Compare forces and masses

Change one condition at a time

To find the effect of mass, compare different masses under the same unbalanced force for the same time, starting with the same motion. To find the effect of force, change the force and keep those other conditions fixed

1. 1Mass and force Mass is measured in kilograms (kg); force is measured in newtons (N). A cart’s total mass includes its load. Mass is not itself a force
2. 2What changes? The condition you deliberately change is the independent variable. Change only one relevant condition when testing its effect
3. 3What is measured? Here, compare the change in speed over a fixed interval. That measured outcome is the dependent variable
4. 4What stays fixed? Controlled variables are the relevant conditions held the same. Time and starting motion matter as well as mass and force

Two carts start at rest and receive the same rightward pull for the same time. All other forces balance and resistance is neglected. One has twice the mass. Which do you predict has the greater increase in speed?

Four runs on the same track

Each setting shows a separate run, starting at rest with the cart centre at 1 m. The current state is always after 3 s. Changing a selector does not add mass or change a pull partway through a run

1. 1Read the motion Speed is in metres per second (m/s), time in seconds (s) and position in metres (m). Because every run starts at rest, the final speed is also its increase in speed
2. 2Keep the scales Initial and current tracks share the fixed 0–10 m scale. Force arrows share one force scale. Cart drawing size is schematic; read the total mass label, not the drawing size
3. 3The forces The right cable pulls constantly; the left cable is slack at 0 N. Horizontal resistance is neglected. Gravity and support balance for each mass and are not drawn
4. 4Model limits These are selected observations, not a live animation. Track ends are display limits, not collision walls. The model is not a practical measurement experiment

Keep mass at 3 kg and compare the two pulls. Then keep the pull at 2 N and compare the two masses. Every run begins at rest and is observed after 3 s. Use the speed readout to compare the increases

Which comparison isolates the effect of changing mass?

Compare the unbalanced effect

For the same mass and interval, a stronger unbalanced force produces a greater change of motion. For the same unbalanced force and interval, a larger mass has a smaller change

1. 1All forces matter If two pulls oppose each other, compare their combined effect. A larger rightward pull does not guarantee a larger change if a leftward pull also increases
2. 2Balanced boundary If all forces balance, neither mass changes speed or direction. Larger equal opposing forces still balance
3. 3Direction matters A greater change can mean more slowing. A stronger effect opposite current motion reduces speed before the object stops; it does not always make the final speed higher

Two 4 kg carts start at rest. Cart A receives a constant 2 N rightward pull and cart B a constant 5 N rightward pull, each for 2 s. The listed pull is the only horizontal force. Gravity and support balance; ignore horizontal resistance. Which gains more speed?

Mass, time and starting motion match. Compare the unbalanced forces

Cart A has total mass 2 kg and cart B 5 kg. Both start at rest and receive a constant 4 N rightward pull for 2 s. The listed pull is the only horizontal force. Gravity and support balance; ignore horizontal resistance. Which gains more speed?

The force, time and start match. Compare the two masses

A greater change can mean more slowing

Two model trolleys, with masses 3 kg and 6 kg, both travel right at 4 m/s. Each receives the same 3 N leftward pull for 1 s. It is the only horizontal force; resistance is neglected and vertical forces balance. Neither stops in that interval

1. 1Principle Under a matched unbalanced force and interval, the smaller mass has the greater change of motion
2. 2Evidence The force points left, opposite the current rightward travel. The masses differ, while force, time and starting motion match
3. 3Conclusion Both slow down, and the 3 kg trolley loses more speed. A greater change here means a lower final speed, not a higher one

Two gliders on a level air track start moving right at 3 m/s. Glider P has mass 2 kg; Q has mass 4 kg. Each receives the same 2 N leftward pull for 1 s, and neither stops. The pull is the only horizontal force; resistance is neglected and gravity and support balance. Which has the higher speed at the end?

Two carts start at rest. Run A uses mass 4 kg and a 2 N rightward pull; run B uses mass 8 kg and a 4 N rightward pull. Each pull is the only horizontal force for 2 s, with resistance neglected and vertical forces balanced. Both end at 1 m/s. Does this prove mass has no effect?

Final speed is not the change in speed

Object A goes from 0 to 3 m/s. Object B goes from 4 to 5 m/s in the same direction. A gains 3 m/s while B gains only 1 m/s, even though B ends faster

1. 1Use each starting value Compare an object’s final speed with its own starting speed. Comparing only final speeds can hide different starting motion
2. 2Keep conclusions conditional Greater mass means a smaller change under the same unbalanced force for the same time. It does not mean a greater mass is always moving more slowly

A 3 kg trolley changes from 0 to 2 m/s right. A 6 kg trolley changes from 4 to 5 m/s right. Which statement correctly compares these recorded changes?

Compare matched conditions

First compare all forces, then check mass, time and starting motion. For the same mass, a stronger unbalanced effect gives a greater change. Under the same unbalanced effect, a greater mass has a smaller change over the same time

1. 1Make a fair comparison Change the condition being tested and keep other relevant conditions fixed. Equal outcomes after two changes do not identify either change’s isolated effect
2. 2Explain with evidence Gliders P (2 kg) and Q (4 kg) both start at 3 m/s to the right. Each receives a 2 N leftward pull for 1 s, and neither stops. Explain why Q ends faster by identifying the matched conditions and its smaller loss of speed. Compare your explanation with the feedback. The app does not mark your prose
3. 3Distinguish outcomes A greater change may be an increase or a decrease in speed. Mass alone does not determine current speed