Forces and Motion

• Average Speed Equation [1.4]


  Average Speed = distance moved / time taken
• Acceleration Equation [1.6]


  Acceleration = (final velocity - initial velocity) / time taken
• Force Equation 1 [1.17]


  Force = mass x acceleration
• Weight Equation [1.18]


  Weight = mass x gravitational field strength (9.8)
• How does the weight of a body act? [1.31P]


  The weight of a body acts through its centre of gravity.
• Momentum Equation [1.25P]


  Momentum = mass x velocity
• Conservation of Momentum [1.27P]


  In a closed system, the total momentum before the collision is equal to the total momentum after the collision.
• Moments Equation [1.30P]


  Moment = force x perpendicular distance from the pivot
• Principal of Moments [1.32P]


  When the total clockwise moment equals the total anticlockwise moment, the object reaches a state of equilibrium (is balanced).
• How do the upward forces on a light beam, supported at its ends, vary with the position of the object placed on the beam? [1.33P]


  Using the Principal of Moments, calculate the clockwise and anticlockwise moments. Rearrange equation for Force.
• Stopping Distance Equation [1.19]


  Stopping distance = thinking distance + breaking distance
• Final Velocity Equation [1.10]


  final velocity squared = initial velocity squared + (2 x acceleration x distance)
• Force Equation 2 [1.28P]


  Force = ((momentum x final velocity) - (momentum x initial velocity)) / time taken
• Velocity-Time Graphs [1.7]


  Gradient equals the acceleration of the object. If gradient is at 0, object is stationary.

  
• Velocity-Time Graphs Accelerating [1.8]


  If the gradient is positive and constant, the object is accelerating constantly.
• Velocity-Time Graph Decelerating [1.8]


  If the gradient is negative, the object has a varying deceleration
• Velocity-Time Graphs Distance [1.9]


  The distance travelled is the area between the gradient and time (x) axis.
• Distance-Time Graphs [1.3]


  Gradient equals the speed of the object. If flat, object is stationary.

  
• Distance-Time Graphs Gradient


  The steeper the gradient, the faster object. If it is flat, it has stopped. Positive gradients means the object is going away from the starting point and vice versa.
• Scalar Quantities [1.13]


  Only have a magnitude and no direction. Eg. Speed
• Vector Quantities [1.13]


  Has a magnitude and direction. Eg. Velocity
• Resultant Force [1.15]


  The sum of all the forces acting on an object. If forces are in opposite directions, subtract them. If they are in the same direction, add them.
• Friction [1.16]


  A force that opposes motion when two surfaces are in contact.
• Forces Acting on Falling Objects [1.21]


  Falling objects experience Weight and Air Resistance.
• The Effect of Air Resistance on Falling Objects [1.21]


  1. Air Resistance increases as the object's speed increases.
  2. This is because the object collides with air particles as it moves through the air.
  3. The faster the object travels, the more collisions it has with the air particles.
• The Effect of Weight on Falling Objects [1.21]


  • The weight of the object does not change.
  • This is because Weight = mass x gfs.
  • The mass and acceleration of freefall does not change so the equation can't change.
• Terminal Velocity


  The constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity
• Terminal Velocity on a Falling Object [1.21]


  1. The object accelerates downwards due to the force of weight.
  2. As the object accelerates, speed increases, so air resistance increases.
  3. Weaker acceleration causes speed to increase at a slower rate.
  4. Both forces are balanced so the resultant force is 0.
  5. The object reaches terminal velocity and travels at constant speed.
• Newton's Third Law [1.29P]


  If an object (A) applies force on another object (B), object B will apply the same size and type of force in the opposite direction on object A.
• Forces [1.14]


  A vector quantity that is either push or pull that acts on an object when it interacts with another object
• Contact Forces


  When 2 objects are physically touching. Eg. friction, air resistance, tension and normal contact force
• Non-Contact Forces


  When objects are separated (not touching). Eg. gravitational force, electrostatic force and magnetic force
• Normal Contact Force


  When you push on a table, your hand doesn't move through it because this force from the table pushes equally on your hand
• Air Resistance


  When an object moves through air and collides with air molecules, causing the object to slow down.
• Tension


  The pulling force that a string or cable exerts when something or someone pulls on it
• What happens during a collision?


  There is a change in momentum. The force of the collision is equal to the rate of the change in momentum.
• Car Safety Features


  • Seatbelts, airbags and crumple zones work to change the shape of the car, increasing the time taken for the collision.
  • They decrease the rate of the change of momentum.
  • This decreases the force of the collision on any passengers.
• Crumple Zone [1.26P]

  • The area of a car that is designed to deform or crumple on impact.
  • They increase the time taken to change the momentum of the driver and passengers in a crash.
  • This reduces the force involved.
• Seat Belt [1.26P]

  • Stops the passenger from tumbling around inside the car in a collision.
  • They stretch a bit in a collision, which increases the time taken for the body's momentum to reach zero.
  • This reduces the forces on it.
• Airbags [1.26P]

  • They increase the time taken for the head's momentum to reach zero.
  • This reduces the forces on it.
  • They also act as a soft cushion and prevent cuts.