Dr. Larry Dennis,

1
Physics 2053C Fall 2001

• Chapter 4
• Forces and
• Newtons Laws of Motion

2
Rescheduling

• Quiz ? Cancelled.
• Lab ? Thursdays lab will meet.
• CAPA ? Due Thursday at noon.

3
The Quiz That Wasnt.
This quiz will be about a car driving along a
level road. The car has a mass of 2000 kg.
  1)     (2 points) If the car is driving
along the level road, what do you know about the
component of the cars velocity in the vertical
direction?   a)     The magnitude of the
vertical component of the cars velocity is
greater than zero, and its direction is
up. b)    The magnitude of the vertical
component of the cars velocity is greater than
zero, and its direction is down. c)     The
vertical component of the cars velocity is
zero.  
4
The Quiz That Wasnt.

• 2. (2 points) If the car is driving along the
  level road, what do you know about the component
  of the cars acceleration in the vertical
  direction.
• The magnitude of the vertical component of the
  cars acceleration is greater than zero and its
  direction is up.
• The magnitude of the vertical component of the
  cars acceleration is greater than zero and its
  direction is down.
• The vertical component of the cars acceleration
  is zero.

5
The Quiz That Wasnt.

• 3) (3 points) If the car travels 600 km in a
  constant direction at a constant speed in 6 hrs,
  what is the speed at which the car travels? Make
  sure you clearly specify what equation you are
  using.

6
The Quiz That Wasnt.

• 4) (3 points) If the car is driving along the
  level road at a constant speed and in a constant
  direction, what do you know about the horizontal
  component of the car's acceleration?
• The magnitude of the horizontal component of the
  acceleration is zero.
• b) The magnitude of the horizontal component of
  the acceleration is greater than zero, and its
  direction is in the same direction as the motion
  of the car.
• c) The magnitude of the horizontal component of
  the acceleration is greater than zero, and its
  direction is opposite to the direction of the
  motion of the car.

7
The Quiz That Wasnt.

• 5) (2 points) If the car is driving along the
  level road with an increasing speed and in a
  constant direction, what do you know about the
  horizontal component of the car's acceleration?
• a) The magnitude of the horizontal component of
  the acceleration is zero.
• b) The magnitude of the horizontal component of
  the acceleration is greater than zero, and its
  direction is in the same direction as the motion
  of the car.
• c) The magnitude of the horizontal component of
  the acceleration is greater than zero, and its
  direction is opposite to the direction of the
  motion of the car.

8
The Quiz That Wasnt.

• 6) (3 points) If the car's speed increases from
  25 m/s to 50 m/s in 3.5 s while continuing to
  travel in a constant direction, what is the
  magnitude of the horizontal component of the
  car's average acceleration? Make sure you
  clearly specify what equation you are using.

9
The Quiz That Wasnt.

• 7) (2 points) If the car is driving along the
  level road with a decreasing speed and in a
  constant direction, what do you know about the
  horizontal component of the car's acceleration?
• The magnitude of the horizontal component of the
  acceleration is zero.
• The magnitude of the horizontal component of the
  acceleration is greater than zero, and its
  direction is in the same direction as the motion
  of the car.
• The magnitude of the horizontal component of the
  acceleration is greater than zero, and its
  direction is opposite to the direction of the
  motion of the car.

10
The Quiz That Wasnt.

• 8) (3 points) If the car's speed decreases from
  40 m/s to 20 m/s in 2.5 s while continuing to
  travel in a constant direction, what is the
  magnitude of the horizontal component of the
  car's average acceleration? Make sure you
  clearly specify what equation you are using.

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Forces

• A push or pull that often gives rise to motion.
• Newtons Laws
• If the force on an object is zero, then its
  velocity is constant.
• The acceleration net force / mass.
• Whenever one object exerts a force on a second
  object, the second object exerts an equal and
  opposite force on the first object.

? F m a
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Vector Nature of Forces

• Forces have
• Magnitude
• Direction
• ?F m a

Vector
Vector
Scalar
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Example Force and Acceleration

• A boy pulls a 10 kg cart across the floor. If he
  pulls with a force of 10 N, the cart moves with
  constant speed. If he pulls with a force of 15 N
  what is the acceleration of the cart?

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Example Force and Acceleration

• A boy pulls a 10 kg cart across the floor. If he
  pulls with a force of 10 N, the cart moves with
  constant speed. If he pulls with a force of 15 N
  what is the acceleration of the cart?

The normal force and weight are equal and
opposite in direction (ay 0).
N W 0
Friction opposes the force of the boy pulling on
the cart.
Fboy f max
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Example Force and Acceleration

• A boy pulls a 10 kg cart across the floor. If he
  pulls with a force of 10 N, the cart moves with
  constant speed. If he pulls with a force of 15 N
  what is the acceleration of the cart?

Fboy f max
When Fboy 10 N, ax 0 Fboy f max
0 f Fboy 10 N
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Example Force and Acceleration

• A boy pulls a 10 kg cart across the floor. If he
  pulls with a force of 10 N, the cart moves with
  constant speed. If he pulls with a force of 15 N
  what is the acceleration of the cart?

Fboy f max
The force of friction does not increase when the
boy increases his force. Fboy f max ? ax
(Fboy f)/m ax (15 N 10 N)/10 kg
0.5 m/s2
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Force and Acceleration

• A boy on his sled is being pulled across the ice
  on a sled, by his sister. She pulls on the rope
  at an angle of 53o with a force of 47 N. Assume
  the ice is frictionless and that the boy an the
  sled have a mass of 42 kg.
• Determine the acceleration of the sled.
• What is the normal force of the ground on the
  sled?

• Step 1
• Draw a free body diagram.
• Identify all the forces.
• Show relevant angles.

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Force and Acceleration

• A boy on his sled is being pulled across the ice
  on a sled, by his sister. She pulls on the rope
  at an angle of 53o with a force of 47 N. Assume
  the ice is frictionless and that the boy an the
  sled have a mass of 42 kg.

Step 2 Use Newtons Laws for forces in the
horizontal and vertical directions.
Horizontal forces. F cos ? max
Vertical forces (acceleration 0). F sin ? N
mg 0
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Force and Acceleration

• A boy on his sled is being pulled across the ice
  on a sled, by his sister. She pulls on the rope
  at an angle of 53o with a force of 47 N. Assume
  the ice is frictionless and that the boy an the
  sled have a mass of 42 kg.
• Determine the acceleration of the sled.

Step 3 Solve for the requested values.
F cos ? max ax F cos ?/m 47 N
cos 53o / 42 kg 47 0.6/42 0.67
m/s2
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Force and Acceleration

• A boy on his sled is being pulled across the ice
  on a sled, by his sister. She pulls on the rope
  at an angle of 53o with a force of 47 N. Assume
  the ice is frictionless and that the boy an the
  sled have a mass of 42 kg.
• What is the normal force of the ground on the
  sled?

Step 3 (continued) Solve for the requested
values.
F sin ? N mg 0 N mg - F sin ?
42 kg 10 m/s2 47 N sin 53o
382 N
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Next Time

• Finish Chapter 4.
• Sample problems.
• Please see me with any questions or comments.

See you Friday.