When an object is stationary, it has an equal force pushing down and up, the downward force being gravity, and the upward force being normal contact force.
e.g:
This object is stationary.
When an object is accelerating, the forward force, the thrust - the motor or pedals of the vehicle, will be more powerful than the backwards opposing forces such as friction and drag.
However, when an object is going at a constant speed, the forward and backward forces will be balanced, e.g: 6 N to 6 N. However, that object could just as easily be stationary as the forces will seem the same on a free-body diagram.
During a collision, there is a change in momentum. The force (of the collision) = change in momentum / time taken.
Car safety features such as seatbelts, air bags, and crumple zones all increase the time taken for the collision, meaning the force is then decreased.
Whichever direction the force is greater in will be the direction the object travels in.
In a collision between two bodies where no external force is applied, momentum is always conserved.
velocity = this is a vector, meaning when you have 2 vehicles travelling in opposite directions, one will have a + value and one will have a -.
The conservation of momentum states that when two bodies collide, the total momentum they have is the same after the collision as it was before.
Hence when working out the total momentum before and after a collision, you know momentum = mass x velocity, therefore the 2 objects' momentum added together before the collision will be equal to the total momentum after. We can use this to work out different problems.
Newton's 3rd Law - To every action there is an equal and opposite reaction.
thanks so much, this was so useful
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