Forces & Momentum

Aprenda jogando

Responda a estas perguntas para ganhar energia, depois pesque e explore. Sem precisar de conta.

Para educadores: slides de aula, resumos de revisão prontos para usar de Forces & Momentum (Physics, HL) — use na sua aula ou rode o tópico como atividade interativa de turma que seus aprendizes jogam como um jogo ao vivo.

Notas de aula

Free-Body Diagrams

A free-body diagram shows all the forces acting on a single object, represented as a point particle at its centre of mass.
Each force is drawn as an arrow: the length gives the magnitude and the direction gives the direction of the force.
Only forces acting on the object are included; forces the object exerts on other things are not shown.
Common forces to include: weight (Fg) always towards the surface of the planet, tension (FT) always away from the mass, normal reaction force (FN) perpendicular to a surface, and friction (Ff) opposite to motion.
Arrows should be roughly to scale so that relative magnitudes are clear.
The resultant force (net force) is the vector sum of all forces acting on the body.
In one dimension, combine forces by adding them with direction signs; in two dimensions, use Pythagoras and trigonometry to find the resultant.

Forces on an object

Newton's First Law

Newton's first law: a body will remain at rest or move with constant velocity unless acted on by a resultant force.
If the resultant force on an object is zero, it is in translational equilibrium.
In equilibrium, forces to the left equal forces to the right, and upward forces equal downward forces.
A resultant force changes an object's motion by speeding it up, slowing it down, or changing its direction.
An object moving at constant velocity has balanced forces and zero acceleration.

Examples of Newton's First Law

Newton's Second Law

Newton's second law: the resultant force on an object is directly proportional to its acceleration, F = ma.
The acceleration always acts in the same direction as the resultant force.
If the resultant force acts along the direction of motion, the object speeds up; if it opposes motion, the object decelerates.
If the resultant force acts at an angle to the direction of motion, the object changes direction.
Newton's second law can also be written as F = Δp/Δt: the resultant force equals the rate of change of momentum.
For a falling object with no drag, acceleration is independent of mass.

Newton's second law in action

Newton's Third Law

Newton's third law: if Object A exerts a force on Object B, then Object B exerts a force on Object A that is equal in magnitude but opposite in direction.
Forces in a third-law pair must: be the same type, have the same magnitude, act in opposite directions, and act on different objects.
A free-body diagram showing two forces on one object is not a third-law pair; it is an example of Newton's first law.
Example: a foot pushes the ground backwards, and the ground pushes the foot forwards with an equal and opposite normal contact force.

Contact and Non-Contact Forces

A contact force acts between objects that are physically touching.
Examples of contact forces: friction, fluid resistance (viscous drag), tension, and normal (reaction) force.
A non-contact force acts at a distance due to a field, without physical contact.
Examples of non-contact forces: gravitational force, electrostatic force, and magnetic force.
Gravitational force is always attractive; electrostatic and magnetic forces can be attractive or repulsive.

Frictional Forces

Friction opposes motion and can prevent a stationary object from moving or reduce the speed of a moving object.
Friction transfers energy by heating, raising the thermal energy of the objects and surroundings.
Static friction acts when a body is stationary; dynamic friction acts when a body is in motion.
Static friction increases in magnitude until movement begins; its maximum value is larger than dynamic friction.
For a constant pushing force, dynamic friction is constant.
Static friction: Ff ≤ μsFN; dynamic friction: Ff = μdFN.
The coefficient of friction is a number between 0 and 1 (not including those values).

Hooke's Law

Hooke's law: the extension of a material is directly proportional to the applied force up to the limit of proportionality.
Hooke's law equation: FH = −kx, where FH is the elastic restoring force, k is the spring constant, and x is the extension.
The spring constant k measures stiffness; a larger k means a stiffer material.
Extension is the difference between stretched and unstretched length: extension = stretched length − unstretched length.
On a force-extension graph, the Hooke's law region is a straight line through the origin; the gradient of this region equals the spring constant k.

Stoke's Law and Buoyancy

Viscous drag is the frictional force between an object and a fluid that opposes relative motion.
Stoke's law: Fd = 6πηrv, where η is fluid viscosity, r is the radius of the sphere, and v is its velocity.
The drag force depends on the speed, size, and shape of the object.
Viscosity is a measure of a fluid's resistance to flow; fluids with low viscosity are easy to pour, high viscosity are difficult to pour.
The rate of flow of a fluid is inversely proportional to its coefficient of viscosity.

Momentum, Impulse, and Collisions

Momentum is the product of mass and velocity: p = mv; it is a vector quantity.
The fundamental SI units of momentum are kg m s⁻¹ (or N s).
Impulse is the change in momentum: Impulse = FΔt = Δp.
The principle of conservation of linear momentum: total momentum before a collision equals total momentum after, provided no external forces act.
In an elastic collision, both momentum and kinetic energy are conserved.
In an inelastic collision, momentum is conserved but kinetic energy is not.
In explosions, momentum is conserved; the total momentum before is zero if the system is initially at rest.
Crumple zones increase the impact time, which reduces the force exerted on the car during a collision.

Conservation of momentum

Circular Motion

Angular velocity ω is the rate of change of angular displacement, measured in rad s⁻¹.
For an object moving in a circle at constant speed, the velocity is constantly changing direction, so it is accelerating.
Centripetal acceleration is directed towards the centre of the circle: a = v²/r = ω²r.
Centripetal force is the resultant force directed towards the centre: F = mv²/r = mω²r.
In non-uniform circular motion, the speed changes as well as the direction, so there is both a centripetal and a tangential component of acceleration.

Circular motion of the International Space Station

Slides

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

CADASTRE-SE — GRÁTIS ENTRAR

Questões de prática

Prévia grátis — 8 de 53 perguntas. Cadastre-se para ver todas.

1.Which of the following is the correct definition of a contact force?
Easy
- AA force which acts between objects that are physically touching
- BA force which acts at a distance without physical contact
- CA force that only acts on objects at rest
- DA force that always opposes motion
2.Which of the following is a non-contact force?
Easy
- AFriction
- BTension
- CNormal reaction force
- DGravitational force
3.A non-contact force acts at a distance, without any physical contact between bodies.
Easy
True or false?
4.Which of the following is the correct equation for dynamic friction?
Easy
- AFf = μs FN
- BFf = μd FN
- CFf = μd / FN
- DFf = FN / μd
5.Which of the following are examples of contact forces? (select all that apply)
Medium
- AFriction
- BTension
- CGravitational force
- DNormal reaction force
- EMagnetic force
6.Static friction increases in magnitude until movement begins.
Easy
True or false?
7.A ball is suspended from a cable and is in translational equilibrium. What does this mean?
Medium
- AThe resultant force on the ball is zero
- BThe ball is accelerating
- CThe ball is moving at constant speed in a circle
- DThe forces on the ball are unbalanced
8.A box is sliding down a slope. Which of the following correctly describes the direction of the frictional force acting on the box?
Medium
- AParallel to the slope, in the opposite direction to the motion
- BPerpendicular to the slope
- CVertically upwards
- DParallel to the slope, in the same direction as the motion

Mais perguntas

9.Match each force with its correct description.
Medium
- Weight
- Tension
- Normal reaction force
- Friction
- Always towards the surface of the planet
- Always away from the mass
- Perpendicular to a surface
- Opposite to the motion of the mass
10.For a Newton's third law force pair, the two forces must act on the same object.
Medium
True or false?
11.Which of the following statements about a Newton's third law force pair are correct? (select all that apply)
Medium
- AThe forces are the same type
- BThe forces have the same magnitude
- CThe forces act on the same object
- DThe forces act in opposite directions
- EThe forces act on different objects
12.A material obeys Hooke's law. Which of the following statements is correct?
Hard
- AThe extension is directly proportional to the applied force up to the limit of proportionality
- BThe extension is inversely proportional to the applied force
- CThe extension is proportional to the square of the applied force
- DThe extension is independent of the applied force
13.A picture frame is hanging from a nail. Which of the following is the correct free-body diagram for the frame?
Hard
- AThree forces: weight downwards, tension from the nail upwards, and tension from the nail at an angle
- BTwo forces: weight downwards and normal force upwards
- CThree forces: weight downwards, normal force upwards, and friction to the left
- DOne force: weight downwards
14.A skier is being towed at constant speed whilst sinking into the snow. Which of the following forces act on the skier? (select all that apply)
Challenge
- AWeight
- BNormal contact force
- CFriction
- DTension from the rope
- EBuoyancy from the snow
15.Match each force symbol with the correct description of the force.
Medium
- F_g
- F_T
- F_N
- F_f
- Weight, always directed towards the surface of the planet
- Tension, always directed away from the mass
- Normal reaction force, perpendicular to a surface
- Frictional force, opposite to the direction of motion
16.Which of the following are correct statements about a Newton's third law force pair? (select all that apply)
Medium
- AThe two forces are the same type of force
- BThe two forces act on the same object
- CThe two forces are equal in magnitude
- DThe two forces act in opposite directions
- EThe two forces act on different objects
17.A car is moving at constant velocity along a straight horizontal road. Which of the following statements are correct? (select all that apply)
Medium
- AThe resultant force on the car is zero
- BThe driving force is greater than the frictional force
- CThe car is in translational equilibrium
- DThe normal contact force equals the weight of the car
- EThe car is accelerating
18.Match each term with its correct definition.
Medium
- Contact force
- Non-contact force
- Static friction
- Dynamic friction
- A force which acts between objects that are physically touching
- A force which acts at a distance, without any physical contact between bodies
- Friction that occurs when a body is stationary on a surface
- Friction that occurs when a body is in motion on a surface
19.Arrange the following stages of an object being pulled on a rough surface from rest until it moves at constant speed.
Medium
- Static friction increases as the applied force increases
- The applied force reaches the maximum static friction
- The object begins to move
- Dynamic friction acts, which is less than maximum static friction
- The object moves at constant speed when applied force equals dynamic friction
20.A wooden block is pulled along a horizontal table at constant speed by a horizontal force. Which of the following statements are correct? (select all that apply)
Hard
- AThe frictional force is dynamic friction
- BThe frictional force is equal in magnitude to the applied force
- CThe normal contact force is greater than the weight of the block
- DThe coefficient of dynamic friction equals the ratio of frictional force to normal contact force
- EThe block is in translational equilibrium
21.Match each equation with the quantity it defines.
Medium
- F = ma
- F_H = -kx
- F_d = 6πηrv
- F_f = μ_d F_N
- Newton's second law
- Hooke's law
- Stoke's law
- Dynamic friction
22.Which of the following are properties of an object moving in a horizontal circle at constant speed? (select all that apply)
Medium
- AThe speed is constant
- BThe velocity is constant
- CThe direction of motion is constantly changing
- DThere is a resultant force directed towards the centre of the circle
- EThe acceleration is zero
23.Place the steps in the correct order to calculate the velocity of two joined railway carriages after a collision, using conservation of momentum.
Medium
- Calculate the initial momentum of each carriage
- Add the initial momenta to find total momentum before collision
- Recognise that total momentum is conserved
- Set total momentum before equal to total momentum after
- Solve for the common velocity of the joined carriages
24.A car collides with a wall and comes to rest. Which of the following statements are correct? (select all that apply)
Hard
- AThe impulse experienced by the car equals its change in momentum
- BThe impulse is equal to the area under the force-time graph
- CA crumple zone increases the impact time
- DA crumple zone increases the force exerted on the car
- EThe change in momentum of the car is zero
25.Match each type of collision with the quantities that are conserved (assuming no external forces).
Medium
- Elastic collision
- Inelastic collision
- Momentum and kinetic energy are conserved
- Momentum is conserved but kinetic energy is not
26.A box is sliding down a rough slope at constant speed. Which of the following forces act on the box? (select all that apply)
Medium
- AWeight
- BNormal contact force
- CFrictional force
- DTension
- EBuoyancy
27.Arrange the steps to find the minimum coefficient of static friction for a block stationary on an incline.
Hard
- Resolve the weight into components parallel and perpendicular to the slope
- Set the parallel component equal to the frictional force
- Set the perpendicular component equal to the normal contact force
- Use F_f ≤ μ_s F_N and rearrange to find μ_s
- State that μ_s must be greater than or equal to the calculated value
28.Match each law with its correct statement.
Medium
- Newton's first law
- Newton's second law
- Newton's third law
- Hooke's law
- A body remains at rest or moves with constant velocity unless acted on by a resultant force
- The resultant force on an object is directly proportional to its acceleration
- If object A exerts a force on object B, then B exerts an equal and opposite force on A
- The extension of a material is directly proportional to the applied force up to the limit of proportionality
29.Order the steps to calculate the average force on a stuntman during landing, given his change in momentum and impact time.
Medium
- Calculate the momentum just before landing
- Calculate the momentum after landing (zero if at rest)
- Find the change in momentum (impulse)
- Use the impulse-momentum theorem: impulse = average force × time
- Rearrange to solve for average force
30.Match each type of force with an example.
Medium
- Gravitational force
- Electrostatic force
- Magnetic force
- Viscous drag
- The force between a planet and a comet
- The attraction between a proton and an electron
- The attraction between the north and south poles of magnets
- The force on an object moving through a fluid
31.Arrange the stages of a car collision with a wall, from before impact to after, in terms of momentum and impulse.
Hard
- The car has initial momentum
- The car collides with the wall and experiences a force
- The force acts over a time interval, producing an impulse
- The impulse equals the change in momentum
- The car comes to rest, so final momentum is zero
32.Two zorbs collide head-on and bounce off each other. Which of the following quantities are conserved during the collision? (select all that apply)
Hard
- ATotal momentum
- BTotal kinetic energy
- CTotal energy
- DThe momentum of each zorb individually
- EThe total mass
33.Order the steps to determine whether a collision is elastic or inelastic.
Hard
- Calculate total kinetic energy before the collision
- Calculate total kinetic energy after the collision
- Compare the two kinetic energy values
- If kinetic energy is unchanged, the collision is elastic
- If kinetic energy decreases, the collision is inelastic
34.A crate is pulled along a rough horizontal floor by a rope at a constant speed. The tension in the rope is 240 N and the crate moves 15 m in 5.0 s. What is the magnitude of the frictional force acting on the crate?
Medium
- A48 N
- B240 N
- C1200 N
- D0 N
35.A 2.0 kg trolley moving at 3.0 m/s collides with a stationary 1.0 kg trolley. They stick together. What is their common velocity after the collision?
Hard
- A1.0 m/s
- B1.5 m/s
- C2.0 m/s
- D3.0 m/s
36.A 1200 kg car travelling at 20 m/s collides head-on with a 2000 kg van travelling at 10 m/s in the opposite direction. They stick together. What is their velocity immediately after the collision?
Hard
- A1.25 m/s in the car's original direction
- B1.25 m/s in the van's original direction
- C2.5 m/s in the car's original direction
- D0 m/s
37.A 0.20 kg ball is dropped and hits the ground at 5.0 m/s, rebounding vertically upwards at 3.0 m/s. The contact time with the ground is 0.10 s. Which of the following statements are correct? (select all that apply)
Hard
- AThe change in momentum of the ball is 1.6 kg m/s upwards.
- BThe impulse on the ball is 1.6 N s upwards.
- CThe average force on the ball is 16 N upwards.
- DThe change in momentum of the ball is 0.4 kg m/s upwards.
- EThe average force on the ball is 4 N upwards.
38.A spring obeys Hooke's law. A force of 12 N produces an extension of 0.030 m. What is the spring constant of the spring?
Medium
- A0.36 N/m
- B2.5 N/m
- C400 N/m
- D360 N/m
39.A force–extension graph for a spring is plotted with force on the y-axis and extension on the x-axis. The linear region has a gradient of 250 N/m. What does this gradient represent?
Medium
- AThe extension of the spring
- BThe spring constant k
- CThe reciprocal of the spring constant, 1/k
- DThe elastic potential energy stored
40.An object moving in a horizontal circle at constant speed has a constant velocity.
Medium
True or false?
41.State the name of the force that occurs within an object when a pulling force is applied to both ends.
Easy
42.Complete the definition of Newton's first law.
Easy
A body will remain at rest or move with constant velocity unless acted on by a ____ force.
43.A block of mass 1.5 kg is moved along a table at constant speed by a force of 5.8 N. Calculate the magnitude of the dynamic coefficient of friction between the block and the table. Use g = 9.81 m s⁻². Give your answer to 2 significant figures.
Medium
44.Complete the equation for Stoke's law by filling in the missing symbols.
Hard
Fd = 6π____rv, where η is the fluid viscosity, r is the radius of the sphere, and v is the velocity of the sphere through the fluid.
45.A 0.50 kg ball moving at 8.0 m/s is brought to rest by a force of 40 N. For how long does the force act? Give your answer in seconds.
Medium
46.A spherical raindrop of radius 0.50 mm falls through air of viscosity 1.8 × 10⁻⁵ Pa s at a speed of 2.0 m/s. Calculate the viscous drag force on the raindrop, using Stoke's law. Give your answer in newtons.
Hard
47.Explain the difference between static friction and dynamic friction.
Medium
48.Outline what is meant by translational equilibrium.
Hard
49.A car is designed with a crumple zone. Explain how the crumple zone reduces the force experienced by the passengers during a collision.
Hard
50.A 0.25 kg ball is swung in a horizontal circle of radius 1.2 m at a constant speed of 4.0 m/s. Calculate the centripetal force acting on the ball and state its direction.
Hard
51.A block of mass 1.5 kg is moved along a table at constant speed by a force of 5.8 N. Calculate the magnitude of the dynamic coefficient of friction between the block and the table. Use g = 9.81 m s⁻². Give your answer to 2 significant figures.
Medium
- A0.39
- B-0.61
- C1.39
- D2.39
52.A 0.50 kg ball moving at 8.0 m/s is brought to rest by a force of 40 N. For how long does the force act? Give your answer in seconds.
Medium
- A-0.9 s
- B2.1 s
- C0.1 s
- D1.1 s
53.A spherical raindrop of radius 0.50 mm falls through air of viscosity 1.8 × 10⁻⁵ Pa s at a speed of 2.0 m/s. Calculate the viscous drag force on the raindrop, using Stoke's law. Give your answer in newtons.
Hard
- A3.4e-7 N
- B-1 N
- C1 N
- D2 N

Unlock all 53 questions & more

Crie uma conta grátis para ver todas as perguntas, os slides, flashcards e resumos de revisão deste tópico.

CADASTRE-SE — GRÁTIS ENTRAR

Provas anteriores

A prática com provas anteriores deste tópico chega em breve.

Em breve