Forces



In this topic, we meet some existing ideas and add to our understanding. Work done is the energy required to move something. It is also the energy required to overcome the friction that was stopping the body from moving. For example, the friction between a box and the floor, the work done is the energy required to overcome friction and move it for a specific distance.

The formula is:
Work Done (J) = Force (N) x Distance moved in that direction(M)

Somethings are not moved along, in fact, they can be lifted or lowered. If an item is lifted, then it gains Gravitational Potential Energy. This can be calculated using another simple formula:

Change in GPE (J) = Mass (Kg) x g (10N/Kg on Earth) x change in height (M)


This can be both positive (item gets higher) or negative (item is lowered). For both GPE and Work Done, the rate at which the item is moved can be calculated in the form of Power. If it moves quickly, it has more power and more slowly, less power. The power can be calculated:
Power (W) = Energy (J) (whether work done or change in GPE) / Time (s)

When an object is moving, it has kinetic energy, another form of energy that we have met before but this time, we can give it values. It can be calculated using the slightly more complicated formula of:
Kinetic Energy (J) = ½ x Mass (Kg) x Velocity2 (m/s)

An object that is moving has an amount of kinetic energy and remember that it will require that exact amount of energy to bring it to a complete stop. Doesn't matter how fast or slow it happens (the power), it will always need that amount of energy.

When moving objects collide, there is a special way of describing their energies and how they react in the collision. We call this measure, the object's Momentum.
It is a vector quantity like velocity so has a specific direction and it is calculated using both the object's mass and its velocity.

Momentum (kgm/s) = Mass (Kg) x Velocity (m/s)

As you can see form the formula, the heavier an object is, the more momentum it will have and the faster an object is travelling, the more momentum it will have. Providing no other external forces are applied, there is a law called The Conservation of Momentum. This means that the total momentum before a collision is equal to the total momentum after a collision. If the cars bounce apart or move off together in one direction, they will have the same momentum as they did when they started.

In an Explosion, objects at the centre will move away in opposite directions. As they are moving apart, one has a positive momentum and the other is negative. Because they are equal and one positive and the other negative, if you add them together, the total is 0 kgm/s.

An example, is a bullet leaving a gun. The gun and the bullet both have the same momentum but as they are in opposite directions, they add up to 0 kgm/s.

"To every action, there is an equal and opposite reaction".

Just like the power measure to tell how quickly an object was being moved, we have Impact Forces to tell us how quickly the momentum of an object is changed. If the object's momentum is reduced quickly, it has a large Impact Force and if it is changed slowly, it has a small Impact Force.
If you are unlucky enough to be in a car crash, you want the forces on your body to be as small as possible. Small impact forces can be achieved by reducing the car's momentum more slowly. To absorb some of the energy, cars have crumple zones. These areas at the front and back of the car help to reduce the impact force and use some of the kinetic energy of the car to bend and twist the metal that makes up these zones. You are also protected by seatbelts, air bags, collapsible steering wheels and a central cage that should not crumple to protect the passengers.


This topic relies on your basic understanding of maths. For example, you should be able to close your eyes and picture a car travelling at 30 miles per hour. If it travels at that speed for 1 hour it will have moved 30 miles. So if it travelled for 30 minutes it will have moved 15 miles and so on.
Speed is the distance an object travels divided by the time it took. Remember to use SI units. In this topic refer only to meters, seconds and meters per second.

Speed = Distance/Time
Velocity is different from speed as it has a direction. You can have a speed of 3m/s but velocity must have a direction e.g. 3m/s north.
Acceleration is the rate of change of velocity or how quickly something speeds up or slows down. It is given by the formula:
Acceleration = (Final velocity - initial velocity) / Time taken

Or more simply: a=(v-u)/t
The units are meters per second per second (m/s2)or ms-2
. If an object is slowing, acceleration will be negative. Distance vs Time and Velocity vs Time graphs are used to display information about journeys. It is important to be able to translate the information on a graph into what a person or car is doing to generate that shape on a graph.

Forces are one of the most fundamental parts of physics and they are always measured in Newtons (N). When objects interact with each other and exert a force, or push off each other, they exert an equal and opposite force. For example, if you lean onto a desk, you are applying a force to the surface and the desk is applying exactly the same amount of force back at you to stop you from falling through it. This is an action and reaction force.
Note that the force due to Gravity, weight, always acts towards the center of Earth-straight down.

If you add up all of the forces acting on a body and the overall force, called the resultant force, is zero, then the body does not move or continues to move at the same speed in the same direction (this means same velocity from the previous page).

If you are still or moving in the direction of a resultant force (which is not 0), you will accelerate in that direction. If you are moving in the opposite direction of the resultant force, you will decelerate until you stop and then begin to accelerate in the direction of the resultant force.

Resultant forces always cause a change in velocity which means that they cause acceleration. Remember, if the velocity doesn't change, there is no resultant force.

This can be calculated by the simple formula, F =m x a
Force in Newtons is equal to the mass in kilograms multiplied by the acceleration in meters per second squared.
When a car travels at a constant velocity, the forces are balanced, no resultant force. The forces of friction and drag trying to slow the car are equal and opposite to the forces from the engine pushing it forward. Linking to the formula above, F=m x a, the faster the car, the more deceleration is needed to stop in a specified distance, the greater the force needed.

Heavier vehicles need more force to decelerate because they have more mass;
- Faster vehicles need more force to decelerate because they need to decelerate more
When you learn to drive, you will see in the highway code thinking distance, braking distance and stopping distance.

-Thinking distance is how far you travel between seeing a hazard and you applying the brakes. This time increases if you are tired or have been on drugs or alcohol. Your reaction time slows with every bit of alcohol so even if you are under the legal limit, you are still more at risk of a collision.
-Breaking distance is how far the car travels from when you apply the brakes to the car coming to a complete stop. This distance goes up if: tyres are worn, brakes are worn, roads are poorly surfaced or on gravel or mud, if there is snow, ice or water on the roads

Finally, we must look back to a fundamental idea which is gravity. Any object that falls to Earth accelerates at about 10 m/s2. On Earth, weight can be calculated using the formula F=m x a. This time, the force due to gravity is equal to the mass of the object multiplied by the acceleration due to gravity on Earth which is 10 m/s2. We can therefore work out that a car with a mass of 1000Kg will have a weight of 10000N. This car will always have a mass of 1000Kg no matter which planet it sits on or even if floating through space but the weight will change.

Finally finally - objects all accelerate to Earth at the same rate BUT air resistance or drag will slow some down. A feather will take longer to hit the floor than a pin because the feather has a lot more air to push out of the way before it gets to the floor.

Key words and terms for this topic: work, friction, gravitational potential energy, power, kinetic energy, elastic potential energy, momentum, conservation of momentum, impact time, crumple zone, speed, velocity, acceleration, deceleration. force, Newton, resultant force, mass stopping distance, thinking distance, breaking distance, weight, gravitational field strength, drag, drag force, terminal velocity, elastic, directly proportional, limit of proportionality, Hooke's law.

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