Forces between objects
- Forces are measured in newtons ( N )
- Objects always exert an equal & opposite force on each other
- Forces have both size and direction
- A force can change the shape of an object or change its motion or state of rest
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- The resultant force is a single force that has the same effect as all the forces acting on an object
- When a resultant force on an object is zero:
- If the object is at rest.
- If an object is moving at the same speed in the same direction.
- When the resultant force on an object is not zero, there is an acceleration in the direction of the force.
- this means that:
- If the object is at rest, it will accelerate in direction of resultant force.
- If the object is moving in the same direction as the resultant force, it will accelerate in that direction.
- If the object is moving in the opposite direction to the resultant force, it will decelerate.
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Force and Acceleration
- A resultant force always causes an acceleration.
- Deceleration =negative acceleration.
- No acceleration = Zero resultant force
- Acceleration = change in velocity
- An object can accelerate by changing its direction even if it's going at a constant speed
- A resultant force is required to change an objects direction
- We can find resultant of an object using equation: F= m x a
- F = resultant force in N
- m = mass in Kg
- a = acceleration in m/s^2
- The greater the resultant force = greater acceleration
- The bigger the mass = bigger the fore needed to give it a particular acceleration
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On the Road
- If a vehicle is travelling at a steady speed, resultant force = not zero. The driving forces are equal and opposite to frictional forces
- The faster the speed of a vehicle = the bigger the decelration needed to stop it in a particular distance = bigger braking force is required
- The stopping distance of a vehicle is the distance it travels during the drivers reaction time (thinking distance) + the distance it travels under the braking force (braking distance)
- The thinking distance is increased if the driver is; tired or under the infulence of alcohol/drugs
- The braking distance can be increased by;
- poorly maintained roads or bad weather conditions
- the conditon of the car e.g. worn tyres/brake pads
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- Weight of a object is the force of gravity on it
- Mass of an object is the quantity of matter in it
- Weight (N) = mass (kg) x gravitational field strength (N/kg)
- If an object falls freely; no other forces are acting on it. This means that the resultant force is the objects’ weight. It accelerates downwards at a constant acceleration of 10ms-2. This is called acceleration due to gravity.
- If an object falls through fluid; the fluid drags on the object. The drag force increases with the objects’ speed. The resultant force on the object is the object’s weight minus the drag force. The acceleration of the object decreases as it falls so the resultant force decreases. The object reaches terminal velocity when the drag force is equal and opposite to the weight. The resultant force is 0 so the acceleration is also 0
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Stretching and Squashing
- Extension is the difference between the length of the spring and its original length.
- Objects that extend then return to their original shape when the forces deforming it are removed are called elastic
- Extension is Directly Proportional to the force applied
- If we apply too big a force, the line begins to curve because we have exceeded the limit of proportionality.
- Objects and materials like this are said to obey Hooke's Law
- Hooke's Law: F = k x e
- F = force appied in N
- k = spring constant in N/m (Newtons per metre)
- e = extention in m
- Stiffer the spring = greater the spring constant
- When an elastic object is streched, work is done. This is stored as Elastic Potential Energy, when the streching force is removed, the EPE is released.
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Force and Speed issues
- Reducing the speed of a vehicle reduced the amount of fuel used to travel a particular distance. This is called fuel economy.
- Reducing the air resistance by making the vehicle more streamlined, this also omproves fuel economy.
- Speed cameras used to discourage drivers from speeding.
- skidding occurs when the brakes on a vehicle are applied to harshly
- Anti skd surfaces used to reduce/prevent skidding.
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