Motion and forces-SP2

• When an object changes its velocity, it is accelearting. As acceleration is a change in a vector quantity, acceleration is also a vector
• In graphs showing the force vector quantities, the lengh of the arrow shows the size
• To work out the resultant of two forces
• If the forces are acting in the same direction, add them
• if they are acting in opposite directions, substract one from the other
• If the resultan of all the forces on an object is zero, we say the forces are balanced
• If there is a non-zero resultant force on an object, the forces are unbalanced

Newtons first law  is that:

a moving object will continue to move at the sane speed abd direction unless an exteral force acts on it

A stationary object will remain at rest unless an external force acts on it

Balance forces will not change the velocity of a object, however a un-balanced force will change in velocity

Circular motion

An object moving in a circle has changing velocity, even though its speed remains the same.

the resultant force that causes the change in direction is called the centripetal force, which acts towards the centre of the circle

Mass is the quanity of matter there is in a object, and inly changes when the object changes

Weight is a measure of the pull of gravity on an object and depends on the strengh of gravity.

On Earth the gravitational field strengh has aa value of 10 newtons per kilogram

The weight of an object can be calculated using the following equation:

weight=mass x gravitational field strengh

Forces on fallin bodies

On earth, a falling object has a force of air resistnce on it as wel as its weight

The acceleration iin direction of a resultant force depends on:

the size of the force(for the same mass, the bigger the force the bigger the acceleration)

the mass of the object( for the same force, the more massive the object the smaller the acceleration)

Calculating forces

The force needed to accelearte particular object can be calculated using the equation:

force= mass x acceleartion

the more massive an object is, the more force is needed to change its velocity.

We define the inertial mass of an object as the force on it devided by the acceleration that force produces

Newtons third law is about the forces on two different objects when they interact with each other, this can happen:

• when objects touch
• at a distance, cuch as the gravitational attraction between the earth an the moon

The force acting on a pair of two interactive objects is called action-reaction forces, these forces are always the same size and are in the opposite direction, furthermore they are the same type of forces

The weight of a object on the ground is equal to the force pushing up on the object.

There is a diference between action-reaction forces and balance forces:

• action-reaction forces act on different objects
• Balance forces all act on the same object

Collisions

• We can apply the idea of action-reaction forces to collisions
• The action-reaction forces that occur during the collision are the same size, but they do not necesarilly have the same effects on the two objects, because of diferent mass

Momentum is a measure of tendency of an object to keep moving- or of how hard it is to stop it moving.

The  momentum of an onject depends on its mass and its velocity.

Momentum=mass x velocity

When moving objects collide the total momentum of both objects is the same before the collision as it is after, this is called the conservation of mass

Remember, momentum is a vector so you need to consider direction when you add the quantities together

• When a driver sees a promblem ahead, their vechile will travel some distance while the driver reacts to the situtaian, this is called the thinking distance
• The car will then still take some time to brake and go to a complete halt, this is called the braking distance
• Stopping distnace= thinking distance + braking distance

Reaction times

• A reaction time is the time between a person reacting to a stimulus and their response
• The typical reaction time to a visual stimulus is 0.25 seconds
• This time can be increased by being tired, ill, under the influence of drugs or alcohol and mobile phone
• A car brakes uses friction to slow the car down. 
• If a vehicle has more mass, more force is needed to decelerate it.
• In a car crash, the vehicles involved come to a stop very quickly
• Crumple zones are built into the front of cars, if the car hits something it takes a little time for this crumbling to happen, so the deceleration of the car is less
• Seat belts hold the passengers into the car, so the effect of the crumple zone reduces the forces on the passengers as well as on the car.