P2.2 - Newton's Laws

Question 1

What can a force do?

Answer 1

Forces can change the shape of an object or change an object’s motion.

Question 2

How can you represent forces?

Answer 2

• You cannot see a force.
• You can only see the effect of a force.
• You can represent forces as arrows in diagrams.
• Forces always have a size and a direction.
• Forces are measured in units of Newtons, N.

Question 3

How can bodies interact to exert forces?

Answer 3

• By directly touching each other - Contact forces.
• At a distance, without touching - Non-contact
  forces.

Question 4

What is a field?

Answer 4

A field is a region where electrical/magnetic/mass bodies experience a force.

Question 5

What happens when two bodies interact to produce a force?

Answer 5

• Each body exerts a separate force on the other.
• The two forces are the same in size, but in
  opposite direction.

Question 6

What is Newtons 3rd Law?

Answer 6

“When two bodies interact, they exert a force upon each other that is equal in size, but opposite in direction”

Question 7

What is a free body diagram and what are they used to calculate?

Answer 7

• A free body diagram, is a diagram that shows all
  the forces acting on a single body.
• Free body diagrams are used to calculate the
  resultant force that acts on a body.

Question 8

What are the 3 steps to draw a free body diagram?

Answer 8

1. Identify all the non-contact pairs.
   2.Identify all the contact pairs.
2. Focus on a single object and draw that object
   with arrows showing all the forces acting on
   that object.

Question 9

How do you calculate a resultant force when forces act in the same plane?

Answer 9

By adding and subtracting the forces.

Question 10

How do you calculate a resultant force when forces act at 90 degrees to each other?

Answer 10

• Draw a force triangle from head to tail.
• The resultant force is the hypotenuse which can
  be calculated either through calculation or by a
  scale drawing.

Question 11

What does it mean to resolve a force and how do you resolve a force?

Answer 11

• To resolve a force means to work out how much
  of it acts in the horizontal plane and how much of
  it acts in the vertical plane.
• Make a triangle where the force is the
  hypotenuse and the components are the sides.
• Use a scale diagram or trig to calculate the
  components.

Question 12

How can you tell if and object is in equilibrium from their vectors?

Answer 12

When the forces acting on an object are in equilibrium, their vectors will form a closed shape when connected nose-to-tail.

Question 13

What is Newton’s first law?

Answer 13

“A body will remain at rest, or continue to move at a uniform velocity, unless acted upon by a resultant force.”

Question 14

What is inertia?

Answer 14

• Inertia is a measure of how difficult it is to
  change the velocity of a body.
• The greater the mass of a body, the greater its
  inertia.
• The more difficult it is to change its velocity the
  more difficult it is to accelerate the body.

Question 15

What is equilibrium?

Answer 15

• A body that does not accelerate (whether its at
  rest or moving at uniform speed), is said to be in
  a state of equilibrium.
• In equilibrium, the resultant force acting on a
  body is zero (in every plane)

Question 16

What is Newton’s second law?

Answer 16

“The acceleration experienced by a body is directly proportional to the resultant force acting upon it, and in the same direction as. And, the acceleration is inversely proportional to mass.”

Question 17

What can a resultant force do to an object?

Answer 17

• Change the speed of an object.
• Change the direction of motion of an object.
• Change both the speed and direction of motion of
  an object.

Question 18

What does Newton’s second law depend on?

Answer 18

• The size of the resultant force.

- The mass (inertia) of the object.

Question 19

What equation do you need to apply in relation to Newton’s second law?

Answer 19

Force(N) = Mass (kg) x Acceleration

Question 20

Why is a body moving in a circle at constant speed still accelerating?

Answer 20

• Constantly changing direction.
• Constantly changing velocity.
• Thus, constantly accelerating - Therefore must
  be subject to a resultant force (centripetal force).
• Centripetal force acts towards the centre of the
  circle and acts only to change the direction of a
  body the velocity (not the size).

Question 21

Draw the components of the forces acting on a person on a slope.

Answer 21

Question 22

What are the two planes we consider of an object on a slope?

Answer 22

1. Plane parallel to the slope - The plane in which
   the box tries to accelerate.
2. Plane perpendicular to the slope - The plane
   in which the normal reaction force acts.

Question 23

What is terminal velocity?

Answer 23

The velocity at which a body falls through a medium, when the drag acting on the body is equal to its weight.

Question 24

The classic terminal velocity question:)

Answer 24

When the parachutist jumps, the drag force acting is small so the parachutist experiences a large acceleration due to the force of his weight.

As his speed increases, the drag force acting on him will increase, so the resultant force will decrease and his acceleration will decrease.

Eventually, the drag force will balance the weight. Acceleration is now zero and the speed is constant. The parachutist reaches a terminal velocity.

When the parachutist opens his parachute, the drag force will suddenly become very large, as the parachutist has a large surface area. This will cause Felix to start to decelerate.

As Felix is now slowing down, the drag force will start to decrease again until it balances his weight. Felix has now reached a new terminal velocity.

Question 25

What is the equation of momentum?

Answer 25

p = m x v

Question 26

Explain the Law of Conservation of Momentum.

Answer 26

• Momentum of a system (group of bodies
  interacting) is always conserved.
• The total momentum before an event is equal to
  the total momentum after the event.

Question 27

What is the difference between an elastic collision and an inelastic collision?

Answer 27

• Elastic collision - No energy transferred to other
  stores.
• Inelastic collision - Energy transferred to other
  stores like sound to a thermal store

Question 28

What are the two specific type of interactions?

Answer 28

• Bodies colliding and moving off together -
  Stationary body with zero momentum is hit and
  both bodies move off together.
• Two stationary bodies, pushing off and moving
  away from each other - Recoil and explosions -
  Two bodies initially at rest end up moving away
  from each other (sometimes in the same plane of
  direction).

Question 29

How do you calculate the force exerted in relation to momentum?

Answer 29

Force (N) = Change In Momentum (kgm/s) / Time (s)

Question 30

What is work done and what are the units of work done?

Answer 30

• Work done is the transfer of energy between
  stores.
• Work is measured in units of Joules, J.

Question 31

What is the equation for work done?

Answer 31

Work done (J) = Force (N) x Distance (m)

Question 32

What does friction do?

Answer 32

Friction always transfers kinetic energy out of a body and transfers to heat whilst the force applied does work to transfer energy into the body.

Question 33

What is power?

Answer 33

Power is the rate of energy transfer.

Question 34

What is the equation for power?

Answer 34

Power (W) = Work done (J) / Time (s)