# Forces and Motion

AQA Physics Unit 2

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## Newtons First Law of Motion

If the forces acting on an object are balanced then the object will either remain stationary or contiue moving at a constant velocity.

Balanced forces = Zero resultant force = Zero acceleration

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## Newtons Second Law Of Motion

An object will always accelerate in the direction of the larger force or resultant force.

Unbalanced force = Acceleration

Examples of acceleration  (These are all accelerations as acceleration is the change in velocity over time. Velocity is in a given direction so if you change direction your velocity also changes meaning that you are accelerating.)

-Stopping

-Starting

-Speeding up

- Slowing down

- Changing direction

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## Newtons Thrid law of Motion

For every force acting on an object, there is an equal and opposite force acting in the opposite direction.

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## Resultant Force

Resultant force is the sum of all the forces acting on an object measured in Newtons. If the resultant force does not equal zero then an object will accelerate in the direction of the larger force e.g.

If the person on the left pulls the rope with a force of 80N and the person on right pulls in 60N then which way will the rope move? You would calculate the resultant force by doing 60N-80N = -20N which just means 20N in the opposite direction, left.

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## Mass, Weight and Gravity

Mass in measure in Kg and is a measure of how much stuff or "matter" is inside of you. Your mass is the same where ever you are you will stil have the same mass.

Your weight however is the force of gravity acting on you. Weight is a force due to gravity and is measured in Newtons. It is calculated by Weight(N)= Mass(kg) x Gravitaional field strength (on earth this is 10N/kg). Your weight can change depending on the pull of gravity on where you are e.g. if you take a trip to the moon your weight is less as the gravitaional field strength is 1.6N/kg. Your mass however is the same.

Weight = Mass x Gravitational field strength

N = Kg x N/kg

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## Obvious points

The larger the force the bigger the acceleration.

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The smaller the mass of the object, the larger the acceleration..

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The larger the mass of the object, the larger the force required to make it accelerate. The larger the mass the smaller the acceleration.

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## Terminal velocity

Friction is a force that acts in the opposite direction to movement. Friction is also known as drag or air resistance and acts to slow down movement. When two materails rub together friction is caused.

Car designers design cars to reduce the frictional force of air resistance. They do this by making cars more streamilined to less the force of air resistance which acts on the car. Sky divers use the opposite of this concept as they use parachutes to increase the the force of air resistance which acts on them.

It is easier for a car at 30mph to maintain a constant speed compared to a car at 70mph. This is because to travel at a constant speed the frictional forces must balance the driving force. At 30mph the driving force and frictional forces are both lower and so can easily balance each other maintaining a constant speed. Whereas at 70mph the driving force is much larger and so needs larger frictional forces to balance out the driving force in order to travel at a constant speed.

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## Drag and Terminal Velocity

Cars and free-falling objects are good examples of this. When a car first sets off from rest the driving force is initially much larger than the drag force. However as the car continues to accelerate the drag force increases until it eventually balances out the driving force. When the two forces are balanced there is zero resultant force and the car will continue travelling at a constat speed. As there is no resultant force the car is said to have reached terminal velocity.

When a sky diver first jumps out of a plane his force of gravity is much larger than the drag force acting on him. As he accelerates down the drag force acting on him increases until it eventually balances out his weight. When the two are balanced there is zero resultant force and the diver will fall at a constant speed - the diver is said to have reached terminal velocity. When the diver opens his parachute,however, his surface area increases and so the drag force acting on him increases. The drag force is now larger than the force of weight and so the diver decelerates. After a while the drag force decreases back down balancing out the force of gravity again and so the diver has reached terminal velocity and will contiue to fall at a constant rate.

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## Drag and terminal velocity

As the force of gravity is the same everywhere it woud make sense to assume that everything falls at the same rate. This would be correct if it wasn't air resistance. Earth has an atmosphere and so has air resistance. Air resistance acts on objects depending upon their shape, surface area and size. This is why a hammer and feather fall at different rates on Earth. As the hammer has a larger mass and a more streamlined shape it will fall faster as by the time air resistance had acted on it, it would have already stopped moving. But with the feather, it has a smaller surface area and is lighter so it is easier for air resistance to act on it, and so it will reach terminal velocity much quicker and continue to fall at a constant rate to the ground wheras the hammer doesn't reach terminal velocity.

On the moon however there is no atmosphere and so everything falls at the same rate e.g. a hammer and a feather.

Note - Why are parties on the moon always rubbish?

Because there's no atmosphere he he he.

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## Stopping Distance

Stopping distance = Braking distance + Thinking distance

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## Thinking distance

Thinking distance is the distance covered by a vehicle from when a hazard was first noticed to when the brakes were first appllied.

Thinking distance is affected by:

- Speed

- Alcohol and drug

Being under the influence of alcohol and drugs may increase your reaction time so the time take for you to recognise a hazard will be long and so your thinking distance is increased.

- Age, increases reaction time

- Tiredness less alert, increased reaction time

- Visibility - If roads are not very clear then its going to take longer to respond to a hazard as you will cover more distance before you see it and brake.

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## Braking distance

Braking distance is the distance travelled by a vehicle after first applying the brakes and until the vehivle actually comes to a stop.

Braking distance is affected by

- Speed

- Poor quallity tyres with poor grip

- Faulty brakes

- Poor road conditions such as loose gravel

- Poor weather conditions such as icy roads which lubricate tyres

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## Work Done

Work done is just energy transferred. Work is only done when an object actually moves a distance, if it does not then no work is done. Work done requires the transferring of energy for example to move a box, chemical energy from your muscles is transferred in kinetic energy to move the box as well as some being transerred into the atmosphere as heat.

Work done (J) = Force (N) x Distance (M)

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## Kinetic energy

Anything that moves posesses kinetic energy. Kinetic energy is a subject of an objects mass and velocity. So a fat man running fast has more kinetic energy than a thin man running fast.

K.e. (J) = 0.5 x Mass (Kg) x Velocity2 (m/s)

When a car stops, its kinetic energy is transferred as heat energy through the friction of the tyres.

This is important in stopping distances as if you double your speed, you are actually increasing your kinetic energy by four times meaning that you are more likely to hit the hazard as you are covering more distance.

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## Gravitaional potential energy

Gravitational potential energy is the energy an object posesses due to its height against the force of gravity.

For something to have GPE energy needs to be transferred. So for an apple to have GPE it needs to be lifted against the force of gravity and put on e.g. a shelf which requires the tranformation of kinetic energy into GPE. If the apple was to lose this GPE it would have to fall off the shelf, the GPE would be transferred into kinetic energy.

GPE (J) = Mass x Gravitaional field strength x vertical height

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## Link between kinetic and gravitaional potential en

When a ball is thrown into the air it has the most kinetic energy at the very begining as after this the kinetic energy is transferred into gravtational potential energy. As the ball rises more and more kinetic energy is converted into gravitational potential energy util the ball reaches its maximum height. Here all kinetic energy has transferred into gravitational potential energy. When the ball falls back down all the gravitational potential energy is converted back into kinetice energy. Some energy is also transferred as heat and sound.

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## Momentum

Momentum is a product of an objects mass and velcocity. Momentum is a vector quantity like velocity. Momentum can be positive or negative depending on the direction. Momentum is always conserved  meaning that the momentum befor a collision is always the same as the momentum after providing no exteranl forces are acting on it.

Momentum (kg m/s) = mass (kg) x velocity (m/s)

A large mass and large velocity = large momentum

Zero velocity = Zero momentum

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## Inelastic collisions

An inelactic collision is where an object collides with another object and the move together at a different velocity.

An elastic collison is where to objects collide and move apart at different velocities.

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## Mometum and forces

Car designers design cars to increase the length of time the change in momentum acts for. This is as if the change in momentum of a collision acts for a longer time then the force which acts on the passengers is less resulting in less serious injury.

Force (N) = Momentum (kg m/s) / time (s)

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## Explosions

Explosions are similar to collisions in that their momentum is always conserved however unlike a collision explosions move apart instead of together. Explosions can have negative and positive momentums.

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