1. PHYS 2010 Nathalie Hoffmann University of Utah
Thursday 5/28
PHYS 2010
Nathalie Hoffmann
University of Utah

2. Forces Vector or scalar quantities? Units?
Important concepts/physical principles?

3. Forces Vector or scalar quantities: Vector
Units: Newtons (N) or 𝑘𝑔∙𝑚/ 𝑠 2
Newton’s laws of motion

4. Newton’s Laws
1st: An object at rest tends to stay at rest, and an object in uniform motion tends to stay in motion with the same speed and in the same direction unless acted upon by a net force.
2nd: 𝑭 𝑒𝑥𝑡 =𝑚𝒂
If there is a net external force acting on an object, it will accelerate.
The direction of the acceleration is in the same direction as the net force.
3rd: If object A exerts a force on object B, object B exerts a force on object A that has the same magnitude but is in the opposite direction. These two forces act on different objects.

5. Newton’s 2nd law 𝑭 𝑒𝑥𝑡 = 𝑭 1 + 𝑭 2 + 𝑭 3 + 𝑭 4 + … =𝑚𝒂
Remember, for 2+ dimensions:
𝐹 𝑒𝑥𝑡,𝑥 =𝑚 𝑎 𝑥
𝐹 𝑒𝑥𝑡,𝑦 =𝑚 𝑎 𝑦

6. Weight
Gravitational force on an object
Source: planet Earth
𝒘=𝑚𝒈

7. Normal Force
Why is it called ‘normal’ force?

8. Normal Force Why is it called ‘normal’ force?
‘Normal’ means perpendicular
Normal force is directed perpendicular to the surface on which object rests

9. Solving Force Problems
Draw a free-body diagram for EACH object
Write Newton’s 2nd law for EACH object
(split into components, if necessary/applicable)
Proceed as usual
Knowns, unknowns. Solve.

10. Practice Problems
An astronaut has a mass of 80.0 kg. What are the astronaut’s mass and weight on Mars where surface gravity is 38.0% of that on Earth?
A tugboat uses its winch to pull up a sinking sailboat with an upward force of 4500 N. The mass of the boat is 200 kg and the water acts on the sailboat with a drag force of 2000 N. Draw a free-body diagram for the sailboat and determine the acceleration of the sailboat.
The distance between two telephone poles is 50 m. When a kg bird lands on the telephone wire midway between the poles, the wire sags 0.15 m. How much tension does the bird produce in the wire? Ignore the weight of the wire.

11. Practice Problems
A locomotive pulls 10 identical freight cars. The force between the locomotive and the first car is 100,000 N, and the acceleration of the train is 2 m/s2. There is no friction to consider. Find the force between the ninth and tenth cars.
Box A weighs 80 N and rests on a table. A rope that connects boxes A and B drapes over a pulley so that box B hangs above the table, as shown in the figure. The pulley and rope are massless, and the pulley is frictionless. What force does the table exert on box A if box B weighs(a) 35 N; (b) 70 N; (c) 90 N?

12. Practice Problems
In the figure, the block on the left incline is 6.00 kg. If θ1 = 60.0° and θ2 = 25°, find the mass of the block on the right incline so that the system is in equilibrium (no acceleration). All surfaces are frictionless.
A child on a sled starts from rest at the top of a 20.0° slope. Assuming that there are no forces resisting the sled’s motion, how long will the child take to reach the bottom of the slope, 210 m from the top?
A 30.0-kg golden retriever stands on a scale in an elevator. Calculate the reading on the scale when the elevator (a) accelerates at m/s2downward, (b) when the elevator cruises down at a steady speed, and (c)when the elevator accelerates at 4.00 m/s2 upward.

13. Practice Problem Answers
P1.1: mass = 80.0 kg, weight = N
P1.2: a = 2.7 m/s
P1.3: T = 410 N
P2.1: 𝐹 9−10 =10000 𝑁
P2.2: a) F = 45 N upward, b) F = 10 N up, c) F = 0 N
P3.1: m = 3.31 kg
P3.2: t = 11.2 s
P3.3: a) 189 N, b) 294 N, c) 414 N