Forces Test Review & Study Guide PDF

# Forces test review & Study Guide ## Forces Standard #1 - Student will be able to **calculate the values of** and **draw accurate free-body diagrams including the force of gravity, normal force, friction, and tension**. **What this means:** - You must be able to identify which of the four main m...

# Forces test review & Study Guide ## Forces Standard #1 - Student will be able to **calculate the values of** and **draw accurate free-body diagrams including the force of gravity, normal force, friction, and tension**. **What this means:** - You must be able to identify which of the four main mechanical forces are acting on an object based on either a description of the force, or a description of the situation. - You'll also have to be able to explain how changing any of the components of a particular force will alter its value, and you must be able to calculate the exact value of each force and the Net Force acting on an object. ## Forces Standard #2 - Students will be able to **apply Newton's First and Second Laws to explain static and dynamic motion**. **What this means:** - You must be able to use the calculated (or given) **Net Force on an object to describe its resulting motion** - both for objects at rest and those that are moving. - You must be able to **calculate the velocity change** of an object based on its Net Force and duration of the forces. - Finally, you must be able to predict how the motion will change if the Net Force changes. ## Important Equations - $F_{net}$ = ma - $F_g$ = mg - $F_f$ = µ$F_N$ - a = $\frac{\Delta v}{\Delta t}$ ## Draw FBD's for the following situations: 1. **A rocket is accelerating upward due to its engines thrust** 2. **A rocket is coasting up after its engines burn out** 3. **A rocket is falling at constant speed under its parachute** 4. **A car slows to a stop at a red light (moving right)** 5. **One team is pulling harder than the other in tug-of-war (draw FBD of the rope)** 6. **A tennis ball is bouncing on the floor (draw FBD while in contact with the ground)** 7. **A pencil sits motionless on the desk** 8. **A hockey puck slides across frictionless ice at a constant speed** ## Questions 1. How do you calculate the weight of an object if you know its mass? 2. How do you calculate the normal force of an object on a surface? 3. How do you calculate the friction force on an object? 4. Which forces (if any) are balanced if: - an object is accelerating to the right? - moving at constant speed to the right. - accelerating upward. - moving upward at constant speed. - not moving 5. A man stands on a scale in an elevator. His mass is 80kg but the scale reads 600 Newtons. What is his acceleration? 6. A boy pulls a 25kg sled across the ground. He pulls with 300N of force, and the coefficient of friction is 0.2. What is the net force, the acceleration, and how much speed does it gain in 3 seconds? 7. Use Newton's 2nd law to explain why a baseball and golf ball will fall at the same speed. 8. Draw FBDs and calculate the values of $F_{net}$ and the acceleration of a 3kg rocket while it is: - Being pushed up by a 100N motor thrust - Coasting to a maximum height after the motor burns out - Falling at a constant speed of -15m/s after the parachute has deployed 9. Mars has an acceleration of gravity of 3.6 m/s/s. How does your mass compare on Earth vs. on Mars? How does your weight compare on Earth vs. on Mars? Which planet would it be easier to lift a weight? Which planet would it be easier to push a sled across a frictionless surface (assuming you could magically get some traction under your feet...) 10. As my father in law pulls his boat out of the lake, the back tires on his truck begin to spin out on the boat ramp. How could we get more traction? # Answers 1. Weight = mass * gravity 2. Normal force = how hard the table/floor/chair must push back. If it's only an object, $F_{normal}$ = weight. But you might have someone pushing down or pulling up on the object, so you have to take that into account as well. 3. Friction = mu * $F_{normal}$ 4. A. no b. yes. C. no. d yes. E yes. 5. -2.5m/s/s 6. 250N to the right (or positive 250N), +10m/s/s, +30m/s 7. A=$F_{net}$/m. g = $F_g$/m. So, if gravity is the only force on a falling object (that is. $F_g$ = $F_{net}$) the acceleration is just g. The other way to think of this is that a more massive object is harder to move... but gravity DOES pull harder on it because it has a greater weight. Those effects cancel. 8. - $F_{motor}$ = 100N - $F_{parachute}$ = 30N - $F_g$ = 30N - $F_{net}$ = 70N - a = 23.33 m/s/s - $F_{net}$ = -30N - a = -10 m/s/s - $F_{net}$ = ON - a = 0 m/s/s 9. Mass is the same. Weight is more on Earth. Mars is easier to lift a weight. Pushing something sideways would be the same. (you're not fighting gravity... only its inertia) 10. $F{friction}$ = mu * $F{normal}$. To increase Ffriction you need to either make the interaction between the tires and the boat ramp stickier (increase the gripatude or the mu) by adding sand, salt, etc. or increase the $F_{normal}$ by putting more weight over the back tires. A couple of kids sitting on the back bumper ought to do it... # Force Analysis Worksheet 1. Free body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces. | Situation | Free Body Diagram | Forces | |---|---|---| | A | [Image of a box with 50N force down and 50N force up] | $F_{net}$ = 0N, A = 50 , B = 50<br> C = | | C | [Image of a box with 200N force down and 200N force up] | $F_{net}$ = 900N, up, C = 200, D = 700 <br>E = | | E | [Image of a box with 300N force left and 300N force right] | $F_{net}$ = 60N, left, E = 300, F = 240<br> G = | | G | [Image of a box with 20N force right and 20N force left] | $F_{net}$ = 30N, right, G = 20, H = 10<br> I = | 2. A 0.8 kg hockey puck is being pushed across the ice with 10 N of force. Determine the net force on the puck if the coefficient of friction between ice and plastic is 0.1. DRAW A FBD! 3. A 20 kg stone rolls across the ground with an acceleration of -0.8 m/s². Calculate all forces acting on the stone, and the coefficient of friction between the stone and grass. DRAW A FBD! 4. A 60 kg water skier is being pulled by rope that she accelerates at 2 m/s². If the coefficient of friction between her and the water is 0.8, what is the tension force? DRAW A FBD!