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2.2.4 Newton's first law

Resultant force

Definition

Resultant force

The single force that has the same effect on an object as all of the forces acting on it combined.

Definition

Balanced forces

Forces acting on an object that cancel each other out so that the resultant force is zero.

  1. Most objects have several forces acting on them at once, and their combined effect decides how the object moves.
  2. The resultant force is the single force that would have the same effect on the object as all of the forces acting together.
  3. The resultant force is also called the net force.
  4. Forces acting along the same line are combined by taking their directions into account.
    1. Forces in the same direction add together.
    2. Forces in opposite directions partly or completely cancel, so the smaller force is taken away from the larger one.
    3. The resultant acts in the direction of the larger total force.
  5. When the forces on an object cancel completely, they are balanced forces and the resultant force is zero.
  6. When the forces do not cancel, they are unbalanced and there is a non-zero resultant force in one direction.
  7. A zero resultant force does not mean that no forces are acting, because several forces can act and still cancel out.

Newton's first law

Definition

Newton's first law

An object remains at rest, or continues to move at a constant velocity, unless a resultant force acts on it.

  1. Newton's first law describes what an object does when the resultant force on it is zero.
  2. If the resultant force on a stationary object is zero, the object stays at rest.
  3. If the resultant force on a moving object is zero, the object keeps moving at the same speed in the same direction, which means at a constant velocity.
  4. An object does not need a forward resultant force to keep moving, it only needs a resultant force to change the way it is moving.
  5. On Earth, moving objects usually slow down because of friction and air resistance, which is why it can seem as if motion needs a force to keep it going.
  6. In deep space, far from any planet or star, a space probe that switches off its engines keeps travelling in a straight line at a constant speed for years.

Uniform velocity

Definition

Uniform velocity

Motion at a constant speed in a constant direction.

  1. An object moving with uniform velocity covers equal distances in equal times without changing direction.
  2. An object at uniform velocity has balanced forces acting on it, so the resultant force on it is zero.
  3. For a car cruising along a straight motorway at a steady 30 m/s30\ \text{m/s}30 m/s, the forward driving force from the engine is exactly balanced by the backward resistive forces of air resistance and friction.
    1. If the driving force is 2500 N2500\ \text{N}2500 N forwards, the resistive forces add up to 2500 N2500\ \text{N}2500 N backwards.
    2. The car has the same velocity at the end of each second as it had at the start, because nothing is unbalanced.
  4. Vertically, the weight of the car is balanced by the normal contact force from the road, so the car neither rises nor sinks.

Changing speed or direction

  1. Velocity is a vector, so it changes whenever the speed changes, the direction changes, or both change.
  2. A change in velocity can only happen when there is a non-zero resultant force on the object.
  3. The way the velocity changes depends on the direction of the resultant force.
    1. A resultant force in the same direction as the motion makes the object speed up.
    2. A resultant force in the opposite direction to the motion makes the object slow down.
    3. A resultant force at an angle to the motion makes the object change direction.
    4. A resultant force on a stationary object makes it start to move in the direction of the resultant.
  4. When a bus driver presses the accelerator, the driving force becomes larger than the resistive forces, so the bus speeds up.
  5. When the driver brakes, the backward forces become larger than any forward force, so the bus slows down.
  6. When a football is kicked sideways as it rolls past a player, the sideways force changes the direction of the ball.
Example

Explaining the motion of a train

  • A train is pulled out of a station by a forward driving force of 90 kN90\ \text{kN}90 kN while resistive forces of 30 kN30\ \text{kN}30 kN act backwards.
  • The forces are unbalanced, because 90 kN90\ \text{kN}90 kN forwards is larger than 30 kN30\ \text{kN}30 kN backwards.
resultant force=90 kN−30 kN=60 kN forwards \text{resultant force}=90\ \text{kN}-30\ \text{kN}=60\ \text{kN}\ \text{forwards} resultant force=90 kN−30 kN=60 kN forwards
  • The resultant force acts in the direction of motion, so the train speeds up.
  • Later the resistive forces rise to 90 kN90\ \text{kN}90 kN, the forces become balanced and the resultant force is zero.
  • By the first law, the train then keeps moving at a constant velocity.
Example

Explaining the motion of a sliding puck

  • An ice hockey puck slides across an ice rink after being struck.
  • The friction between the puck and the ice is very small, so the resultant force on the puck is almost zero.
  • The puck therefore travels a long way in a nearly straight line at a nearly constant speed.
  • When the puck hits the barrier, the barrier exerts a large force on it, so the puck changes direction and bounces back.
Practical

Gliders on a linear air track

  • Aim: to observe how gliders move when the resultant force on them is almost zero, and how their velocities change only when they collide.
  • Apparatus: a linear air track with an air blower, two gliders, two interrupt cards, two light gates connected to a data logger, a pair of magnetic or spring buffers that fit on the gliders, and a spirit level.
  • Method:
    • Switch on the blower so that air streams out of the small holes along the track and lifts each glider on a thin cushion of air, which almost removes friction.
    • Level the track by adjusting its feet until a glider placed in the middle stays still and does not drift towards either end.
    • Fix an interrupt card upright on each glider and set up the two light gates about 50 cm50\ \text{cm}50 cm apart along the track.
    • Give one glider a gentle push and let it pass through both light gates without touching it again.
    • Record the speed shown at each light gate and compare the two values.
    • Place the second glider at rest between the two gates, push the first glider towards it and watch the collision.
    • Record the speeds of both gliders through the gates before and after the collision.
    • Repeat with the buffers swapped so that the gliders either bounce apart or stick together.
  • Results: a single glider records almost the same speed at both light gates, and each glider changes speed or direction only during the short time the gliders are in contact.
  • Conclusion: with a resultant force of almost zero the gliders move at a nearly uniform velocity, and their velocities change only when the collision gives each glider a resultant force.
  • Watch out: a track that is not level gives the glider a small resultant force down the slope, so it speeds up or slows down between the gates and the result looks like evidence against the first law.
  • Safety: keep fingers clear of the gliders during collisions and do not lean on the track, as it is easily bent.
Practical

Balloon glider on a string

  • Aim: to see how a resultant force changes the velocity of an object, and how the object moves when that force stops.
  • Apparatus: a balloon, a drinking straw, sticky tape, about 5 m5\ \text{m}5 m of smooth nylon string, two clamp stands, a metre rule or tape measure, a balloon pump and a phone to film the motion.
  • Method:
    • Thread the string through the straw, then tie the ends to the two clamp stands so that the string is tight and level across the bench.
    • Inflate the balloon with the pump, pinch the neck closed and tape the balloon to the straw with the neck pointing along the string.
    • Mark the string every 50 cm50\ \text{cm}50 cm with small pieces of tape and set the phone to film the whole length of the string.
    • Hold the balloon at the starting mark, then release the neck without pushing the balloon.
    • Use the video, played frame by frame, to note the time at which the balloon passes each mark.
    • Repeat three times with the balloon inflated to the same size each time, measured by wrapping a string around its widest part.
  • Results: while air escapes, the balloon covers more distance in each equal time interval, and once the balloon is empty it covers less distance in each interval until it stops.
  • Conclusion: escaping air gives the balloon a forward resultant force that makes it speed up, and once the air runs out, friction from the string gives a backward resultant force that slows it down.
  • Watch out: a sagging string makes the balloon climb or fall, so part of the change in speed is caused by gravity rather than by the escaping air.
  • Safety: wear eye protection, because a balloon can burst when it is over-inflated.
Common Mistake
  • Do not say that a moving object needs a resultant force to keep it moving, because a resultant force is only needed to change its velocity.
  • Do not say there are "no forces" on an object moving at constant velocity, because the forces are present but balanced.
  • Do not forget that a change in direction is a change in velocity, so it also needs a resultant force.
Exam technique

Explaining motion with the first law

  • Start by stating whether the forces are balanced or unbalanced, and say which force is larger.
  • Then state the size of the resultant force, using the phrase "resultant force is zero" for balanced forces.
  • Finish with the effect on the motion: stays at rest, constant velocity, speeds up, slows down or changes direction.
  • In a practical question about an air track, link the cushion of air to reduced friction and so to a resultant force close to zero.
Self review
  • What is meant by the resultant force on an object?
  • State Newton's first law.
  • What can you say about the forces on a car moving at a steady velocity on a straight road?
  • Give three ways in which the velocity of an object can change.
  • Why does a glider on a linear air track move at a nearly constant velocity?
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2.2.4 Newton's first law Revision Guide

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  2. /Physics
  3. /2.2.4 Newton's first law

Revision notes for OCR GCSE Physics 2.2.4 Newton's first law: explanations and worked examples.

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