State Newton's first law .

Newton's first law states that a body will remain at rest or move with constant velocity unless acted on by a resultant force.

State Newton's second law .

Newton's second law states that the resultant force on a body is equal to the rate of change of momentum, and the change in momentum is in the same direction as the resultant force.

State Newton's third law .

Newton's third law states that if body A exerts a force on body B, then body B exerts a force on body A of equal magnitude but in the opposite direction.

What two conditions must a Newton's third law force pair satisfy?

The two forces must act on different objects and be of the same type (e.g. both gravitational or both frictional).

If the forces on a body are balanced, the resultant force is .......... , so the body is either at rest or moving with constant velocity.

If the forces on a body are balanced, the resultant force is zero , so the body is either at rest or moving with constant velocity.

True or False? A Newton's third law force pair can act on the same object.

False. Newton's third law force pairs must always act on two different objects .

What type of quantity is momentum, and what does this mean for its sign?

Momentum is a vector quantity, having both magnitude and direction. It can therefore be positive or negative depending on the object's direction of motion relative to the chosen positive direction.

The linear momentum of an object remains .......... unless the system is acted on by an external resultant force.

The linear momentum of an object remains constant unless the system is acted on by an external resultant force.

True or False? A heavier object always has greater momentum than a lighter object.

False. Momentum depends on both mass and velocity , so a light, fast-moving object can have the same momentum as a heavy, slow-moving one (e.g. a tennis ball and a brick can have equal momentum).

What are the units of impulse?

N s (newton seconds), equivalent to kg m s -1 .

True or False? Impulse is equal to change in momentum.

True. Impulse is equal to change in momentum . Therefore, change in momentum can be used to indirectly measure impulse.

Why does the force on an object become negative when it rebounds off a collision?

Because it is directed in the opposite direction to the object's initial velocity — the force is produced by the object it has collided with.

A small force acting over a long time has the .......... effect (impulse) as a large force acting over a short time.

A small force acting over a long time has the same effect (impulse) as a large force acting over a short time.

True or False? Decreasing the time over which a change in momentum occurs reduces the force applied to the object.

False. Increasing the time over which a change in momentum occurs reduces the force experienced by the object.

Why is the impulse on an umbrella greater in hail than in rain?

Hailstones bounce back off the umbrella, causing a greater change in momentum than raindrops, which simply splatter and stop. A greater change in momentum means a greater impulse.

How is impulse found from a force-time graph?

Impulse is equal to the area under the force-time graph .

How can you determine the area under a curved force-time graph?

Count the squares underneath the curve.

How can you determine the area under a force-time graph made up of straight lines?

Split the graph into sections (e.g. triangles, rectangles, trapeziums) and take the sum of the areas of each section.

State the principle of conservation of momentum .

The principle of conservation of momentum states that the total momentum before a collision is equal to the total momentum after a collision, provided no external force acts.

True or False? Internal forces change the total momentum of a system.

False. Internal forces , such as tension between colliding objects, do not change a system's total momentum. Only external forces (for example friction or weight ) can change it, which is why momentum is conserved provided no external force acts.

Define a closed or isolated system.

A closed or isolated system is one on which no external forces act, so its total momentum is conserved .

A swimmer dives forward off a stationary boat. Using conservation of momentum, explain what happens to the boat.

Before the dive, the total momentum of the diver-boat system is zero . Since no external forces act horizontally, momentum must remain zero after the dive, so the boat moves backwards to balance the diver's forward momentum.

Define an elastic collision .

An elastic collision is one in which kinetic energy is conserved (as well as momentum).

Define an inelastic collision .

An inelastic collision is one in which kinetic energy is not conserved (although momentum is still conserved).

How can you determine whether a collision is elastic or inelastic?

Compare the total kinetic energy before and after the collision. If it is unchanged, the collision is elastic; if it has decreased, the collision is inelastic.

What typically happens to two objects in an inelastic collision?

They stick together and move with a common final velocity after the collision.

What quantity is always conserved in collisions and explosions?

Momentum is always conserved in both collisions and explosions.

How should you treat two objects that stick together after a collision, when calculating momentum?

Treat them as a single object with mass equal to the sum of the two individual masses.

In two-dimensional collision problems, momentum is resolved into its horizontal and .......... components.

In two-dimensional collision problems, momentum is resolved into its horizontal and vertical components.

True or False? An explosion, such as a gun recoiling, conserves kinetic energy.

False. Momentum is always conserved in an explosion, but kinetic energy is not necessarily conserved .

A LevelPhysicsOCRFlashcards3. Forces & MotionNewton's Laws of Motion & Momentum

Newton's Laws of Motion & Momentum (OCR A Level Physics): Flashcards

Exam code: H556

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FrontNewton's Three Laws of Motion
State Newton's first law.

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Cards in this collection (40)

State Newton's first law.
Newton's first law states that a body will remain at rest or move with constant velocity unless acted on by a resultant force.
State Newton's second law.
Newton's second law states that the resultant force on a body is equal to the rate of change of momentum, and the change in momentum is in the same direction as the resultant force.
State Newton's third law.
Newton's third law states that if body A exerts a force on body B, then body B exerts a force on body A of equal magnitude but in the opposite direction.
What two conditions must a Newton's third law force pair satisfy?
The two forces must act on different objects and be of the same type (e.g. both gravitational or both frictional).
Show how $F = m a$ follows from Newton's second law for an object of constant mass.
$F = \frac{Δ p}{Δ t} = \frac{m v - m u}{Δ t} = \frac{m (v - u)}{Δ t} = m a$
since the mass is constant.
If the forces on a body are balanced, the resultant force is .........., so the body is either at rest or moving with constant velocity.
If the forces on a body are balanced, the resultant force is zero, so the body is either at rest or moving with constant velocity.
True or False?
A Newton's third law force pair can act on the same object.
False.
Newton's third law force pairs must always act on two different objects.
Define momentum.
Momentum is defined as the product of an object's mass and velocity: $p = m v$
What type of quantity is momentum, and what does this mean for its sign?
Momentum is a vector quantity, having both magnitude and direction. It can therefore be positive or negative depending on the object's direction of motion relative to the chosen positive direction.
The linear momentum of an object remains .......... unless the system is acted on by an external resultant force.
The linear momentum of an object remains constant unless the system is acted on by an external resultant force.
What are the SI units of momentum?
kg m s^-1
True or False?
A heavier object always has greater momentum than a lighter object.
False.
Momentum depends on both mass and velocity, so a light, fast-moving object can have the same momentum as a heavy, slow-moving one (e.g. a tennis ball and a brick can have equal momentum).
Define impulse.
Impulse is the change in momentum of an object: $I = F ∆ t = ∆ p = m v - m u$
Define force in terms of momentum.
Force is defined as the rate of change of momentum of a body: $F = \frac{∆ p}{∆ t}$
What are the units of impulse?
N s (newton seconds), equivalent to kg m s^-1.
True or False?
Impulse is equal to change in momentum.
True.
Impulse is equal to change in momentum. Therefore, change in momentum can be used to indirectly measure impulse.
Why does the force on an object become negative when it rebounds off a collision?
Because it is directed in the opposite direction to the object's initial velocity — the force is produced by the object it has collided with.
A small force acting over a long time has the .......... effect (impulse) as a large force acting over a short time.
A small force acting over a long time has the same effect (impulse) as a large force acting over a short time.
True or False?
Decreasing the time over which a change in momentum occurs reduces the force applied to the object.
False.
Increasing the time over which a change in momentum occurs reduces the force experienced by the object.
Why is the impulse on an umbrella greater in hail than in rain?
Hailstones bounce back off the umbrella, causing a greater change in momentum than raindrops, which simply splatter and stop. A greater change in momentum means a greater impulse.
True or False?
The equation $I = F ∆ t$ can be used when the force varies with time.
False.
This form of the impulse equation only applies when the force is constant.
How is impulse found from a force-time graph?
Impulse is equal to the area under the force-time graph.
How can you determine the area under a curved force-time graph?
Count the squares underneath the curve.
How can you determine the area under a force-time graph made up of straight lines?
Split the graph into sections (e.g. triangles, rectangles, trapeziums) and take the sum of the areas of each section.
For the following graph, what is the impulse of the force acting for a time $T$ ?
The impulse of the force is $I = \frac{F_{m a x} T}{2}$
Impulse is the area under a force-time graph, therefore:
- $I = (\frac{F_{m a x}}{2} \times \frac{T}{2}) + (\frac{F_{m a x}}{2} \times \frac{T}{2})$
- $I = \frac{F_{m a x} T}{2}$
State the principle of conservation of momentum.
The principle of conservation of momentum states that the total momentum before a collision is equal to the total momentum after a collision, provided no external force acts.
What is the conservation of momentum equation for two objects, A and B, that collide?
$p_{A i} + p_{B i} = p_{A f} + p_{B f}$
- $p_{A i}$ and $p_{B i}$ are the initial momenta of A and B
- $p_{A f}$ and $p_{B f}$ are the final momenta of A and B
- All momenta are measured in kg m s^-1
True or False?
Internal forces change the total momentum of a system.
False.
Internal forces, such as tension between colliding objects, do not change a system's total momentum. Only external forces (for example friction or weight) can change it, which is why momentum is conserved provided no external force acts.
Define a closed or isolated system.
A closed or isolated system is one on which no external forces act, so its total momentum is conserved.
For two balls, A and B, colliding head-on, conservation of momentum gives $m_{A} u_{A} = m_{B} v_{B} - m_{A} v_{A}$ . The minus sign shows that ball A's final velocity is in the .......... direction to its initial motion.
For two balls, A and B, colliding head-on, conservation of momentum gives $m_{A} u_{A} = m_{B} v_{B} - m_{A} v_{A}$ . The minus sign shows that ball A's final velocity is in the opposite direction to its initial motion.
Trolley A has a mass of 0.80 kg and travels at 3.0 m s^-1 towards a stationary trolley B, which has twice the mass of trolley A. The trolleys collide head-on and stick together. Using conservation of momentum, calculate the common velocity of both trolleys after the collision.
$p_{b e f o r e} = (0.80 \times 3.0) + (1.60 \times 0) = 2.4 {kg m s}^{- 1}$
$p_{a f t e r} = (0.80 + 1.60) v = 2.40 v$
Since momentum is conserved, $p_{b e f o r e} = p_{a f t e r}$ , so:
$v = \frac{2.4}{2.40} = 1.0 {m s}^{- 1}$
A swimmer dives forward off a stationary boat. Using conservation of momentum, explain what happens to the boat.
Before the dive, the total momentum of the diver-boat system is zero. Since no external forces act horizontally, momentum must remain zero after the dive, so the boat moves backwards to balance the diver's forward momentum.
Define an elastic collision.
An elastic collision is one in which kinetic energy is conserved (as well as momentum).
Define an inelastic collision.
An inelastic collision is one in which kinetic energy is not conserved (although momentum is still conserved).
How can you determine whether a collision is elastic or inelastic?
Compare the total kinetic energy before and after the collision. If it is unchanged, the collision is elastic; if it has decreased, the collision is inelastic.
What typically happens to two objects in an inelastic collision?
They stick together and move with a common final velocity after the collision.
What quantity is always conserved in collisions and explosions?
Momentum is always conserved in both collisions and explosions.
How should you treat two objects that stick together after a collision, when calculating momentum?
Treat them as a single object with mass equal to the sum of the two individual masses.
In two-dimensional collision problems, momentum is resolved into its horizontal and .......... components.
In two-dimensional collision problems, momentum is resolved into its horizontal and vertical components.
True or False?
An explosion, such as a gun recoiling, conserves kinetic energy.
False.
Momentum is always conserved in an explosion, but kinetic energy is not necessarily conserved.

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