forces

• Scalar quantity

  A quantity that has magnitude (size) but no direction
• Vector quantity

  A quantity that has both magnitude (size) and direction
• Resultant force

  A single force that has the same effect as all the forces combined
• Balanced forces

  Forces of the same size acting in opposite directions
• Unbalanced forces

  Forces where the force in one direction is bigger than the force in the other
• Newton's First Law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force
• A force is a push or a pull that can make an object change its shape, speed up, slow down or change direction
• Different types of forces

  • Contact forces (friction, air resistance, normal/upthrust, tension & compression)
  • Non-contact forces (magnetic, gravitational, electrical, nuclear)
• Spot the forces on the aeroplane

  • Lift (caused by air flowing over wings)
  • Weight
  • Thrust (from engines)
  • Drag (air resistance)
• Spot the forces on the boy

  • Upwards force from stool
  • Weight
• Spot the forces on the car

  • Upwards force from road
  • Weight
  • Forwards force from engine
  • Friction
  • Air resistance
• Spot the forces on the gymnast

  • Upwards forces from rings
  • Weight
• Spot the forces on the diver

  • Upthrust
  • Weight
  • Drag (water resistance)
  • Forwards force from flippers
• The length of the arrows represents the size of the force
• If the upthrust and weight forces are the same size but acting in opposite directions, the resultant force is zero
• Calculating resultant force

  1. Subtract the opposing forces

  2. Add the forces in the same direction
• Increasing the upthrust increases the resultant force upwards
• The horizontal resultant force on the diver is acting forwards (to the left in the photo)
• The vertical resultant force on the diver is acting upwards
• Calculating vertical resultant force on diver

  Upthrust (1200 N) - Weight (1000 N) = 200 N upwards
• Calculating horizontal resultant force on diver

  Forwards force (200 N) - Drag (50 N) = 150 N forwards (or to the left)
• Calculating resultant force on car

  Forwards force from engine (2500 N) - Friction (500 N) - Air resistance (1000 N) = 1000 N forwards (or to the right)
• Spot the forces on the skier

  • Normal force (force at right angles to the ground)
  • Weight
  • Air resistance
  • Friction
• There is no specific forwards force on the skier, but part of her weight acts to pull her down the hill due to the slope
• t

  Mass × gravitational field strength
• t = 1.0 kg × 10 N/kg = 10 N
• Falling objects

  If there is no air resistance, what is the acceleration of each object?
• Calculating acceleration

  Force = mass × acceleration, so acceleration = force / mass
• Acceleration of 0.01 N / 0.001 kg = 10 m/s^2
• Acceleration of 10 N / 1 kg = 10 m/s^2
• There is a greater force on the hammer than the feather, but the greater mass of the hammer also means it needs a greater force to make it accelerate. The two effects cancel each other out. Both the hammer and the feather have the same acceleration. (As long as there are no other forces acting.)
• Air resistance

  The amount of air resistance depends on the size of the object, its shape, and on how fast it is moving.
• When they are first dropped, the air resistance will be zero because they are not moving.
• After 0.2 seconds they will both have a velocity of about 2.9 m/s. The air resistance will be about 0.009 N.
• At this point the air resistance is the same because they are moving at about the same speed. But the air resistance is a greater proportion of the weight of the balloon.
• The acceleration of the balloon is slightly less.
• This table shows how the speeds and forces change as the two objects fall.

  • Time (s)
  • Air resistance (N)
  • Resultant force (N)
  • Acceleration (m/s^2)
• Although the resultant force on the two objects is different, the initial acceleration is the same because the bowling ball has a higher mass and so needs a bigger force to accelerate it.
• The air resistance is similar for both objects.
• However this air resistance is nearly 10% of the weight of the balloon, but only about 0.1% of the weight of the ball.