Edexcel IGCSE Physics: Double Science 复习笔记：1.2.4 Unbalanced Forces

• Forces can combine to produce
• • Balanced forces
  • Unbalanced forces

   

• Balanced forces mean that the forces have combined in such a way that they cancel each other out and no resultant force acts on the body
  • For example, the weight of a book on a desk is balanced by the normal force of the desk
  • As a result, no resultant force is experienced by the book, the book and the table are equal and balanced

   

A book resting on a table is an example of balanced forces

• Unbalanced forces mean that the forces have combined in such a way that they do not cancel out completely and there is a resultant force on the object
  • For example, imagine two people playing a game of tug-of-war, working against each other on opposite sides of the rope
  • If person A pulls with 80 N to the left and person B pulls with 100 N to the right, these forces do not cancel each other out completely
  • Since person B pulled with more force than person A the forces will be unbalanced and the rope will experience a resultant force of 20 N to the right

   

A tug-of-war is an example of when forces can become unbalanced

Unbalanced Forces, Mass & Acceleration

• When forces combine on an object in such a way that they do not cancel out, there is a resultant force on the object
• This resultant force causes the object to accelerate (i.e. change its velocity)
  • The object might speed up
  • The object might slow down
  • The object might change direction

   

• The relationship between resultant force, mass and acceleration is given by the equation:

F = m × a

• Where:
  • F = resultant force, measured in Newtons (N)
  • m = mass, measured in kilograms (kg)
  • a = acceleration, measured in metres per second squared (m/s²)

   

Part (a)

Step 1: List the known quantities

• • Initial velocity = 0 m/s
  • Final velocity = 27 m/s
  • Time, t = 3 s

   

Step 2: Calculate the change in velocity

change in velocity = Δv = final velocity − initial velocity

Δv = 27 − 0 = 27 m/s

Step 3: State the equation for acceleration

Step 4: Calculate the acceleration

a = 27 ÷ 3 = 9 m/s²

Part (b)

Step 1: List the known quantities

• • Mass of the car, m = 900 kg
  • Acceleration, a = 9 m/s²

   

Step 2: Identify which law of motion to apply

• • The question involves quantities of force, mass and acceleration, so Newton's second law is required:

   

F = ma

Step 3: Calculate the force required to accelerate the car

F = 900 × 9 = 8100 N

Part (a)

Step 1: List the known quantities

• • Initial velocity, u = 20 m/s
  • Final velocity, v = 0 m/s
  • Time, t = 0.1 s

   

Step 2: Calculate the change in velocity of the car (and the passenger)

change in velocity = Δv = final velocity − initial velocity = v − u

Δv = 0 − 20

Δv = −20 m/s

Step 3: Calculate the deceleration of the car (and the passenger) using the equation:

Step 4: Calculate the deceleration

a = −20 ÷ 0.1

a = −200 m/s²

Part (b)

Step 1: List the known quantities

• • Mass of the passenger, m = 70 kg
  • Acceleration (deceleration, in this case), a = −200 m/s²

   

Step 2: State the relationship between resultant force, mass and acceleration

• • This question involves quantities of force, mass and acceleration, so the appropriate equation for this case is:

   

F = m × a

Step 3: Calculate the decelerating force

F = 70 × −200

F = −14 000 N