Forces

• A force is any push or pull that changes the motion, shape or size of an object.
• Forces can be classified as contact forces (pushing/pulling) or non-contact forces (gravity).
• Contact forces are caused by direct physical contact between two objects.
• Non-contact forces do not require direct physical contact between two objects to act on them.
• Contact forces are caused by direct physical interaction between objects.
• Non-contact forces do not require direct physical contact to act on an object.
• Gravitational force pulls all objects towards Earth's centre with a strength proportional to mass.
• The magnitude of a force is measured in Newtons (N), where one Newton equals the force needed to accelerate a mass of one kilogram at a rate of one metre per second squared.
• Examples of contact forces include friction, tension, air resistance, normal contact force, and spring force.
• The magnitude of a force is measured in Newtons (N), with one Newton being equal to the force required to accelerate a mass of one kilogram at a rate of one metre per second squared.
• Weight is the gravitational force acting upon an object due to its mass.
• Friction is a contact force that opposes relative movement between surfaces in contact.
• Weight is the gravitational force acting on an object due to its mass.
• Friction is a type of contact force that opposes relative movement between surfaces.
• Gravitational attraction is a type of non-contact force that pulls objects towards one another due to their mass.
• Mass is the amount of matter in an object, while weight depends on both mass and gravity.
• Equation to calculate an object’s momentum: Momentum = Mass x Velocity
• Unit used for momentum: kg m/s (kilogram metres per second)
• In a closed system, the total momentum before a collision is equal to the total momentum afterwards
• Equation linking change in momentum, force, and time: Force x Time = Change in Momentum (F Δt = mΔv)
• The force experienced in a collision is equal to the rate of change of momentum
• If an object’s change of momentum is fixed, the only way to reduce the force it experiences is to increase the length of time over which the change of momentum occurs
• Explanation of how a seatbelt improves a passenger’s safety during a collision:

  • Passenger must decelerate from the vehicle’s velocity at impact to zero, undergoing a fixed change of momentum
  • The force experienced is equal to the rate of change of momentum
  • Seatbelts increase the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced
• Distance is a scalar quantity, meaning it does not require a specific direction
• Total displacement of an object that moves 3 meters to the left and then 3 meters back to its initial position is zero
• Speed of sound is typically 330 m/s
• Typical values for human speeds:

  • Walking speed: 1.5 m/s
  • Running speed: 3 m/s
  • Cycling speed: 6 m/s
• Equation linking distance, speed, and time:

  • Distance = Speed x Time
  • Units: Distance (m), Speed (m/s), Time (s)
• Object traveling at a constant speed in a circle cannot have a constant velocity because velocity involves direction, which continuously changes in circular motion
• Speed can be calculated from a distance-time graph by determining the gradient of the graph
• To calculate speed at a given time from a distance-time graph for an accelerating object, draw a tangent to the curve at the required time and calculate the gradient of the tangent
• Equation for the average acceleration of an object:

  • Acceleration = (Change in Velocity)/(Time Taken)
  • Units: Acceleration (m/s²), Velocity (m/s), Time (s)
• Distance traveled by an object from a velocity-time graph is equal to the area under the graph
• Acceleration of an object in free fall under gravity near Earth's surface is approximately 9.8 m/s²
• Resultant force acting on an object at terminal velocity is zero because the object is moving at a constant speed and not accelerating
• Newton's first law for a stationary object:

  • If the resultant force on a stationary object is zero, the object will remain at rest
• Newton's first law for a moving object:

  • If the resultant force on a moving object is zero, the object will remain at constant velocity
• Newton’s first law for a moving object:

  If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)
• Braking forces and driving forces when a car is traveling at constant velocity:

  The braking forces are equal to the driving forces
• If an object changes direction but remains at a constant speed, is there a resultant force?

  Since there is a change in direction, there is a change in velocity and so there must be a resultant force