# force and acceleration

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## Force and acceleration

• Reslutant force causes an acceleration
• If no accelration, reslutant force is 0
• Acceleration is change in velocity
• Resultant force is needed to make object change direction

• Resultant force: f=mxa
• f is the resultant force in newtons, N
• m is the mass in kilograms kg
• a is the acceleration in m/s2 (squared)

The greater the reusltant force on an object, the greater itas accleleration. The bigger the mass of an object, the bigger the force needed to give it an particular acceleration

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## vehicle

• If a vehicle is travelling at a steady speed, the resultant force is 0
• faster the speed of the vehicle, the faster the deccelration needed to stop it- so bigger the braking force needed
• Stopping distance:the distance the vehicle travels during the drivers reaction time(thinkng distance) plus the distance it travels under the braking force (braking distance)
• The thinking distance: increased if the driver is: tired, under the influence of drugs or alchohol
• The braking distance can be increased by:
• poorly maitained roads or bad weather conditions
• condition of the car e.g. worn tyres, worn brakes
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## Falling objects

• The weight of an object is the force of gravity on it
• Its mass is the quantity of matter in it
• If an object falls freely, the resultant force acting on it is the force of gravity. It'll make and object close to the earth's surface accelerate at about 10m/s2
• W=mxg
• W is the weight in newtons, N
• m is the mass in kilograms, kg
• g is the acceleration due to gravity m/s2

If the object is on earth not falling, g is called the gravitational field strenght- units are N/kg (newtons per kilogram)

• The faster the object falls, the bigger the drag force becomes, until eventually it will be equal to the weight of the object. The resultant force will become 0, so the body stops accelerating.
• Terminal velocity of a falling object: the velocity it reaches when it is falling in a fluid.
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## stretching and squashing

• Extension: the difference between the length of the spring and its original length.

The spring constant of a spring is the force per unit extension needed to stretch it.

Hookes law:

• The extension of a spring is directly proportional to he force applied to it, provided the limit of proportionality is not exceed.
• f=kxe
• f is the force applied in newtons,N
• k is the spring constant of the spring in newtons per metre, N/m
• e is the extension in metres, m
• the stiffer the spring the greater its spring constant
• When elastic object stretched work is done, stored as elastic potential energy in the object
• when stretching force is removed, stored energy is released
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## Force and speed issues

fuel economy of road vechiles can be improved by: reducing the speed or fitting a wind deflector

Average speed cameras are linked in pairs to measure the average speed of a vechile.

Anti-skid surfaces increase the friction between a car tyre and the road surface.

• reduces skids
• or prevents skids
• rougher than normal road surfaces
• used near traffic lights and road juction, where drivers are likely to brake
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