Forces and Acceleration

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Resultant Forces

Resultant Force is the overall force on a point or object

  • In most real situation there are at least two forces acting on an object along any direction.
  • The overall effect of these forces will decide the motion of the object (accelerate, decelerate or stay at a steady speed).
  • If you have a number of forces acting at a single point, you can replace them with a single force.
  • If the forces all act along the same line the overall effect is found by just adding or subtracting them.
  • The overall force you get is called the resultant force.
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Resultant Forces example

All forces on the book are balanced

  • The force of gravity (or weight) is acting downwards.
  • This causes a reaction force from the surface pushing on the object.
  • This is the only way it can be balanced.
  • Without the reaction force, it would accelerate downwards due to the pull of gravity.
  • The Resultant force on the book is zero


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A Resultant Force means a change in velocity

A Resultant Force means a change in velocity

  • If there is a resultant force acting on an object, then the object will change its state of rest or motion. - It causes a change in the object's velocity.

Example: A tug of war team

  • Forces are basically pushes and pulls in this. If they act on an object they can cause it to change speed or direction.
  • Each team pulls on the rope and puts a force on it.
  • If one team is stronger than the other, then the resultant force will be in the direction of the strongest team.
  • The stronger team is putting a 200N force on the rope, and the weaker a 100N force on it. Therefore there will be a 100N resultant force in the direction of the stronger team.
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An object needs a Force to start moving

An object needs a Force to start moving

If the resultant force on a stationary object is zero, the object will remain stationary.

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No Resultant Force means no change in velocity.

No Resultant Force means no change in velocity.

  • When anything is moving at a constant velocity then the forces on it must all be balanced.
  • To keep a steady speed, there must be zero resultant force.
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A Resultant Force means acceleration

A Resultant Force means acceleration

  • If there is a non zero resultant force, then the object will accelerate in the direction of the force.
  • This acceleration can take 5 different forms: Starting, stopping, speeding up, slowing down, and changing direction.
  • On a force diagram, the arrows will be unequal
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Formula for resultant force

Formula for resultant force:

F=ma     or     a=F/m

m= mass in kilograms (kg)

a= acceleration in meters per second squared (m/s2)

F= resultant force in newtons (N)

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Reaction Forces are Equal and Opposite

When two objects interact, the forces they excery on each other are equal and opposite.

  • That means if you push a something, like a shopping trolley, the trolley will push back against you, just as hard.
  • As soon as you stop pushing, so does the trolley.

With two ice skaters, if they push against each other they will both feel an equal same sized force, in the opposite direction and so accelerate away from each other. 

Skater A's  mass is 55 kg and skater B's is 65kg. Therefore skater A will accelerate more than skater B, becuase of her smaller mass (a=F/m) 

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