P8

• What is a force?

  A force is a push, pull, or twist that can cause an object to change its velocity, shape, or direction.
• What is a balanced force?

  A balanced force occurs when the net force acting on an object is zero. This means the object is either at rest or moving at a constant velocity.
• What is a contact force?


  A force that requires two objects or materials to be touching in order to have an effect
• Which of the following forces is considered a Contact force?

  Frictional force. A Contact force requires physical contact between the objects for it to have an effect.
• What is a non-contact force?

  A non-contact force is a force that can act on an object without the two objects or materials needing to be in physical contact. Examples include gravitational, magnetic, and electrostatic forces.
• What happens if an object is stationary and an unbalanced force acts on it?

  The object will start to move.
• What are the 3 possible effects if an object is moving and an unbalanced force acts on it?

  1. The object could speed up.
  2. The object could slow down.
  3. The object could change direction.
• What is a "resultant force"?


  A resultant force is a single force that can replace all of the other forces acting on something.
• A force will cause a free body to...

  change velocity.
• Newton's First Law of Motion

  Every object travelling at constant speed in a straight line (or at rest) will continue at this same speed, unless a resultant force acts on it.
• An object in space will continue at the same speed forever, unless it is acted upon by the gravity of a celestial body.

  This is an application of Newton's first law of motion, which states that an object will continue at constant velocity (including zero velocity, i.e. rest) unless acted upon by an unbalanced force.
• Simplifying force diagrams


  • Draw all the forces acting on an object in a diagram
  • Look for pairs of forces acting in the same direction
  • Replace these pairs with a single resultant force
  • The resultant force is the sum of the individual forces acting in the same direction
• What can a force acting on an object cause?

  A force acting on an object can cause it to turn about a pivot.
• Turning forces and moments

  • A force acting on an object can cause it to turn about a pivot point
  • If the force acts to the left of the pivot, it causes an anticlockwise turn (the left goes down)
  • The turning force is called a moment
• Ways to Increase the Moment of a Force

  1. Increase the distance from the force to the pivot
  2. Increase the force applied
• Moment Formula


  Moment = Force (N) x Distance (cm or m)
• Units for Measuring Moments

  • Newton centimetre (Ncm)
  • Newton metre (Nm)
• Levers


  • Practical applications of moments or turning forces
  • A spanner is a lever or force multiplier
  • Muscle force is multiplied by the length of the spanner and converted into a large turning moment to move a nut
  • Levers allow a small force to be converted into a large turning force
• Gears and Moments


  • Gears are a practical application of moments and turning forces
  • The size difference (radius) between gears decides whether the moment is increased or decreased
  • Larger gears increase the moment, while smaller gears decrease the moment
  • This allows gears to multiply the effect of a turning force
• Gear Ratios


  • A small gear driving a larger gear increases the turning effect (moment)
  • But this also slows down the speed of rotation
  • The size difference, or radius, of the gears decides whether the moment is increased or decreased
• Large gear driving small gear

  • Decreases the turning effect (or moment)
  • Increases the speed of rotation
• Center of Mass


  The point at which all the mass of an object can be concentrated. It is the average position of the mass in the object.
• Center of Mass in Regular Shaped Solids

  • For a regular shaped solid, the center of mass will always be found where the lines of symmetry intersect.
  • This center of mass can be located either inside or outside the object, depending on the shape.
• The Suspension Method for Finding Center of Mass

  • When an object is freely suspended and allowed to hang, the center of mass will lie on a vertical line directly below the point of suspension.
  • Hang the object from one point or edge and draw the vertical line.
  • Choose another point or edge to suspend it from and draw the new vertical line.
  • Repeat until the vertical lines intersect - this point of intersection is the center of mass.
• Principle of Moments


  • When a seesaw is balanced, the total clockwise moment = the total anti-clockwise moment
  • This is known as the principle of moments
  • For balance, the total clockwise moment = total anti-clockwise moment
• Moment of a load

  Moment of load = load × distance of load from tower
• Parallelogram of Forces


  • To find the resultant force of two forces that do not act in the same direction
  • Draw a parallelogram with the two force vectors as the sides
  • The resultant force is represented by the diagonal of the parallelogram
• **Parallelogram of Forces**


  • When two forces do not act in the same direction, you cannot simply add or subtract them
  • Instead, you need to use the parallelogram of forces method:
  • Draw a scaled diagram on graph paper, with the forces at the correct size and angles
  • Construct the other sides of the parallelogram, keeping everything accurate and to scale
  • The resultant force is represented by the diagonal of the parallelogram
• Breaking Down a Single Force Using the Parallelogram of Forces

  • If two forces can give a single resultant force, then a single force can be broken down into two forces
  • The single force being broken down is represented by the diagonal of the parallelogram
  • The two new forces it is broken down into are represented by the sides of the parallelogram
• Finding Vertical and Horizontal Components of a Force

  • Draw a scale diagram of the single force as the diagonal of a rectangle
  • The sides of the rectangle represent the vertical and horizontal components of the force
  • Measure the lengths of the sides to determine the values of the vertical and horizontal force components