IGCSEPhysicsEdexcelPhysics (Modular)Unit 1Revision Notes1. Forces & MotionMomentumForces & Momentum

Forces & Momentum (Edexcel IGCSE Physics (Modular))

Revision Note

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Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Forces & momentum

  • When a force acts on an object that is moving, or able to move, the object will accelerate (or decelerate)

    • This causes a change in momentum

Rate of change in momentum

  • The resultant force acting on an object is defined by the equation:

F space equals space m a

  • Momentum is calculated using the equation:

space p space equals space m v

  • Change in momentum is given as:

increment p space equals space m v space minus space m u

  • Combining these equations gives:

force space equals space fraction numerator change space in space momentum over denominator time end fraction

F space equals space fraction numerator open parentheses m v space minus space m u close parentheses over denominator t end fraction

  • Where:

    • F = resultant force, measured in newtons (N)

    • a = acceleration, measured in metres per second squared (m/s2)

    • m = mass, measured in kilograms (kg)

    • ∆p = change in momentum, measured in kilogram metres per second (kg m/s)

    • v = final velocity, measured in metres per second (m/s)

    • u = initial velocity, measured in metres per second (m/s)

    • t = time, measured in seconds (s)

  • Remember to consider the direction of object's motion

    • If you take the initial direction as positive then the reverse direction is negative

  • Force can also be described as the rate of change of momentum on a body

    • The rate of change describes how a variable changes with respect to time

  • The shorter the time over which momentum changes, the bigger the force 

  • So, force and time are inversely proportional to each other

Examiner Tips and Tricks

When two quantities are inversely proportional, it means that as one increases, the other decreases by a proportional amount

  • If one is doubled, the other is halved

  • If one is decreased by a factor of 4, the other is increased by a factor of 4

Worked Example

A tennis ball hits a racket twice, with a change in momentum of 0.5 kg m/s both times.

During the first hit, the contact time is 2 s and during the second hit, the contact time is 0.1 s

Determine which tennis racket experiences the greatest force from the tennis ball.

2-4-we-different-contact-times

Answer:

Step 1: Calculate the force during the first hit

F space equals space fraction numerator increment p over denominator t end fraction

F space equals space fraction numerator 0.5 over denominator 2 end fraction

F space equals space 0.25 space straight N

Step 2: Calculate the force during the second hit

F space equals space fraction numerator increment p over denominator t end fraction

F space equals space fraction numerator 0.5 over denominator 0.1 end fraction

F space equals space 5.0 space straight N

Step 3: State your answer

  • The tennis racket experiences the greatest force from the ball during the second hit

Worked Example

A car of mass 1500 kg hits a wall at an initial velocity of 15 m/s and rebounds with a velocity of 5 m/s. The car is in contact with the wall for 3 seconds.

Calculate the average force experienced by the car and state the direction of the force.

Answer:

Step 1: List the known quantities and assign direction

  • Mass of car, m = 1500 kg

  • Velocity before collision, u = 15 m/s

  • Velocity after collision, v = −5 m/s

  • Time of impact, t = 3 s

Step 2: Draw a diagram of the collision

  • The diagram should include:

    • The velocity before and after the collision

    • The direction of motion before and after the collision

2-4-we-solution-diagram

Step 3: Write out the force and momentum equation

F space equals space fraction numerator open parentheses m v space minus space m u close parentheses over denominator t end fractionp = mv

Step 4: Substitute in the known values to calculate

F space equals space fraction numerator open parentheses 1500 space cross times space minus 5 close parentheses space minus space open parentheses 1500 space cross times space 15 close parentheses over denominator 3 end fraction

F space equals space fraction numerator negative 7500 space minus space 22 space 500 over denominator 3 end fraction

F space equals space fraction numerator negative 30 space 000 over denominator 3 end fraction

F space equals space minus 10 space 000 space straight N

  • The minus sign means the direction of the force is to the left or in the opposite direction to the car's initial motion

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    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.