# Explaining motion P4. Big picture How forces arise How forces arise Friction and normal reaction Friction and normal reaction Adding forces Adding forces.

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Explaining motion P4

Big picture How forces arise How forces arise Friction and normal reaction Friction and normal reaction Adding forces Adding forces Describing and summarizing motion Describing and summarizing motion Explaining the motion of objects Explaining the motion of objects Work Work Energy Energy

How forces arise Forces arise from an interaction between two objects Forces arise from an interaction between two objects Always come in pairs Always come in pairs The two forces in an interaction pair are always equal and opposite and act on different objects The two forces in an interaction pair are always equal and opposite and act on different objects

How things start moving To make a vehicle/person start moving it needs to push against the ground To make a vehicle/person start moving it needs to push against the ground When it pushes on the ground the ground pushes back and it will start to move When it pushes on the ground the ground pushes back and it will start to move

Friction Friction is an unusual force Friction is an unusual force It adjusts its size in response to the situation – up to a limit It adjusts its size in response to the situation – up to a limit This limit depends on the objects and the surfaces involved This limit depends on the objects and the surfaces involved The force of friction arises due to lots of tiny welds that have to be broken as an object slides against another The force of friction arises due to lots of tiny welds that have to be broken as an object slides against another

Reaction of surfaces If an object is placed on a surface it squashes or distorts the surface If an object is placed on a surface it squashes or distorts the surface The surface exerts a reaction force on the object The surface exerts a reaction force on the object

Adding forces If there is a force acting on an object and it is not moving there must be another force balancing the first one If there is a force acting on an object and it is not moving there must be another force balancing the first one If they balance we say the “resultant force” is zero If they balance we say the “resultant force” is zero

Speed Average speed = distance / time Average speed = distance / time Instantaneous speed – when average speed is measured over very short time intervals Instantaneous speed – when average speed is measured over very short time intervals Speed cameras detect speeding cars Speed cameras detect speeding cars

Motion graphs Distance – time graph: gradient/slope shows speed Distance – time graph: gradient/slope shows speed Speed – time graph: gradient shows acceleration Speed – time graph: gradient shows acceleration Velocity – time graph: also shows direction of motion Velocity – time graph: also shows direction of motion

Force and change of momentum Momentum = mass x velocity Momentum = mass x velocity Change of momentum caused by a force: Change of momentum caused by a force: Change of momentum = force x time Change of momentum = force x time (time is for how long the force acts) Conservation of momentum – in an interaction the total change in momentum is zero Conservation of momentum – in an interaction the total change in momentum is zero

Car Safety In a collision the force on passengers can be great. Cars are designed to reduce these forces: Crumple zones – increase the collision time Crumple zones – increase the collision time Seat belts – stretch to make the change of momentum longer Seat belts – stretch to make the change of momentum longer Air bags – cushion impact to reduce your momentum slowly Air bags – cushion impact to reduce your momentum slowly

Factors involved Collision time – the size of force on the car depends on the time the collision lasts Collision time – the size of force on the car depends on the time the collision lasts Momentum – the bigger the time, the smaller the force Momentum – the bigger the time, the smaller the force In summary, the longer it takes to reduce the passenger’s speed to zero, the smaller the force they experience.

Laws of motion Law 1 – if the resultant force acting on an object is zero, the momentum of the object does not change Law 1 – if the resultant force acting on an object is zero, the momentum of the object does not change Law 2 – if there is a resultant force acting on an object, the momentum will change (c.o.m.=r.f x time) and is in the same direction Law 2 – if there is a resultant force acting on an object, the momentum will change (c.o.m.=r.f x time) and is in the same direction

Motion Stationary objects have a resultant force that is zero Stationary objects have a resultant force that is zero Objects moving at a constant speed also have a resultant force that is zero Objects moving at a constant speed also have a resultant force that is zero Speeding up or slowing down- overall resultant force exists Speeding up or slowing down- overall resultant force exists

Work done When a force causes movement of an object, work is done Use the equation: work done by a force = force × distance moved by the force (joule, J) (newton, N) (metre, m)

Change of energy The energy of a moving object is called kinetic energy As an object falls, its gravitational potential energy decreases

Work and change of energy (cont) Understand that when work is done on an object, the energy of the object increases and When work is done by an object, the energy of the object decreases according to the relationship: change in energy = work done (joule, J)

From potential to kinetic energy When an object is lifted to a higher position above the ground, work is done by the lifting force against the gravitational force acting on the object (its weight); this increases the object’s gravitational potential energy (GPE); use the equation: change in GPE = weight × vertical height difference (joule, J) (newton, N) (metre, m)

Changes in kinetic energy When work is done to make an object move faster the kinetic energy increase. When work is done to make an object move faster the kinetic energy increase. Change in energy = work done Change in energy = work done So, So, change in energy = force x distance However, some work is wasted due to the force of friction.

Conservation of energy When an object falls it – When an object falls it – Loses gravitational potential energyLoses gravitational potential energy Gains kinetic energyGains kinetic energy If friction is small enough to ignore thenIf friction is small enough to ignore then Amount of GPE lost = amount of KE gained We use this formula to calculate KE: Gain in KE = ½ mass x velocity squared

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