. If the object was moving, it stays at a constant velocity (the same speed in the same direction) and constant momentum. If the resultant force is not zero, its momentum changes in the direction of the force.

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## Change in Momentum and Force

The change in momentum depends on the force

1) When a resultant force acts on an object, it causes a change in momentum in the direction of the force.

2) The change of momentum it causes is proportional to the size of the force and the time it acts for:

Change in Momentum = Resultant force x Time for which the force acts

(kg m/s)                              (N)                                  (s)

e.g.   Q:  A rock with mass 1Kg is travelling through space  at 15 m/s. A comet hits the rock, giving it a resultant force of 2500N  for 0.7 seconds. Calculate the rock's initial momentum, and then the change in its momentum resulting from the impact with the comet.

A: 1 x 15 = 15 Kg m/s     Change of momentum = force x time = 2500 x 0.7 = 1750 Kg m/s

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## Change in momentum and Car Safety

3) So, the bigger the force and the longer it acts for, the bigger the change in momentum.

4) If someone's momentum changes very quickly, the forces on the body will be very large, and more likely to cause injury.

Car Safety features reduces forces

1) If you rearrange the equation you get force = change in momentum / time.

The greater the time for a change in momentum, the smaller the force.

2) So if your momentum changes slowly, the forces acting on your body are small and you're unlikely to be hurt.

3) In a collision, you can't really affect the change in momentum-whatever you do, the car's mass and its change in velocity stay the same. However, the average force on an object can be lowered by slowing the object down over a longer time.

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## Car Safety

Safety Features in a car increase the collision time to reduce the forces on the passengers.

Crumple Zones

Crumple on impact, increasing the time taken for the car to stop.

Air Bags

Also slow you down more gradually.

Seat Belts

Stretch slightly, increasing the time taken for the wearer to stop. This reduces the forces acting on the chest.

Cycle and Motorcycle Helmets